50 - 555 Circuits

Save 50 - 555 Circuits (more than 101 Circuits) as: .doc (5.6MB) or .pdf (2.7MB) (26-5-2019)
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For our other free eBooks,
Go to: 1 - 100 Transistor Circuits
Go to: 101 - 200 Transistor Circuits
Go to: 100 IC Circuits

For more data on the 555, see these pages:

555-Page 1    for CD users:   555-Page 1
555-Page 2                                555-Page 2
555-Page 3                                555-Page 3
555-Test                                     555-Test

To learn about the development and history of the 555, go to these links:
http://semiconductormuseum.com/Museum_Index.htm - a general discussion about the development of the transistor
http://semiconductormuseum.com/Transistors/LectureHall/Camenzind/Camenzind_Index.htm - history of the 555 - Page1
http://www.semiconductormuseum.com/Transistors/LectureHall/Camenzind/Camenzind_Page2.htm - history of the 555 - Page2
http://www.semiconductormuseum.com/Transistors/LectureHall/Camenzind/Camenzind_Page3.htm - history of the 555 - Page3
http://www.semiconductormuseum.com/Transistors/LectureHall/Camenzind/Camenzind_Page4.htm - history of the 555 - Page4
http://www.semiconductormuseum.com/Transistors/LectureHall/Camenzind/Camenzind_Page5.htm - history of the 555 - Page5
http://www.semiconductormuseum.com/Transistors/LectureHall/Camenzind/Camenzind_Page6.htm - history of the 555 - Page6
http://www.semiconductormuseum.com/Transistors/LectureHall/Camenzind/Camenzind_Page7.htm - history of the 555 - Page7
http://www.semiconductormuseum.com/Transistors/LectureHall/Camenzind/Camenzind_Page8.htm - history of the 555 - Page8
http://www.semiconductormuseum.com/Transistors/LectureHall/Camenzind/Camenzind_Page9.htm - history of the 555 - Page9
http://www.semiconductormuseum.com/Transistors/LectureHall/Camenzind/Camenzind_Page10.htm - history of the 555 - Page10

For a list of every electronic symbol, see: Circuit Symbols.

For more articles and projects for the hobbyist: see TALKING ELECTRONICS WEBSITE

See TALKING ELECTRONICS WEBSITE

email Colin Mitchell: talking@tpg.com.au

This set of 555 projects has increased to the largest on the web, with many circuits designed by me as a request
from readers. The whole site has been copied by many other sites without any reference or acknowledgement to
me. But you can see the circuits are mine by the way they have been drawn and none of the other sites keep
updating with the latest additions.
Over one million readers have viewed this article and many have saved it as .pdf
This is the only site that teaches you how to learn and remember how a 555 works and this is very important
when you want to design a new circuit. The secret is: you can hold the 555 with the output LOW by putting
a high on pin 6, but when pin 2 goes LOW, the output goes HIGH. Pin 2 dominates over pin 6.

CONTENTS
You will find many of these circuits "talked-about" and produced on bread-board (with videos) on:
http://www.555-timer-circuits.com and:
http://www.555-timer-circuits.com/forum

Active High Trigger
Active Low Trigger
Aircraft Navigation Beacon
Alarm Sounds (4 sounds)
Alarm-1 (Home Alarm-1)
Alarm 4-Zone
Amplifier using 555
Animated Display
Audio Frequency Meter
Automatic Curtain Closer
Automatic Garden LED Strip
Astable Multivibrator
Basic 555 Oscillators
Battery Charger (voltage doubler)
Beep short
Bi-Coloured LED
Bike Turning Signal
Bi-Polar LED Driver
Bi-Stable 555
Building the Circuits
Burglar Alarm 4-Zone
Capacitor Charge Pump
Car Lights Flasher - warning flasher
Car Tachometer
Charge Controller - prevents overcharge
Charge Pump
Chaser - 3 Chase   NEW
Clark Zapper
Clap ON-OFF
Clicks Uneven
Calculator   555   7555
CMOS 555
Constant Current
Continuity Tester
Crossing Lights
Crossing Lights - Jim's MkIV
Current Limiting Throttle
Curtain Closer
Dark Detector
DC to DC Converter 3.7v to 5v
Delay - also called a Timer
Delay before turn-on
Dice   LED Dice Cct-2
Dice to 7-Segment Display
Display - Animated
Divide by 2
Dog-Bark Stopper
Doorbell for front door and back door
Driving A Bi-Coloured LED
Driving A Relay
Driving White LEDs
Duty Cycle 1:1   (50%)
Fade-IN Fade-OUT LEDs
Fading LED
Fastest 555 Oscillator
First Cycle of a 555
Flasher
Flashing Indicators
Flashing Railroad Lights
Flip Flop see also Toggle
Four Alarm Sounds
Frequency Divider
Frequency Meter
Function of each 555 pin
H-Bridge
H-Bridge Push-Pull - high current
H-Bridge with PWM
Headlight Flasher - faulty circuit
Headlight Selector
Hee Haw Siren
Hee Haw Siren with one 555
Higher Sinking Current
High Frequency 555 Oscillator
Home Alarm-1
How to use the 555
Hulda Clark Zapper
Hysteresis
Improving the output of a 555
Increasing Sinking Current
Increasing Output Push-Pull Current
Inverter 12v to 240v
Inside the 555
Jammer for TV
Jim's Crossing Lights
Joy Stick and Servo
Kitt Scanner
Knight Rider
Laser Ray Sound
Latch
Latch A Relay
Latch - using transistors
LED Dice   LED Dice Cct-2
LED Dimmer
LED Flasher
LED Flasher using LM386
LED Flasher with LDR
LED FX
LED Strip Illuminator
Level Shifter
Light Controlled Screamer Siren
Light Detector
Lights - Traffic Lights
Listen to an RGB LED
LMC555 CMOS 555
LM386 LED Flasher
LM558 - Quad 555
Loco Stop
Logic Probe
Long Delay Timer - NEW CHIP $8.00
Long Duration Timer
Low Current Timer - 7555 CMOS 555
Low Frequency 555 Oscillator
Low Power 555
Machine Gun
Make any 555 Project on a PC board
Mark-Space Ratio
Memory Cell see also Toggle Flip Flop
Mercury Switch Detector - faulty circuit
Metal Detector
Missing Audio Detector
Missing Pulse Detector - faulty circuit
Model Railway Time
Monostable 555
Morse Keyer
Mosquito Repeller
Motor Controller (stepper Motor)
Motor Controller (servo motor)
Motor Over-run
Motor PWM
Multivibrator - Astable
Music Box
Negative Voltage

Normally Closed Trigger
One-Shot 555
Organ
Phase Control with XR-L555 (low power)
Pin 4 Reset
Pin 5 - using pin 5
Police Lights 1,2,3
Police Siren
Powering A Project
Pulse Extender
Pulser - 74c14
Push-ON Push-OFF
Push Pull
Push-Pull - high current
PWM Controller - FET buffer
PWM - transistor buffer - and via pin 5
see also Motor PWM
Quad 555 - LM558
Quadrupler - 4 times!
Railroad Lights (flashing)
Railway Time
Rain Alarm
Ramp Generator
Reaction Timer Game
Replacing 556 with two 555's
Replacing TTL 555 with CMOS 555
Reset Pin
Resistor Colour Codes
Re-triggerable 555
Reversing A Motor
RGB LED - driving a tri-coloured LED
Roulette
Rubbish 555 The 555 is RUBBISH
Schmitt Trigger
Screamer Siren - Light Controlled
Servo Controller
Servo Tester
SET or RESET?    Note this !!!!
Shoot-through
Short Beep
Simplest 555 Oscillator
Sinewave Output
Siren 100dB
Solar Tracker
Square Wave Oscillator
Stepper Motor Controller
Stun Gun
Substituting a 555 - Part 1
Substituting a 555 - Part 2
Supply (170v) for Nixie Tubes
Surface Mount 555
Switch Debounce
Tachometer
Tap ON Tap OFF
TE555-1 Stepper Motor Controller
TE555-2 Animated Display
TE555-3 4 Alarm Sounds
TE555-4 Dice
TE555-5 LED Fx
Throttle - simple
Throttle with Current Limiting
Ticking Bomb
Tilt Switch
Timer - 5 seconds
Toggle A Relay
Toggle 555 555 ON/OFF   Flip Flop
Touch Switch
Touch ON-OFF   - more Touch Sw Ccts
Toy Organ
Traffic Lights
Traffic Lights - 4 way
Transistor Tester
Trigger Timer - 74c14
Tripler - & quad
Turning Signal
TV Remote Control Jammer
Useless Machine
Uneven Clicks
Universal 555
Up/Down Fading LED
Using the 555
VCO (Voltage Controlled Oscillator)
Voltage Doubler see also Battery Charger
Voltage Tripler
Voltage Inverter
Voltage Multiplier x10times
Warning Flasher - car lights flasher
Water Level Detector
Wailing Siren
What is the 555?????
Zapper (Dr Clark)
Zapper - Voltage Multiplier
Zener Diode Tester
2 Minute Timer - 74c14
3x3x3 Cube
3-Chase    Chaser NEW
3 secs HIGH 60 secs LOW
3.7v to 5v DC to DC converter -no work!
4 Alarm Sounds
4 way Traffic Lights
4-Zone Burglar Alarm
4 Pin 4 Reset
1-10 Minute Auto Turn Off
5 Seconds Delay
5 Pin 5 - using pin 5
10 Minute Timer - 74c14
12v DC to 12v DC Battery Charger
12v DC to 19v DC
12v to 240v Inverter
50% Duty Cycle
100dB Siren
170v Supply for Nixie Tubes
555's - a list of substitutes
555+4017 Display $4.50
555 Amplifier
555 CMOS version LMC555
555 Kit of Components
555 Printed Circuit Board
555 Pinout
555 Pins - Remembering the pins
555 Mistakes (No-No's)
555 on 24v
555 Timer Calculator
555 VCO
556 Dual Timer
558 Quad 555
7555 CMOS Calculator
7555 Low Current Timer (delay)

WHAT IS THE 555?
The 555 is an 8-pin chip that can be turned into lots of different things (building blocks).
The circuit inside the 555 is just an amplifier with 2 inputs and an output.
The parts you add to the chip determine the final result (effect).
In most cases you add a capacitor and resistor to produce a circuit known as a TIME DELAY CIRCUIT and the chip has a detection-pin and an amplifier (also called a power amplifier) to produce an output.
It is not a "timer," "delay" or "oscillator" but is capable of being converted into these and maybe 100 other circuits. Just call it the "555"

THE 555 CHIP

INTRODUCTION
This e-book covers the 555.
The 555 is everywhere and it is one of the cheapest and most-rugged chips on the market.
It comes as a TTL 555 and will operate from 4v to about 16-18v. (but don't use less than 5.5v) It costs from 20 cents (eBay) to $1.20 depending on the quantity and distributor. The circuitry inside the chip takes about 10mA - even when the output is not driving a load. This means it is not suitable for battery operation if the chip is to be powered ALL THE TIME.
The 555 is also available as a CMOS chip (ICM7555 or ICL7555 or TLC555) and will operate from 2v to 18v and takes 60uA when the circuitry inside the chip is powered. The "7555" costs from 60 cents (eBay) to $2.00. But the output driving capability is a lot less than the TTL 555.
We call the TTL version "555" and the CMOS version "7555." This is called ELECTRONICS JARGON.
The 555 comes as a single timer in an 8-pin package or a dual timer (556) in a 14 pin package.
The 7555 comes as a single timer in an 8-pin package or a dual timer (7556) in a 14 pin package.

The 555 and 7555 are called TIMERS or Timer Chips. They contain about 28 transistors and the only extra components you need are called TIMING COMPONENTS. This is an external resistor and capacitor. When a capacitor is connected to a voltage, it takes a period of time to charge. If a resistor is placed in series with the capacitor, the timing will increase. The chip detects the rising and falling voltage on the capacitor. When the voltage on the capacitor is 2/3 of the supply the output goes LOW and when the voltage falls to 1/3, the output goes HIGH.
We can also do other things with the chip such as "freezing" or halting its operation, or allowing it to produce a single HIGH-LOW on the output pin. This is called a "ONE-SHOT" or MONOSTABLE OPERATION - but it still takes 10mA while "sitting around".
When the chip produces an output frequency above 1 cycle per second, (1Hz), the circuit is called an OSCILLATOR and below one cycle per second, it is called a TIMER.
But the chip should not be called a "555 Timer," as it has so many applications. That's why we call it a "555." (triple 5)
Another thing you have to be aware of is the voltage on output pin 3. It is about 1-2v LESS THAN rail voltage and does not go to 0v (about 0.7v for 10mA and up to 1900mV for 200mA sinking current). For instance, to get an output swing of 10v you will need a 12.6v supply. In "electronic terms" the 555 has very poor sinking and sourcing capabilities.
One way to understand how the chip operates is to remember that pin 7 goes LOW when pin 3 (the output pin) goes LOW. When pin 3 goes HIGH, pin 7 goes "open circuit" (it does not go HIGH - it goes HIGH IMPEDANCE).
When pin 4 is taken LOW, (it needs to be as low as 0.5v) the chip stops operating, but it still takes 10mA.

For photos of nearly every electronic component, see this website: https://www.egr.msu.edu/eceshop/Parts_Inventory/totalinventory.php

You can also search the web for videos showing the 555 in action.
Here are a few:
Making A 555 LED Flasher – Video Tutorial
Three 555 LED Flasher
555 Timer Flasher
Fading LED with 555 timer

Each website has lots more videos and you can see exactly how the circuits work. But there is nothing like building the circuit and that's why you need to re-enforce your knowledge by ACTUAL CONSTRUCTION.

Learning Electronics is like building a model with Lego bricks. Each "topic" or "subject" or "area" must be covered fully and perfectly, just like a Lego brick is perfect and fits with interference-fit to the next block. When you complete this eBook, you can safely say you will have mastered the 555 - one more "building block" under your belt and in the process you learn about DC motors, Stepper motors, servos, 4017 chips, LEDs and lots of other things. Any one of these can take you off in a completely different direction. So, lets start . . .

Colin Mitchell
TALKING ELECTRONICS.
talking@tpg.com.au

To save space (and get everything on a single page) we have not provided lengthy explanations of how any of the circuits work. This has already been covered in TALKING ELECTRONICS Basic Electronics Course, and can be obtained on a CD for $10.00 (posted to anywhere in the world). See Talking Electronics website (http://www.talkingelectronics.com) for more details on the 555 by clicking on the following four pages: 555-Page 1 555-Page 2 555-Page 3 555-Test
Many of the circuits have been designed by Colin Mitchell: Music Box, Reaction Timer Game, Traffic Lights, TV Remote Control Jammer, 3x3x3 Cube, while others are freely available on the web. But this eBook has brought everything together and covers just about every novel 555 circuit. If you think you know everything about the 555, take the 555-Test and you will be surprised!

SI NOTATION
All the schematics in this eBook have components that are labelled using the System International (SI) notation system. The SI system is an easy way to show values without the need for a decimal point. Sometimes the decimal point is difficult to see and the SI system overcomes this problem and offers a clear advantage.
Resistor values are in ohms (R), and the multipliers are: k for kilo, M for Mega. Capacitance is measured in farads (F) and the sub-multiples are u for micro, n for nano, and p for pico. Inductors are measured in Henrys (H) and the sub-multiples are mH for milliHenry and uH for microHenry.
A 10 ohm resistor would be written as 10R and a 0.001u capacitor as 1n.
The markings on components are written slightly differently to the way they are shown on a circuit diagram (such as 100p on a circuit and 101 on the capacitor or 10 on a capacitor and 10p on a diagram) and you will have to look on the internet under Basic Electronics to learn about these differences.

Here's a 555 built from transistors by Eric Schlaepfer in honor of Hans Camenzined, who invented the 555:

You can build your own 555 from this Instructable:
http://www.instructables.com/id/Build-Your-Own-555-Timer/

Or buy a kit for $35 from Evil Mad Scientist:
http://shop.evilmadscientist.com/productsmenu/tinykitlist/652

This kit is ideal for a school science project.
Here is the circuit and assembly instructions.

WARNING!!
Before making any 555 project, remember the chip takes about 10mA ALL THE TIME and cannot be turned off.
The output (the "driving power") of a 555 is about 200mA while the CMOS versions are only about 50mA.
The chip is not really suited for doorbells and circuits as the 10mA will rapidly drain the battery.
The 555 does not like 5v supply. Use 6v supply (minimum) when designing a circuit and then see if it works on 5v. This is a REAL TRAP !!!!!

THE 555 IS RUBBISH
This headline is just to grab your attention.
There is one more problem with the 555. It is not suited to deliver a current above 100mA on a 12v supply.
WHY?
This has never been covered before.
If you put a 555 on 12v and create a square-wave output (50:50) with a load of 100R, the output voltage will be 10v and the current will be 100mA when the output is HIGH and 0mA when the output is LOW. This means the average current through the load is 50mA. But the current taken by the circuit will be more than 50mA. Where does the extra current go ????
It goes into the circuitry of the 555 and heats up the chip.
When the load is 50R, the current will be 200mA and 0mA with an average of 100mA. But the current from the supply will heat up the chip so hot that you cannot hold it.
Everyone thinks it is the loss of 2v due to the output switching HIGH and LOW, but that would only account for about 400mW. The chip is actually dissipating up to 1200mW and that's why it will burn your finger.
The 555 was initially developed as a TIMER. It provides time-delays up to about 30 minutes.
During a timing interval it might be able to deliver 200mA to a load but when it is used as an oscillator, the transistors inside the chip consume about 50% of the current and cause the chip to heat up.
NOW YOU KNOW.

SURFACE MOUNT 555
The 555 is also available in SURFACE MOUNT packages.

There are 5 different sizes and you will need to find out the "foot-print" before making a Printed Circuit board.
All the features of the surface mount version are the same as the through-hole version, but you must remember the output does not "go to rail" or 0v. When the current through the output pin is less than 10mA, the output will go as low as 250mV. But when the current increases to 100mA to 200mA, the output will not go lower than 2.5v.
When the output is HIGH and trying to deliver 200mA, it will be
2.5v lower than the supply voltage. That's why you need to make a prototype circuit first and check the output voltage as well as the heating of the chip. Max current is 200mA.

NEW! FROM TALKING ELECTRONICS
A new range of 555 chips have been designed by Talking Electronics to carry out tasks that normally need 2 or more chips.
These chips are designated: TE 555-1, TE555-2 and the first project to use the TE 555-1 is STEPPER MOTOR CONTROLLER TE555-1.

It's a revolutionary concept. Instead of using an old 8-pin TTL 555 chip, you can use a new TE555-1,2,3 8-pin chip and save board space as well as components. These new chips require considerably less external componentry and the possibilities are endless. Depending on the circuit, they can have a number of timing and frequency outputs as well as a "power-down" feature that consumes almost no current when the circuit is not operating. See the first project in this series: STEPPER MOTOR CONTROLLER TE555-1.
See also: Stepper Motor Controller project

See also TE 555-2 TE555-3 TE 555-4 TE555-5

555 TIMER CALCULATOR
A program to work out the values for a 555 in Astable or Monostable mode is available from Andy Clarkson's website:
http://555-timer-circuits.uk/
555-Timer.zip (987KB). Name a folder: "555 Timer." Unzip and run "555 Timer setup.exe"
Setup will produce a desktop icon. Click on icon for program. Set the voltage for the 555 then use the Astable or Monostable tabs to design your circuit. Read the Help screen to understand the operation of: "Hold Output" and "smallest."

Here is another 555 calculator by Jeff:

555 Timer Calculator

555diagram

This calculator will provide a number of component values for the delay you want or the frequency.
You can specify “10ms” or “4.5kHz” in the text fields.

How often do you want the timer to produce an output? seconds
What frequency do you want the timer to operate at? Hertz

7555 CMOS CALCULATOR   see 7555
The 555 comes in a low-power CMOS version. The drive-current from pin 3 is less than the TTL "555."
At 5v, a 7555 will deliver 2mA and sink only 8mA
At 12v a 7555 will deliver 10mA and sink 50mA
At 15v a 7555 will deliver 100mA and sink 100mA
Use the following 7555 calculator to find the OUTPUT FREQUENCY in Astable mode or OUTPUT TIME in Monostable mode:
7555 CMOS Calculator

Here's a 555 made with 22 transistors by Malcolm Faed. See his video.

See his Electric Vehicle website.

How are your powers of observation?
Can you find the LED:

THE POWER SUPPLY

Sometimes you will see a circuit as shown in the first diagram with 12v or +12v on the top rail and 0v or a negative sign or the word "negative" on the bottom rail. In this case the word negative means earth or "chassis of a car" and we commonly refer to this as "negative earth" or "negative chassis."
In the second diagram, the output from a power supply has a positive 12 volts and a negative 12v with the 0v rail in the middle. In this case the negative 12v rail is twelve volts BELOW the earth rail and that's why we call it the NEGATIVE RAIL.
This means that when you hear "Negative Rail," you need to work out if it means the negative terminal of a battery (as in the first case - meaning 0v or earth) or if the voltage is below zero volts (as in the second case).

CD of Talking Electronics website
A CD of the whole website is available for $10.00 posted to anywhere in the world. The CD comes with a small bag of electronic components including surface-mount items.

Log in to your PayPal account
and send a payment to:
talking@tpg.com.au for $10.00 AUD

Or email Colin Mitchell:   talking@tpg.com.au for details on sending for the CD plus other kits etc.

SQUARE WAVE OSCILLATOR KIT
A Square Wave Oscillator Kit is available from Talking Electronics for under $10.00. See full details of circuit below.
   (This link will send an email to Colin Mitchell and you will be advised of costs and how to send money via Paypal or credit card.)
Or email Colin Mitchell:   talking@tpg.com.au

555 KIT
A kit of components to make many of the circuits described in this eBook is available for $10.00 plus $7.00 post.
Or email Colin Mitchell:   talking@tpg.com.au

The kit contains the following components:
(plus extra 30 resistors and 10 capacitors for
experimenting), plus:

2 - 220R
2 - 1k
2 - 4k7
2 - 10k
2 - 33k
2- 100k
2 - 1M
1 - 10k mini pot
1 - 100k mini pot
2 - 10n
2 - 100n
1 - 10u electrolytic
1- 100u electrolytic
2 - 1N4148 signal diodes
2 - BC547 transistors
1 - BC557 transistor
1 - 555 timer chip
1 - 8 pin IC socket
1 - red LED
1 - green LED
1 - orange LED
1 - mini 8R speaker
1 - mini piezo
1 - LDR (Light Dependent Resistor)
1 - 10mH inductor
1 - push button
1 - tactile push button
1 - Experimenter Board (will take 8, 14 and 16 pin chips)

Components for 555 Kit

There are more components than you think.
All kits contain more components than on the list.
555 Kit: $10.00 plus $7.00 postage

You can also get a breadboard and 30 jumper leads for $5.00 so you can plug the parts into the board and re-use them for another circuit:

The photo shows a number of components fitted to the breadboard.

This is the breadboard you get.
These are the 30 jumper leads that come with the breadboard

Here is a project from Franz Bachler, using a 555 and 4017 to drive 4 orange LEDs. He has
used the bread-board to make a very neat project. The only problem is: he has cut the leads of the components and some of them will only fit between three holes. This may be a problem when you make the next project.
Email: Colin Mitchell to get the breadboard for $5.00 with the TRANSISTOR KIT or 555 KIT (see above).
You can make this project with MAKE A 555+4017 DISPLAY. The kit has 3 different displays that plug into the main board to get a range of different effects.

This is what you can do with the breadboard.

MAKE ANY 555 PROJECT
3 Printed circuit boards: MAKE ANY 555 PROJECT are available for $10.00 post free to ANYWHERE IN THE WORLD !!!.
email Colin Mitchell:   talking@tpg.com.au

A set of components for MAKE ANY 555 PROJECT is just $5.00 extra
See Guitar Tuner Project
and Automatic Garden LED Strip Project

NEW !

5 Different boards are now available for $10.00 USD posted.

Please log into your paypal account and send a payment to
talking@tpg.com.au for $10.00   USD

And send your name and address and I will send the item(s). Let me know that you want the 5 different 555 boards for $10.00 posted.

Colin

The parts include:
2 - 220R
2 - 1k
2 - 4k7
2 - 10k
2 - 33k
2 - 47k
2- 100k
2 - 1M
1 - 10k mini pot
1 - 100k mini pot
2 - 10n
2 - 100n
1 - 10u electrolytic
2- 100u electrolytic
1 - 1N4148 signal diodes
3 - BC547 transistors
3 - 555 timer chips
3 - 8 pin IC sockets
1 - red LED
1 - green LED
1 - orange LED
3 - mini piezos
1 - LDR (Light Dependent Resistor)
3 - slim tactile push buttons
Kit: $5.00    [VALUE: $6.50]

These parts will help you build many of the 555 circuits presented below.

3 x Make any 555 Project PC boards $10.00 plus $5.00 components: $15.00 (post FREE)

MAKE A 555+4017 DISPLAY $4.50
Make you own RUNNING LED DISPLAY with this 555+4017 PC board. It comes with 3 display boards that plug into the main board to produce an expanding or compressing effect, as well as a 6-LED KITT SCANNER.      email Colin Mitchell:   talking@tpg.com.au

The Scanner - called Kitt Scanner

555 EXPAND

You also get COMPRESS - all instantly interchangeable
A set of components for this project is just $7.50 extra. This includes 44 LEDs, 20 surface mount diodes, 50k pot, 2u2, 22k, 555, 4017, IC sockets, 11 machine pins, 33 double-ended pins, slide switch, and battery snap.
Make 555+4017 Project (with 3 display boards) $4.50
Set of components: $7.50
plus $4.00 postage.

We have everything for the hobbyist and engineer to build a 555 circuit on a Printed Circuit Board. This is the latest edition to the range:

555 MATRIX BOARD $1.50each
Make you own 555 project with this 555 Matrix Board. Get 4 boards for $5.00 plus $4.00 postage. email Colin Mitchell: talking@tpg.com.au

555 MATRIX BOARD
The 555 IC is already "wired up" and some of the lands are connected to make fitting components easy. The overlay on the board shows you the wiring.
Get the set of components above for $5.00 extra and you will be able to make 3 different projects.

Click HERE to order your 3 x 555 PC boards for $10.00 post free. Use your own components and build lots of different circuits . . . set of components . . . just $5.00 extra !!
The set of boards has been increased to FIVE for $10.00 and you will get 5 when you place an order.

LONG DELAY TIMER
The 555 does not produce times longer than about 60 minutes and a new chip on the market produces times up to 1,000 hours. It is called C005. Here is the prototype where we tested the chip:

Back of the timer module showing the two wires for the pot.

Connect the two top pads to produce 1 to 10 hours

A kit is available for $8.00 (plus postage) and comes with all components and a printed circuit board with all parts clearly shown on the overlay.
The pot is marked with 1 to 10 hours and this will be ideal for most applications. The tactile button starts the timing and the buzzer sounds when the time is up. Two output pins are also provided on the PC board to connect to other devices to turn them ON.
The output of the board turns ON a device such as a relay, after say 5 hours, and the contacts on the relay will turn the device OFF. The buzzer on the board will let you know the 5 hours is up.
The project is connected to 3 x AA cells (4.5v) (in a battery box)
Consumption when timing: 100uA
Output: pull-up 3mA pull-down 30mA
Connecting the pads at P1 and also the pads at P2 increases the delay by 8 times, making the timing up to 80 hours. (but don't let P1 pads touch P2 pads). Changing the pot to 20M creates 1,000 hours.
email Colin Mitchell: talking@tpg.com.au for payment for the kit via paypal.

HOW A 555 WORKS
To design a 555 circuit you must know how a 555 IC works. Here is an explanation:

The capacitor charges between 33% and 66% of tail voltage. When the capacitor is 66%, pin 6 detects the voltage and makes pin 7 go LOW. The capacitor discharges to 33% and pin 2 detects the voltage and makes pin go HIGH (actually pin7 goes OPEN) and the capacitor charges to 66% to repeat the cycle.

LET'S START:
1. Connect the 555:

UNKNOWN STATE

We start with pins 2 and 6 NOT CONNECTED. This is called the UNKNOWN STATE because the chip is not getting any information and the output is UNKNOWN.
The inputs are called "FLOATING." Inputs must NEVER be "FLOATING."

2. Connect Pins 2 and 6:
We start by connecting pins 2 and 6 as shown and then connect the power:

Read the notes to understand this.

This is also called the UNKNOWN STATE because Pin 2 detects a LOW to make the output HIGH and pin 6 detects a HIGH to make the output LOW.
The output will be HIGH or LOW depending on which pin is detected first.
The output is controlled by a BUILDING BLOCK called a FLIP FLOP or TOGGLE or SWITCH or BI-STABLE LATCH. It will remain "locked" or in a stable state and it will take a voltage of about 33% or more of rail voltage to change the state via pins 2 or 6.
In most cases the output will "flip over" to the state with the output LOW. If you delay pin 2 by 1uS, the output will go HIGH, indicating pin 2 is now detected to make the output HIGH.

3. Toggle (or change) the output:
The output can be changed by using pin 2 or 6.

Pin 2 and Pin 6 change output

The output changes immediately, the animation is a "slow-down."

4. The output:
If we make pin 2 LOW and Pin 6 HIGH, both pins are trying to change the output of the chip and pin 2 WINS !!! The output is HIGH !!

Pin 2 WINS !!

4a. The RESET PIN
Pin 4 is the reset pin. When it is taken to the positive power rail, the chip "works." When it is taken to 0v rail, the chip "freezes" and pin 3 instantly goes LOW, (even if it is halfway through a timing cycle). But the current taken by the chip remains at 10mA and you cannot use this pin to "turn off the current taken by the chip."
If pin is unconnected, it will allow the chip to operate as the internal circuitry puts a "High" (950mV) on the pin.

But the Reset Pin is more complex.
When nothing is connected to pin 4, it has a voltage of about 950mV on it and it will deliver about 350uA. This is due to the circuitry inside the chip.
It must be taken below 500mV to turn the chip off.
A 2k7 resistor connected to pin 4 will turn the chip off.
A 3k3 resistor connected to pin 4 will NOT turn the chip off.
This means pin 4 is generating a voltage (and current) and we can use this feature to turn the chip OFF via a Light Dependent Resistor (LDR).
If we connect an LDR to pin 4, it will require a very bright light to reduce the resistance of the LDR to less than 3k3.
By adding a 10k to the circuit, we help the LDR produce a combined resistance less than 3k3 and the circuit will work in a normal lounge-room or bedroom. When the LDR sees illumination from a room-light, it will turn the chip off.
You can try values as low at 4k7 to make the chip MORE sensitive.
In the following circuit, both pin 2 and pin 6 activate the chip to make the output HIGH and LOW "at the same time" but Pin 2 has control over pin 6 and pin 2 makes pin 3 (the output pin) HIGH.

5. The Schmitt Trigger:
Pin 2 detects 33% of rail voltage and pin 6 detects 66%. This gives a gap of 33% between the two. This gap is called the HYSTERESIS GAP.
Here is a circuit to show how the gap works:

The Hysteresis Gap

The 555 is wired as a SCHMITT TRIGGER. A Schmitt Trigger has a gap between HIGH and LOW input so the signal has to rise and fall a large percentage of rail voltage to make the output change. This means noise on the input will not alter the output as noise has a small amplitude. This type of circuit "cleans up" noisy inputs.
This circuit is also called AN INVERTER, A BUFFER and AN AMPLIFIER

5a. The Schmitt Trigger:
Here is a circuit with the 555 wired as a Schmitt Trigger. It is an oscillator with a square-wave output and operates by detecting 33% and 66% of rail voltage on the capacitor to change the state of the output.
The first diagram shows the rectangular 555 and the second diagram shows it as a triangular shape which is the same shape for a 74c14 (40106) Hex Schmitt Trigger.

6. One-Shot or Switch Debouncer:
The 555 can take the noisy pulses from a switch and produce a single CLEAN pulse.
The circuit is also called PULSE EXTENDER.

7. The 555 as a Delay (Timer):
The 555 can be used as a timer up to 10 minutes. This circuit is also called a DELAY.
To start timing, the START button is pressed briefly and the output of the chip goes LOW. At the expiration of 10 minutes, the output goes HIGH and the red LED illuminates.
A simple application may be for a cooking operation in a shop.
If a product needs to be cooked or heated etc, the button can be pressed and the LED illuminates when the time has expired.

When calculating the time-duration for the circuit above, the capacitor charges from 0v to 2/3 rail voltage.

In this circuit we can see pin 2 is taken LOW when the power is applied and quickly goes HIGH. Pin 2 has control over pin 6 as explained in THE 555 PINS below.
The action of Pin 2 going LOW "activates the 555" and makes pin 3 HIGH. This only applies because pin 6 is LOW.
Because pin 2 goes HIGH after 0.1secs, it has done its job of turning ON the 555 and now we wait for pin 6 to go HIGH to turn the chip OFF. If pin 2 remained LOW, the chip would NEVER turn OFF.

8. LONG Duration Timer:
The 555 can be used as a timer up to 10 minutes, but the accuracy of this duration is not reliable and will vary enormously according to the temperature of the day and the leakage of the electrolytic. If you make the time too long, the output may never go HIGH. Reliability stops at about 1 minute, so to get a long duration we connect the 555 to a chip called a "divider-chip" or "counter-chip."
They are called divider chips because they are designed to take a high frequency and reduce it to a lower frequency by using a number of flip flops.
A flip-flop has the ability to divide a signal by 2. In other words, the output oscillates at half the input frequency.
If you connect a chain (or train) of flip flops, the signal gets divided by 2, 4, 8, 16, 32, 64, 128 etc.
Thus 1 minute will turn into 2 minutes . . . 128 minutes.
This type of chip is called a BINARY DIVIDER CHIP.
You can also connect a 555 to a divide-by-10 counter (CD4017) to get 2 minutes, 3, 4, 5, 6, 7, 8, 9, 10.
If your timing is 61 seconds, the 10 minutes will be 10 minutes and 10 seconds.
Here are 2 LONG DURATION TIMING CIRCUITS:

LONG DURATION TIMER -1 will produce a HIGH on pin 3 after 1 minute. Or pin 11 after 10 minutes. The timer start when the power is applied, providing the 220u is fully discharged. If not, the 1st minute will not be accurate. The timer will keep cycling.

CD 4024 LONG Duration Timer:

You have to test the circuit to see if the counter-chip advances on the HIGH or LOW of the 555 and the actual timing for each pin may be different to the circuit above. The circuit is just a sample of how to connect the two chips.

The circuits above will CYCLE. In other words, the outputs will "start all over again" after the longest time-interval has occurred.
To FREEZE the output and make it stay HIGH, you need the following modification:

9. Delay before turn-on:
Pin 3 comes on LOW and goes HIGH after a few seconds:

In other words, Pin 2 "turns the chip ON" when it sees a LOW

10. Delay before turn-off:
Pin 3 comes on HIGH and goes LOW after a few seconds:

In other words, Pin 6 "turns the chip OFF" when it sees a HIGH

Don't forget, the chip takes 10mA ALL THE TIME because it is connected to the supply. These two circuits show how pins 2 and 6 control the chip.

11. HIGH for 3 seconds every 60 seconds
Here's a very simple circuit to create a HIGH for 3 seconds and a LOW for 60 seconds:

12. Level Shifter
This circuit produces a maximum 5.6v on the output and minus 0.7v when the output is LOW. The electro on the output charges to about 8v when the output is HIGH and when it goes low, this 8v would produce a negative 8v on the output, but when the output goes below 0.7v, the zener turns into an ordinary diode and conducts to discharge the electro.
This guarantees 0v on the output as pin 3 of the 555 does not go to 0v and sometimes the device being driven by pin 3 does not fully turn off due to this.

13. PIN 3 - THE OUTPUT PIN
The output of the 555 does not rise to rail voltage or fall to 0v.
The actual value of the HIGH and LOW will depend on the supply voltage and the load on the output. The output can be as much as 2v less than rail voltage and up to 1v above the 0v rail.
Because of the it can fail to turn OFF an NPN transistor connected to the output and it will fail to turn OFF a PNP transistor.

14. FIRST CYCLE
The first cycle of a 555 is twice as long as all the other cycles because the timing capacitor has to charge from 0v to 2/3supply. All the other cycles require the capacitor to charge from 1/3 supply voltage to 2/3.
The following circuit charges the capacitor quickly when the project is turned ON:

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HOW THE 555 WORKS
Remember this . . .
Always draw a 555 with the pins as shown in the diagram.
Pin 3 is output. It pulls HIGH (200mA max). It pulls LOW (200mA max).
Pin 7 is IN PHASE with pin 3. But it only pulls LOW (100mA max). When it is HIGH it is open-circuit - "high-impedance." Pin 7 can be connected to Pin 3 to get 300mA LOW.
Make Pin 2 HIGH. Now Pin 6 detects 66% of rail voltage to make Pin 3 LOW.
If Pins 2 and Pin 6 are connected, chip goes LOW when both pins go HIGH and chip goes HIGH when pins are LOW.
Make Pin 2 LOW. Chip stays HIGH. Pin 6 has NO effect.
Pin 4 is RESET. Must go to 0.8v to reset the chip.

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THE 555 PINS
Here is the identification for each pin:

When drawing a circuit diagram, always draw the 555 as a building block, as shown below with the pins in the following locations. This will help you instantly recognise the function of each pin:

Pin 1 GROUND. Connects to the 0v rail.
Pin 2 TRIGGER. Detects 1/3 of rail voltage to make output HIGH. Pin 2 has control over pin 6. If pin 2 is LOW, and pin 6 LOW, output goes and stays HIGH. If pin 6 HIGH, and pin 2 goes LOW, output goes LOW while pin 2 LOW. This pin has a very high impedance (about 10M) and will trigger with about 1uA.
Pin 3 OUTPUT. (Pins 3 and 7 are "in phase.") Goes HIGH (about 2v less than rail) and LOW (about 0.5v above 0v rail) and will deliver up to 200mA.
Pin 4 RESET. Maybe internally connected HIGH via 100k in some chips, but NOT others!!. Must be taken below 0.8v to reset the chip.
Pin 5 CONTROL. A voltage applied to this pin will vary the timing of the RC network (quite considerably).
Pin 6 THRESHOLD. Detects 2/3 of rail voltage to make output LOW only if pin 2 is HIGH. This pin has a very high impedance (about 10M) and will trigger with about 1uA.
Pin 7 DISCHARGE. Goes LOW when pin 6 detects 2/3 rail voltage but pin 2 must be HIGH. If pin 2 is HIGH, pin 6 can be HIGH or LOW and pin 7 remains LOW. Goes OPEN (HIGH) and stays HIGH when pin 2 detects 1/3 rail voltage (even as a LOW pulse) when pin 6 is LOW. (Pins 7 and 3 are "in phase.") Pin 7 is equal to pin 3 but pin 7 does not go high - it goes OPEN. But it goes LOW and will sink about 200mA. You can connect pin 7 to pin 3 to get a slightly better SINK capability from the chip.
Pin 8 SUPPLY. Connects to the positive rail.

555 in a circuit - note the circle on the chip to identify pin 1
This is sometimes called a "push-out-pin" (hole) to push the chip out of the cavity when it is being moulded and sometimes it has no importance. But in this case it represents pin 1.

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SHOOT-THROUGH:
The 555 (the TTL version, such as NE555, SE555) has a problem that both the output transistors turn ON when the output changes from HIGH to LOW or LOW to HIGH. This only occurs for a very short period of time (about 2 nanoseconds) and is called SHOOT THROUGH. This is when a high current flows through a circuit during a part of a cycle. It flows through the collector-emitter junction of transistors Q22 and Q24.
This type of problem may damage a component and it may also put a "blip" (glitch, spike) on the rail voltage.
Since this is generally a very high current, if you have a small filter capacitor (electrolytic) on the rail, the shoot-through will take a large amount of energy from the capacitor and extend the width of the time when the voltage is low. The capacitor will take an extra amount of time to recover. This spike may now be wide enough for the rest of the circuit to detect the glitch and create a problem.
That's why you need a large-value electrolytic (100u) across the 555 and try to avoid using it with counting-chips and microcontrollers.

The schematic of the 555 shows the two output transistors can be active when Q20 turns ON and starts to turn on Q24, while Q22 is still turned ON via Q21 and R12. This problem does not affect the operation of the 55, but may affect sensitive IC's in the same project.

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THE SIMPLEST 555 OSCILLATOR
The simplest 555 oscillator takes output pin 3 to capacitor C1 via resistor R1.
When the circuit is turned on, C1 is uncharged and output pin 3 is HIGH. C1 charges via R1 and when Pin 6 detects 2/3 rail voltage, output pin 3 goes LOW. R1 now discharges capacitor C1 and when pin 2 detects 1/3 rail voltage, output pin 3 goes HIGH to repeat the cycle.
The amount of time when the output is HIGH is called the MARK and the time when the output is LOW is called the SPACE.
In the diagram, the mark is the same length as the space and this is called 1:1 or 50%:50%.
If a resistor and capacitor (or electrolytic) is placed on the output, the result is very similar to a sinewave.

C1 to POSITIVE RAIL
C₁ can be connected to the positive rail. This is not normal practice, however it does work.
The output frequency changes when the capacitor is changed from the negative rail to the positive rail. Theoretically the frequency should not change, but it does, and that's why you have to check everything. The frequency of operation in this arrangement is different to connecting the components via pin7 because pin3 does not go to full rail voltage or 0v. This means all the output frequencies are lower than those in the "555 Frequency Calculator."
The table shows the frequency for the capacitor connected to the 0v rail and 12v rail:

C₁ to 0v rail			C₁ to 12v rail
1k	1n	505kHz	1k	1n	255kHz
1k	10n	115kHz	1k	10n	130kHz
1k	100n	23kHz	1k	100n	16kHz
10k	1n	112kHz	10k	1n	128kHz
10k	10n	27kHz	10k	10n	16kHz
10k	100n	3700Hz	10k	100n	1600Hz

CHANGING THE MARK-SPACE RATIO
This ratio can be altered by adding a diode and resistor as shown in the following diagrams. In the first diagram, the 555 comes ON ("fires-up") with pin 3 low and pin 2 immediately detects this low and makes pin 3 HIGH. The 10n is quickly charged via the diode and 4k7 and this is why the MARK is "short." When the capacitor is 2/3Vcc, pin 6 detects a HIGH and the output of the 555 goes LOW. The 10n is discharged via the 33k and this creates the long-duration SPACE (LOW). The second diagram creates a long-duration HIGH:

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THE THREE BASIC 555 OSCILLATORS
The 3 basic 555 oscillators are shown in these circuits.
The basic points to remember are these:
Pin 7 goes LOW when pin 3 goes LOW.
Pin 6 detects a HIGH
Pin 2 detects a LOW
Pin 3 can be used instead of the supply-rail to deliver a HIGH and instead of pin 7 to deliver a LOW to the timing section made up of pins 2&6 and "C" and a timing resistor.

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HOW TO REMEMBER THE PINS:

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THE FASTEST 555 OSCILLATOR
The highest frequency for a 555 can be obtained by connecting the output to pins 2 and 6. This arrangement takes about 5mA and produces an output as shown. The max frequency will depend on the supply voltage, the manufacturer, and the actual type of 555 chip.

View the output on a CRO. Our 555 "Test Chip" produced a frequency of 300kHz at 5v and also at 12v. (CMOS versions will operate at a higher frequency.) Note the very short LOW TIME.

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INSIDE THE 555

Note: Pin 7 is "in phase" with output Pin 3 (both are low at the same time).
Pin 7 "shorts" to 0v via a transistor. It is pulled HIGH via R1.
Maximum supply voltage 16v - 18v
Current consumption approx 10mA
Output Current sink @5v = 5 - 50mA @15v = 50mA
Output Current source @5v = 100mA @15v = 200mA
Maximum operating frequency 300kHz - 500kHz

Faults with Chip:
Consumes about 10mA when sitting in circuit
Output voltage can be up to 2.5v less than rail voltage
Output can be 0.5v to 1.5v above ground
Sources up to 200mA
Some chips sink only 50mA, some will sink 200mA

A NE555 was tested at 1kHz, 12.75v rail and 39R load.
The Results:
Output voltage 0.5v low, 11.5v high at output current of 180mA
The "test chip" performance was excellent.

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HOW TO USE THE 555
There are many ways to use the 555. They can be used in hundreds of different circuits to do all sorts of clever things. They can also be used as three different types of oscillators:
(a) Astable Multivibrator - constantly oscillates
For frequencies above 1 cycle per second, it is called an oscillator (multivibrator or square wave oscillator).
For frequencies below 1 cycle per second it is called a TIMER or DELAY.
(b) Monostable - changes state only once per trigger pulse - also called a ONE SHOT
(c) Voltage Controlled Oscillator - called a VCO.

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THE ASTABLE (or FREE RUNNING)
MULTIVIBRATOR

The capacitor C charges via R1 and R2 and when the voltage on the capacitor reaches 2/3 of the supply, pin 6 detects this and pin 7 connects to 0v. The capacitor discharges through R2 until its voltage is 1/3 of the supply and pin 2 detects this and turns off pin 7 to repeat the cycle.
The top resistor is included to prevent pin 7 being damaged as it shorts to 0v when pin 6 detects 2/3 rail voltage.
Its resistance is small compared to R2 and does not come into the timing of the oscillator.

The following graph applies to the Astable circuit:

Using the graph:
Suppose R1 = 1k, R2 = 10k and C = 0.1u (100n).
Using the formula on the graph, the total resistance = 1 + 10 + 10 = 21k
The scales on the graph are logarithmic so that 21k is approximately near the "1" on the 10k. Draw a line parallel to the lines on the graph and where it crosses the 0.1u line, is the answer. The result is approx 900Hz.

Suppose R1 = 10k, R2 = 100k and C = 1u
Using the formula on the graph, the total resistance = 10 + 100 + 100 = 210k
The scales on the graph are logarithmic so that 210k is approximately near the first "0" on the 100k. Draw a line parallel to the lines on the graph and where it crosses the 1u line, is the answer. The result is approx 9Hz.

The frequency of an astable circuit can also be worked out from the following formula:

frequency =	1.4
	(R₁ + 2R₂) × C

555 astable frequencies
C	R₁ = 1k R₂ = 6k8	R₁ = 10k R₂ = 68k	R₁ = 100k R₂ = 680k
0.001µ	100kHz	10kHz	1kHz
0.01µ	10kHz	1kHz	100Hz
0.1µ	1kHz	100Hz	10Hz
1µ	100Hz	10Hz	1Hz
10µ	10Hz	1Hz	0.1Hz

0.001µ = 1n
0.01µ = 10n
0.1µ = 100n

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HIGH FREQUENCY OSCILLATORS
360kHz is the absolute maximum as the 555 starts to malfunction with irregular bursts of pulses above this frequency. To improve the performance of the oscillator, a 270R and 1n can be added as shown in the second circuit:

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LOW FREQUENCY OSCILLATORS - called TIMERS
	If the capacitor is replaced with an electrolytic, the frequency of oscillation will reduce. When the frequency is less than 1Hz, the oscillator circuit is called a timer or "delay circuit." The 555 will produce delays as long as 30 minutes but with long delays, the timing is not accurate.

555 Delay Times:
C	R₁ = 100k R₂ = 100k	R₁ = 470k R₂ = 470k	R₁ = 1M R₂ = 1M
10µ	2.2sec	10sec	22sec
100µ	22sec	100sec	220sec
470µ	100sec	500sec	1000sec

The following circuits show a 1-5 minute timer and 10 minute timer:

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CMOS 555
A low power version of the 555 is available from many manufacturers and basically it is a CMOS version of the TTL 555 device.
The CMOS 555 has the same pinouts as the TTL version and can be fitted into the same 8 pin socket but if the circuit needs more current than can be supplied by the CMOS version, it will not produce the same results.
It is the low current capability of the CMOS version that will be the major reason why you cannot directly replace the TTL version with the CMOS version.
It will operate from 1v (only some manufacturers) to 15v and will work up to 3MHz in astable mode.
Current consumption @5v is about 250uA (1/4mA)
But the major thing to remember is the output current capability.
At 2v, the chip will only deliver 0.25mA and sink only 1mA.
At 5v, the chip will deliver 2mA and sink only 8mA
At 12v the chip will deliver 10mA and sink 50mA
At 15v the chip will deliver 100mA and sink 100mA

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SQUARE WAVE OSCILLATOR KIT:

A square wave oscillator kit can be purchased from Talking Electronics for approx $10.00
See website: Square Wave Oscillator
It has adjustable (and settable) frequencies from 1Hz to 100kHz and is an ideal piece of Test Equipment.
(This link will send an email to Colin Mitchell and you will be advised of costs and how to send money via Paypal or credit card.)

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Bi-stable or "Latch" or "2-state" 555
The bi-stable 555 has two steady states. SET turns ON the LED and RESET turns the LED off. The 555 comes on in reset mode as Pin2 does not see a LOW to SET the 555.
See also: Divide By Two

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Monostable or "One Shot" or Pulse Extender
When the circuit is turned on, the output is LOW and a brief negative pulse on pin 2 will make the output go HIGH for a period of time determined by the value of R and C. If pin 2 is low for longer than this period, the output will remain HIGH while pin 2 is LOW and immediately go LOW when pin 2 goes HIGH.

CIRCUIT OPERATION
When the circuit is turned on, the capacitor is uncharged. Pin 6 sees a LOW and pin 2 sees a HIGH.
Remember: Pin 2 must be LOW to make the output HIGH.
Pin 6 must be HIGH to make the output LOW.
Neither pin is "controlling the chip" at start-up.

SET OR RESET?
The 555 contains a FLIP-FLOP and the output can be either HIGH or LOW if the voltage on pins 2 and 6 are not at the correct levels.
If pin 6 is HIGH and pin 2 is LOW - the output will be HIGH - pin 2 is sending a "set" message
If pin 6 is HIGH and pin 2 is HIGH - the output will be LOW - pin 6 is sending a "reset" message
If pin 6 is LOW and pin 2 is LOW - the output will be HIGH - pin 2 is sending a "set" message
If pin 6 is LOW and pin 2 is HIGH - the output will be HIGH or LOW - because no pin is sending a "set" or "reset" message.

When pin 2 see a LOW pulse, the chip goes to SET mode and the output goes HIGH. Pin 7 goes OPEN and capacitor C charges via R. When pin 6 sees 2/3 rail voltage, the chip goes to RESET mode with pin 3 and 7 LOW. The capacitor instantly discharges via pin 7 and the circuit waits for a negative pulse on pin 2.

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Re-triggerable 555
In the previous circuit, the output is LOW until pin 2 sees a brief LOW. The output goes HIGH and remains HIGH until pin 6 sees a HIGH via the charging of the capacitor. In other words the 555 extends the pulse according to the value of R and C.
The following diagrams show the output when the switch is pressed immediately, at half-time at the end of time and after the time has expired:

The switch is pushed and released quickly in the circuit above. Note the HIGH time for the output.

The switch is pushed and released slowly in the circuit above. Note the HIGH time for the output is the same as before.

The switch is pushed and released just when the time is expiring. Note the HIGH time for the output is the same as before. The switch does not extend (re-trigger) the 555.

To re-trigger the 555, the capacitor must be discharged. To do this we add a diode as shown so the capacitor discharges each time the switch is pressed:

Re-triggering the 555

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THE 555 AS A VOLTAGE CONTROLLED OSCILLATOR (VCO)
By adjusting the voltage on pin 5, (the CONTROL pin) the frequency of the oscillator can be adjusted quite considerably. See Police Siren for an application.

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THE 555 AS A RAMP GENERATOR
When a capacitor is charged via a constant current, the waveform across it is a ramp.

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FREQUENCY DIVIDER
A 555 can be used to divide a frequency by almost any division.
It works this way:
A 555 is set-up to produce the required output frequency.
Pin 2 is then taken to the input frequency and this turns the 555 into a Monostable Multivibrator.
The circuit will detect a LOW on pin 2 to start the timing cycle and pin 3 will go HIGH. The 555 will not respond to any more pulses on pin 2 until pin 6 detects a HIGH via the charging of the capacitor. The value of C and the 1M pot need to be adjusted to produce the desired results.

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DIVIDE BY 2
A 555 can be used to divide-by-2
When pins 2 and 6 are connected, they detect 1/3 and 2/3 of rail voltage. When the detected voltage is below 1/3, the output goes HIGH and when the voltage is above 2/3, the output goes LOW.
The push switch detects the output voltage and after a short period of time the electrolytic will charge or discharge and it will be HIGH or LOW.
If the switch is pressed for a short period of time, the output will change. If the switch is kept pressed, the output will oscillate at a low frequency.

This circuit can also be used as a Push-ON Push-OFF.

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LOGIC PROBE
A 555 can be used to make a very simple LOGIC PROBE. It connects to the circuit you are testing via a red and black alligator clip and pins 2 and 6 detect 1/3 and 2/3 of the voltage on the chip.
Pins 2 and 6 provide a high impedance input and the red and green LEDs indicate HIGH and LOW.
The only problem is one of the LEDs will be illuminated when the probe is not touching anything and that's why this circuit is not a really GENUINE Logic Probe. But it will do in an emergency or if you want to build something quickly to test a project.

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LM386 LED FLASHER
If you don't have a 555, you can use an LM386 IC to flash a LED. This will show how an LM386 chip works and and how to use chips for purposes other than their normal intent.
An LM386 IC is an 8-pin 250mW power amplifier with a differential input, similar to an op-amp.
Here is the internal circuit:
Normally pin 3 has zero volts on it, but the 100k puts about 35% of rail voltage on the pin.
This means pin 3 has an input impedance of about 300k.
The actual voltage does not matter, as long as it is slightly higher than zero volts, so pin 2 can have an effect when it goes lower than pin 3.
When the circuit is turned on, the 100u is uncharged and zero volts is applied to pin 2.
This means pin 2 is lower than pin 3 and the output goes HIGH to illuminate the LED.
The 100u charges via the 10k and after a short period of time the voltage on pin 2 is higher than pin 3 and it makes the output go LOW. The LED turns OFF and the 100u starts to discharge via the 10k to repeat the cycle.
The chips has a gain of 20, but this is not enough for this circuit to flash a LED. By joining pins 1 and 8, the gain is increased to 200 or more and the circuit works successfully.

LED FLASHER USING LM386 IC

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"No-No's"
Here are some mistakes to avoid:
1. Pin 7 gets connected to the 0v rail via a transistor inside the chip during part of the operation of the 555. If the pot is turned to very low resistance in the following circuit, a high current will flow through the pot and it will be damaged:

2. The impedance of the 100u electrolytic will allow a very high current to flow and the chip will get very hot. Use 10u maximum when using 8R speaker. (The temp of the chip will depend on the frequency of the circuit.)

3. The reset pin (pin 4) is internally tied HIGH via approx 100k but it should not be left floating as stray pulses may reset the chip.

4. Do not draw 555 circuits as shown in the following diagram. Keep to a standard layout so the circuit is easy to follow.

5. Here's an example from the web. It takes a lot of time to work out what the circuit is doing:

The aim it to lay-out a circuit so that it shows instantly what is happening. That's why everything must be in recognised locations.

Here is the corrected circuit: From this diagram it is obvious the circuit is an oscillator (and not a one-shot etc).

6. Don't use high value electrolytics and high resistances to produce long delays. The 555 is very unreliable with timing values above 5-10 minutes. The reason is simple. The charging current for the electrolytic is between 1 - 3 microamp in the following diagram (when the electro is beginning to charge) and drops to less than 1 microamp when the electro is nearly charged.
If the leakage of the electro is 1 microamp, it will never fully charge and the 555 will never "time-out."

7. Do not connect a PNP to the output of a 555 as shown in the following diagram. Pin 3 does not rise high enough to turn the transistor OFF and the current taken by the circuit will be excessive. Use an NPN driver.

8. Do not connect pin 7 directly to the capacitor. Pin 7 connects to the 0v rail during part of a cycle and this will discharge the capacitor very quickly and produce a very brief "low" on the output.
This may upset the clocking of a chip (such as a CD4017).
Use a resistor of at least 1k to increase the width of the LOW.

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555's
Here is a list of 555's from different manufacturers plus the range of low voltage, low current 555's. The normal 555 is called a TTL or Transistor-Transistor-Logic chip and it consumes about 10mA when "sitting and doing nothing." It will work from 4v to 18v.
A low current version is available from the list below, (called a CMOS version) and consumes about 10uA to 100uA. Some of these chips work from 1.5v to 15v (ZSCT1555 = 9v max) but they can sink and source only about 100mA (less than 30mA at 2v).
The 555 is the cheapest and the others cost about double.
The normal 555 oscillates up to 300kHz. A CMOS version can oscillate to 3MHz.
You need to know the limitations as well as the advantages of these chips before substituting them for the normal 555:

Manufacturer	Model	Remark
Custom Silicon Solutions	CSS555/CSS555C	CMOS from 1.2V, IDD < 5uA
ECG Philips	ECG955M
Exar	XR-555
Fairchild Semiconductor	NE555/KA555
Harris	HA555
IK Semicon	ILC555	CMOS from 2V
Intersil	SE555/NE555/ICM7555
Lithic Systems	LC555
Maxim	ICM7555	CMOS from 2V
Motorola	MC1455/MC1555
National Semiconductor	LM1455/LM555/LM555C
National Semiconductor	LMC555	CMOS from 1.5V
NTE Sylvania	NTE955M
Raytheon	RM555/RC555
RCA	CA555/CA555C
STMicroelectronics	NE555N/ K3T647
Talking Electronics	TE555-1, -2, -3, -4	email Talking Electronics $2.50 ea
Texas Instruments	SN52555/SN72555; TLC555	CMOS from 2V
Zetex	ZSCT1555	down to 0.9V (9v max)

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REPLACING A 556 WITH TWO 555's
Here is a handy reference to replace a 556 dual timer with two 555's:

The table shows the pin numbering for each timer:

	555	556 - Timer 1	556 - Timer 2
Ground (–)	1	7	7
Trigger	2	6	8
Output	3	5	9
Reset	4	4	10
Control	5	3	11
Threshold	6	2	12
Discharge	7	1	13
Vcc (+)	8	14	14

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REPLACING A 558 WITH FOUR 555's
This is an obsolete chip. No-one stocks it.
The LM558 only has 100mA output current (the LM555 has up to 200mA).
The output only goes LOW. In other words, the output only SINKS.

I suggest only using 555's in all projects. You can easily remember the pin numbering on a 555 and this makes servicing and testing and fault-finding easy. A circuit using a "558" (NE55, LM558) can be converted to using 4 x LM555's by referring to the following image:

The NE558 is a very messy chip and it seems the pin that is equivalent to pin 6, changes to pin 7 to discharge the capacitor when 66% of rail voltage is reached.

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SUBSTITUTING A 555 - part 1
Although a 555 is a wonderful chip, it may not be the best choice for some applications.
You may require an oscillator frequency higher than 1MHz, or a very low quiescent current. You may also need 4 or more 555's to get the timing and delays you require.
Here are some circuits to help you substitute a 555.

The 74c14 IC contains 6 Schmitt Trigger gates and each gate can be used to replace a 555 in SOME circuits. The voltage for a 74c14 is 3v to 15v. Maximum output current per gate is 15mA. Max frequency of operation: 2MHz - 5MHz. Quiescent current is 1uA if all inputs are 0v or rail voltage.

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SUBSTITUTING A 555 - part 2
If you need a special function or special effect, it may take 2, 3 or more 555's to do the job. The 74c14 has 6 gates and can create 6 "building blocks."
Here are some circuits to show its versatility:

2 MINUTE TIMER
The relay is energized for a short time, 2 minutes after the push-button is pressed. The push-button produces a brief LOW on pin 1, no matter how long it is pushed and this produces a pulse of constant length via the three components between pin 2 and 3.
This pulse is long enough to fully discharge the 100u timing electrolytic on pin 5.
The 100k and electrolytic between pins 6 and 9 are designed to produce a brief pulse to energize the relay.

OUTPUT AFTER 2 MINUTES
Here is another very similar circuit. Use either the active HIGH or Active LOW switch and if the Active LOW switch is used, do not connect the parts or gate between pins 1 and 2 to the rest of the circuit.

PULSER
The 74c14 can be used for lots of different circuits. In the following design, the output produces 3mS pulses every second. The circuit is adjustable to a wide range of requirements.

TRIGGER TIMER
The next design interfaces a "Normally Open" and "Normally Closed" switch to a delay circuit.
The feedback diode from the output prevents the inputs re-triggering the timer (during the delay period) so that a device such as a motor, globe or voice chip can be activated for a set period of time.

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BUILDING THE CIRCUITS
The fastest way to put a circuit together is on BREADBOARD. The cheapest and best bread-board has power-rails and sets of "tie-points" or "holes" as shown in this photo:

Connect the components with hook-up wire (called jumpers) by stripping the ends to expose the wire at both ends. Or you can use 0.5mm tinned copper wire (make sure the jumpers do not touch each other).
Do not cut the leads of the components as you may want long leads on another project.

Neatness is not important. The important thing is to build as many circuits as possible as each one will help you understand how the 555 works and how the external circuitry modifies the signal to produce the resulting effect. There is a point-to-learn in every circuit.

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POWERING A PROJECT
The safest way to power a project is with a battery. Use 4 x AA cells in a holder or a 9v battery if you only want to use the project for a short period of time.
If you want to use a 555 project for a long period of time you will need a "power supply."
The safest power supply is a Plug Pack (wall-wort, wall wart, wall cube, power brick, plug-in adapter, adapter block, domestic mains adapter, power adapter, or AC adapter). The adapter shown in the diagram has a switchable output voltage: 3v, 6v, 7.5v, 9v, 12v) DC and is rated at 500mA. The black lead is negative and the other lead with a white stripe (or a grey lead with a black stripe) is the positive lead.
This is the safest way to power a project as the insulation (isolation) from the mains is provided inside the adapter and there is no possibility of getting a shock.
The rating "500mA" is the maximum the Plug Pack will deliver and if your circuit takes just 50mA, this is the current that will be supplied. Some pluck packs are rated at 300mA or 1A and some have a fixed output voltage. All these plug packs will be suitable.
Some Plug Packs are marked "12vAC." This type of plug pack is not suitable for these circuits as it does not have a set of diodes and electrolytic to convert the AC to DC. All the circuits in this eBook require DC.

PROJECTS
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TOY ORGAN
This circuit produces a tone according to the button being pressed. Only 1 button can be pressed at a time, that's why it is called a monophonic organ. You can change the 1k resistors to produce a more-accurate scale.

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TICKING BOMB
This circuit sound just like a ticking bomb.

This project can be constructed on our MAKE ANY 555 PROJECT printed circuit board.

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METAL DETECTOR
This circuit detects metal and also magnets. When a magnet is brought close to the 10mH choke, the output frequency changes. The photo shows the circuit made by a reader:

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GUITAR TUNER
This circuit is designed to produce a tone of 440Hz.
To do this: Set the potentiometer to about half a turn with a small screw driver. Used an electronic keyboard and select the square wave tone, then turn on the Guitar Tuner circuit. Hold the A4 key down on the keyboard and use the screwdriver to turn the potentiometer until the frequencies match.

This project can be constructed on our MAKE ANY 555 PROJECT printed circuit board.

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UNEVEN CLICKS
This circuit produces two clicks then a short space before two more clicks etc. Changing the voltage on pin, 5 via the diode, adjusts the timing of the chip.

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FLASHING RAILROAD LIGHTS
This circuit flashes two red LEDs for a model railway crossing.

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SCREAMER
This circuit will produce an ear-piercing scream, depending on the amount of light being detected by the Light Dependent Resistor.

This project can be constructed on our MAKE ANY 555 PROJECT printed circuit board.

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LASER RAY
This circuit produces a weird "Laser Ray" sound and flashes a white LED at approx 5Hz:

Here is a video of the sound. It sounds much better with a large speaker.

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LED DIMMER
This circuit will adjust the brightness of one or more LEDs from 5% to 95%.

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MOTOR PWM
See also: PWM Controller
The speed of a motor can be adjusted by this circuit, from 5% to 95%.

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Throttle - simple
The speed of a motor can be adjusted by this circuit, from 5% to 95%. The circuit uses a FET and no heatsink is needed. This circuit is available from Talking Electronics as a fully assembled module. Click HERE for details.

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Throttle - with Current Limiting
The maximum current the circuit will deliver is controlled by the second pot. When it is rotated fully clockwise the current is limited to 1.4Amps. When it is turn anticlockwise the current is limited to 100mA. This allows you to drive large or small locos and also allows you test your track for short-circuits at turn-outs etc.
It is designed for those who have a 12v @ 10Amp supply. If a short-circuit occurs with the type of supply, the throttle module will be damaged. With current limiting, the current will be limited to 1.4Amp and even though it will stress the power diodes, they will not be destroyed.
This circuit is available from Talking Electronics as a fully assembled module. Click HERE for details

The current-limiting circuit controls the mark-space ratio of the 555 by shifting the setting created by the 100k pot and the charging and discharging of the 100n capacitor via the two signal diodes.
The current across the two supply diodes is monitored by the PNP transistor and they have a voltage of about 0.7v across them when passing current. This voltage will turn on any PNP transistor so we create a voltage divider to the base with a pot. The resistor in series with the pot sets the maximum current for the circuit as the pot allows the circuit to increase the current to infinity when it has its smallest value. So we create the lowest value with a series resistor.
As the pot is increased in value, the maximum current reduces to less than 100mA, because the PNP transistor can be turned on at a lower current through the diodes and now we have our two extremes.
As the PNP transistor turns ON, it turns on the NPN transistor and this is causes the voltage on pin 5 to reduce and change the mark-space ratio.
This is the first time such a feature of the 555 has been used and shows the versatility of this chip. This is a proprietary circuit from Talking Electronics and the important component values have been omitted from the diagram as the module is available fully built and tested.

Using pin 5
30 Billion 555 IC's have been used and almost no-one has discussed the use of pin 5.
It controls when the two amplifiers come into operation.
The inputs of the two amplifiers are connected to pin 2 and pin 6. These amplifiers are called OPERATIONAL AMPLIFIERS and have two inputs and one output. It is the difference in voltage on each of the input lines of each op-amp that makes them change state. These amplifiers have what we call a VERY HIGH INPUT IMPEDANCE. This means a very weak signal will be detected. As long as the signal has a very good amplitude, (voltage) it will be detected. That's because the op-amps are internally connected in such a way that they only respond to a large amplitude. The op-amp circuitry will respond to a very small change in voltage but the 555 IC has different internal circuitry set by three resistors so that when pin 2 goes below 30% of the voltage supplied to the chip, it will change state and when it goes higher than 30% NOTHING HAPPENS. It only works when going below 30%
When the voltage is above 70%, on pin 6, the chip will change to the opposite state and when the voltage goes LOW on pin 6, nothing happens.
The chip need both pin 2 and 6 to be connected to the input voltage for it to change states.
This is how output pin 3 changes from HIGH to LOW. It sees voltage-levels on pins 2 and 6.
But we can change these trigger-values (they are called levels, extremes, you can even call them digital values, trigger points, trigger values, inputs or control voltages) by adjusting the voltage on pin 5.
Pin 5 controls these "trigger voltages" by shifting them up and down. It controls the point at which pins 2 and 6 activate the chip.
And that's what we do in the circuit above.
Pin 5 normally has about 70% of the value of the supply on it and by reducing this voltage to almost zero, we can create a very big change in the frequency of operation of the 555 when it is wired as an oscillator.
I am not going into any technical details as you can build your own circuit and see the changes.
All I will say is pin 5 is classified as a low-impedance input as it has some 5k resistors connected to it and it will appear to have a resistance of much less than 2k, with a voltage of about 60% of rail voltage.
You can supply a voltage to this pin up to rail voltage or take pin to 0v without damage.

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PWM
See also: PWM Controller
The output of these circuits can be adjusted from 5% to 95%.

PWM via pin 5
The ratio of the HIGH time to the LOW time can be adjusted by changing the voltage on Pin 5. This is called PULSE-WIDTH adjustment or PULSE-WIDTH -MODULATION.
It does not give the wide range of the circuits above, but is handy for some applications.

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VOLTAGE DOUBLER
A voltage higher than the supply can be created by a "Charge-Pump" circuit created with a 555, diodes and capacitors as shown in the following circuit. The output will deliver about 50mA

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VOLTAGE TRIPLER and QUADRIPLER!
A voltage of about 3 times the supply (minus about 5v) can be created by the following circuit. The output will deliver about 50mA. Pin 3 goes LOW to about 0.5v and HIGH to about 11v.
When pin 3 goes LOW, capacitor "a" charges via diode "a" to about 11v.
When pin 3 goes HIGH, capacitor "a" has about 11v across it, plus the voltage on pin 3. The 22v on the positive lead of capacitor "a" passes through diode "b" to charge capacitor "b'' with 21v - 12v = 9v. This produces a voltage of 21v on the anode of diode "c." When pin 3 goes LOW, both capacitors "a" and "c" will charge via diodes "a" and "c" with capacitor "a" being charged via diode "a" and capacitor "c" charged via diode "c." When pin 3 goes HIGH, the 9v across capacitor "c" will be added to the 22v to charge capacitor "d" to 31v.

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INCREASING OUTPUT PUSH-PULL CURRENT
Some 555's do not swing rail-to-rail when 200mA is being delivered and the chip gets very hot when trying to deliver 200mA.
The solution is to add a push-pull output. The following arrangement has been chosen as it swings almost rail-to-rail but two faults need to be addressed.
Both transistors turn on during the brief interval when pin 3 is travelling from high to low or low to high.
This means the two transistors will put a "short" across the power rail.
The addition of the 4R4 will allow a high current to flow but the transistors will not be damaged. In addition, green LEDs on the base of each transistor reduces the time when both transistors are ON.
The animation shows how the transistors are turned on and off and deliver a high current to the load. The animation shows how NPN and PNP transistors follow an input signal in a push -pull arrangement using positive and negative supply rails. This is not the same as our circuit however the basic effect applies. The output is inverse of pin3 but pin3 only needs to deliver 10-50 milliamp and the transistors can deliver 1 amp or more to the load. This allows the 555 to be kept cool.

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NEGATIVE VOLTAGE
A negative supply can be produced by a "Charge-Pump" circuit created with a 555, diodes and capacitors as shown in the following circuit. The output will deliver about 50mA.

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STUN GUN - Voltage Multiplier
This circuit produces a very high voltage and care must be used to prevent getting a nasty shock.  The transformer can produce over 1,000v and the 8-stage multiplier can produce up to 20,000v

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12v to 240v INVERTER
This circuit will produce 240v at 50Hz. The wattage will depend on the driver transistors and transformer.

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170v SUPPLY FOR NIXIE TUBES
This circuit produces approx 170v for Nixie tubes and other neon tubes. It is a switch-mode boost circuit.

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12v DC to 12v DC BATTERY CHARGER (and 19v see below)
You cannot charge a 12v battery from a 12v battery. The battery being charged creates a "floating charge" or "floating voltage" that is higher than the charging voltage and the charging stops.
The following circuit produces a voltage higher than 12v via a CHARGE PUMP arrangement in which the energy in an electrolytic is fed to a battery to charge it.
The circuit produces about 900mA "charge current" and the diodes and transistors must be fitted with heat sinks. The LEDs are designed to prevent the two output transistors turning ON at the same time. The lower output transistor does not start to turn on until the voltage is above 5v and the top transistor does not turn on until the voltage drops 4v from the positive rail. This means both transistors will be turned on ONLY when the voltage passes a mid-point-gap of 4v. In our circuit, this time is very short and and the transition is so fast that no current (short-circuit current) flows via the two output transistors (as per our test).
The electrolytic charges to about 10v via the lower transistor and top diode. The top BD679 then pulls the negative of the 2200u electrolytic towards the 12v6 rail and the positive is higher than 12v6 by a theoretical 10v, (about 9v in our case) however we need the ENERGY IN THE ELECTROLYTIC and in our circuit it is capable of delivering a current of about 900mA. This energy is passed to the battery via the lower diode.  Most batteries should not be charged faster than the "14-hour-rate." This basically means a flat battery will be charged in 14 hours. To do this, divide the AHr capacity by 14 to get the charge-rate. For example, a 17AHr battery should be charged at 1.2A or less. For lower-capacity batteries, the 2200u can be reduced to 1,000u. Charging is about 80% efficient. In other words, delivering 120% of the AHr capacity of a battery is needed to fully charge it.

        This circuit will deliver about 150mA

12v DC to 19v DC
This circuit can also be used to charge some of the older-style laptops that need a 19v supply. Remove the 12v battery from the circuit above and connect the cathode of the lower 1N4004 to the input of the laptop.

3.7v DC to 5v DC
This circuit will produce about 5v from a 3.7v Li-Ion cell:
A 555 will not work on 3.7v - 0.7v = 3v     You may be lucky to get it to "pump up" the output to get 5v and this will activate the 555. BUT some chips will not work.   The 7555 IC may work on 3.0v - but I would not guarantee anything.

5v DC to 7v DC
This circuit will produce about 7v from a 5v USB port or 5v cell-phone charger to trickle charge a 6v battery.
The voltage of the battery will rise to about 7v when charging (called the "floating charge") and that's why you need to "jack-up" the voltage from the USB port to charge the battery. The circuit is limited to about 45mA with a 22u electrolytic and a battery can take up to a week to charge.
Circuit design by: budiyantosamuel90@gmail.com
                        This circuit will deliver about 45mA

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555 AMPLIFIER
The 555 can be used as an amplifier. It operates very similar to pulse-width modulation. The component values cause the 555 to oscillate at approx 66kHz and the speaker does not respond to this high frequency. Instead it responds to the average CD value of the modulated output and demonstrates the concept of pulse-width modulation. The chip gets very hot and is only for brief demonstrations.

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LIGHT DETECTOR
This circuit detects light falling on the Photo-cell (Light Dependent Resistor) to turn on the 555 and create a tone that is delivered to the speaker. Pin 4 must be held below 0.7v to turn the 555 off. Any voltage above 0.7v will activate the circuit. The adjustable sensitivity control is needed to set the level at which the circuit is activated. When the sensitivity pot is turned so that it has the lowest resistance (as shown in red), a large amount of light must be detected by the LDR for its resistance to be low. This produces a voltage-divider made up of the LDR and 4k7 resistor. As the resistance of the LDR decreases, the voltage across the 4k7 increases and the circuit is activated.
When the sensitivity control is taken to the 0v rail, its resistance increases and this effectively adds resistance to the 4k7. The lower-part of the voltage-divider now has a larger resistance and this is in series with the LDR. Less light is needed on the LDR for it to raise the voltage on pin 4 to turn the 555 on.

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DARK DETECTOR
When the level of light on the photo-cell decreases, the 555 is activated. Photo-cells (Photo-resistors) have a wide range of specifications. Some cells go down to 100R in full sunlight while others only go down to 1k. Some have a HIGH resistance of between 1M and others are 10M in total darkness. For this circuit, the LOW resistance (the resistance in sunlight) is the critical value.
More accurately, the value for a particular level of illumination, is the critical factor. The sensitivity pot adjusts the level at which the circuit turns on and allows almost any type of photo-cell to be used.

The next circuit turns on a relay for a short period of time when the LDR does not detect sufficient light:

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SHORT BEEP
This circuit produces a short beep when the input goes from HIGH to LOW. The input can stay LOW for any length of time but the output will always be a short beep.

This circuit produces a short beep when the input goes from LOW to HIGH. The length of the beep-tone depends on the value of the 1u electrolytic.

This circuit produces a short beep when the LDR does not receive any illumination.
The length of the beep-tone depends on the value of the 1u and the 10k connected to it.

Don't forget:
The 555 still takes 10mA when it is "turned off" via pin 4.
Pin 4 merely stops the output changing state.

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FLIP FLOP and MEMORY CELL
When output pin 3 is HIGH, the 220n charges through the 220k to 6v. When pin 3 is LOW, the 220n discharges through the 220k to 0v. Pressing the switch upsets the 3v created by the two 10k voltage dividers, triggering the flip flop inside the 555 and changing the state of the output from HIGH to LOW or vice-versa. The output of the 555 drives a transistor to turn a globe on and off.
The second circuit is a Memory cell and is the basis of the memory in a computer. The SET button turns on the globe and the RESET button turns the globe off.
It works like this: When the circuit is turned on, pin 6 does not see a high and pin 2 does not see a low, so the 555 starts in reset mode.

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CAR TACHOMETER
A 555 is configured as a monostable or one shot in this project. The period of the 555 is determined by the 47k and the capacitor from pin 6 to ground (100n). Time "T" = 1.1 RC or 1.1 X 50,000 X 0.1 ^{X10 -6} = 0.0055 or 5.5 mS (milli-seconds).
The 555 receives trigger pulses from the distributor points. These are limited by the 1k and 5v zener diode. These are AC coupled to the trigger input through the 100n coupling capacitor. The 50mA meter receives pulses of current through the 200k pot to show a reading.

Integration of the current pulses produces a visible indication of the cars engine speed on the 0-1mA meter.
Supply is taken from the cars 12v system and for the 555 it is reduced to a regulated 9v by the 15 ohm resistor in conjunction with the 9v zener diode. Note: the 10u electrolytic must be placed physically as close as possible to supply pin 8.

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FREQUENCY METER
This circuit will detect audio frequencies and display them on a meter (actually called a "movement"). Connect the circuit to the output of an amplifier. It is best to detect one frequency at a time. Integration of the audio frequency produces a visible indication on the 0-1mA meter.

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SERVO TESTER
This circuit can be used to manually turn a servo clockwise and anti-clockwise. By pushing the forward or reverse button for a short period of time you can control the rotation of the servo. It will also test a servo.
Here is a photo of a kit from Cana Kit for $10.00 plus postage (it is a slightly different circuit) and a motor and gearbox, commonly called a "servo." The output shaft has a disk or wheel containing holes. A linkage or push-rod is fitted to a hole and when the disk rotates, the shaft is pushed and pulled. The shaft only rotates about 180° to actuate flaps or ailerons etc.

A pot can be used to control the position of the servo by using the following circuit. It produces a positive pulse between about 0.9 milliseconds and 2.1 milliseconds. The off period between pulses is about 40 milliseconds. This can be shortened by reducing the value of the 3M3 resistor.

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JOY STICK
This circuit will control a servo from a Joy Stick:

The supply for the servo must be separate from the 555 section because the 555 picks up noise from the servo and makes the motor vibrate all the time.
This circuit works in the "Y-direction." Build another circuit for the "X-direction."
Adjust the 10k pot for the centre of travel and use the Joy Stick to produce left and right action.
The Joy Stick comes in a set of 37 Sensors for less than $20.00 on eBay and you can see all the sensors HERE.

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USELESS MACHINE
Here is a fun project using a servo motor and a circuit similar to the SERVO TESTER project above. It is available on the INSTRUCTABLES website. Before you do anything, watch the video:
http://www.instructables.com/id/The-Most-Useless-Machine

The Instructables website contains all the construction details. The circuit diagram shows the toggle switch is clicked towards the lid of the box and this starts the servo motor. The servo has an arm that comes out of the box and clicks the switch to the opposite position. This reverses the servo and the arm retreats into the box and hits the limit switch that turns the circuit off.
You may have to adjust the value of the 15k and 27k resistors and you will also see other videos on the Instructables website to help you with construction.
As the website says: "It's the most useless invention, but everyone wants one."

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TRANSISTOR TESTER

The 555 operates at 2Hz. Output pin 3 drives the circuit with a positive then zero voltage. The other end of the circuit is connected to a voltage divider with the mid-point at approx 4.5v. This allows the red and green LEDs to alternately flash when no transistor is connected to the tester.
If a good transistor is connected, it will produce a short across the LED pair when the voltage is in one direction and only one LED will flash. If the transistor is open, both LED’s will flash and if the transistor is shorted, neither LED will flash.

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SWITCH DEBOUNCE
The output goes HIGH for 100mS when the switch is pressed.

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INCREASING OUTPUT
CURRENT
The 555 will deliver 200mA to a load but the chip gets extremely hot (12v supply). The answer is to use a buffer transistor.
For 200mA, use a BC547 or equivalent.
For 500mA use a BC337 or equivalent
For 1A, use a TIP31 or equivalent.
For 3A - 5A use a BD679 or equivalent with heatsink
For 5A to 10A use TIP3055 with heatsink

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IMPROVING THE SINKING OF A 555
The output of a 555 goes low to deliver current to a load connected as shown in the circuit below. But when the chip is sinking 200mA, pin 3 has about 1.9v on it. This means the chip does not provide full rail voltage to the load.
This can be improved by connecting pin 7 to pin 3. Pin 7 has a transistor that connects it to 0v rail at the same time when pin 3 is LOW. They can both be connected together to improve sinking capability. In this case the low will be 800mV for 200mA instead of 1900mV, an improvement of 1100mV. This will add 1v1 to the load and also make the chip run cooler.

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CONSTANT CURRENT
The 555 will deliver 200mA to a load but this might be too much. You can add a dropper resistor (current limiting resistor) but the current will reduce as the supply voltage drops.
To provide a constant output current to a device such as an IR LED, the following circuit can be used. The current will be constant for any supply voltage but the best range will be 7v to 12v.
The current is determined according to the value of R. You can use this table:

R
Current

5R6 100mA

10R 60mA

22R 30mA

47R 15mA

100R 6mA

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DRIVING A BI-COLOUR LED
Some 3-leaded LEDs produce red and green. This circuit alternately flashes a red/green bi-coloured LED:

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BI-POLAR LED DRIVER
Some 2-leaded LEDs produce red and green. These are called Bi-polar LEDs. This circuit alternately flashes a red/green bi-polar LED:

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DRIVING A TRI-COLOUR LED
This circuit drives each colour slowly from dark to full brightness and each oscillator has a different frequency. This creates a total mixture of colours and allows the RGB LED to produce all its different colours. Some LEDs (diffused LEDs) work better than others. Change the 220k/22k etc for each oscillator to get a different mixing effect.
The output pin 3 goes HIGH immediately but the base is taken high slowly because the 100u takes time to charge via the 22k etc resistor). The transistor is an emitter-follower and it delivers more and more current through the 470R to increase the brightness. Then the process reverses.

There are many different flickering and flashing LEDs on the market via eBay.
They contain a microscopic microcontroller chip and current limiting resistor. Many of them work on a voltage from 3v to 6v and you can hear the oscillator turning ON and OFF to produce the different effects by building the following circuit:
This means you have a choice. You can buy a 4-leaded RGB LED and activate the individual colours or buy a 2-leaded RGB LED and let the microcomputer inside the LED do all the flickering.
The circuit above will let you hear all the "ramping and oscillating" needed to produce the effects.

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ZENER DIODE TESTER
This circuit will test zener diodes up to 56v. See Talking Electronics website, left index, 200 Transistor Circuits (circuits 1-100) and go to Zener Diode (making) to see how to make a zener diode and how to create a zener voltage from a combination of zeners.
Place the zener across the terminals in the circuit below and read the value across it with a multimeter set to 50v range.

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WAILING SIREN
By pressing the button, the wailing sound increases. Releasing the button decreases the wailing.
The circuit automatically turns off after about 30 seconds.

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CONTINUITY TESTER
This circuit will detect low resistances and high resistances to produce a tone from the speaker.
It will detect up to 200k and the circuit automatically turns off when the probes are not used.

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MORSE KEYER
This circuit will help you master the art of keying Morse Code:

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ACTIVE LOW TRIGGER
This circuit sits LOW and the output goes HIGH when the push-button is pressed. When the normally-closed push button is pressed, it opens and the uncharged 1u will be pulled to nearly 0v rail via the 10k and this will take pin 2 LOW to make output pin 3 HIGH for the duration determined by the 22u and 100k. If the push-switch stays open, the 1u will charge via the 100k and eventually the output of the 555 will go low.
But normally the switch must be pressed for a short period of time so that the timing components (100k and 22u) make the output go HIGH for a short period of time. This circuit is called an ACTIVE LOW TRIGGER

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ACTIVE HIGH TRIGGER
This circuit produces a HIGH output via a HIGH trigger:

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MACHINE GUN
This circuit produces a sound very similar to a machine gun:

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LATCH
This circuit is a LATCH and remains ACTIVE when the push-button has been pressed for an INSTANT and released.

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TOGGLE 555
This circuit will toggle the output each time the switch is pressed. The action cannot be repeated until the 10u charges or discharges via the 100k.

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TAP ON TAP OFF
This circuit will toggle the output each time the piezo is tapped very hard. The action cannot be repeated until the 10u charges or discharges via the 100k (up to about 8 seconds).
The piezo must be good quality (available from Talking Electronics). Don't complain if your circuit does not work. The piezo must produce a high output.

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TAP ON TAP OFF-2
To increase the sensitivity of the piezo, a transistor can be added. It does not matter which way around the transistor is connected and the piezo can be connected to the collector and base. Try experimenting with the different arrangements and see if there is any improvement.

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CLAP ON - OFF
At long-last we have a clap switch using a 555.
The output going HIGH and LOW provides two different voltages for the BC547 and also pins 2 and 6. The transistor simply provides a signal and this alters the voltage on pins 2 and 6.
Pin 3 will provide a lowish voltage on pin 2 and 6 and the transistor will simply lower this voltage to turn the 555 ON. When the 555 is ON, the voltage on pins 2 and 6 will be higher but not high enough to turn the 555 OFF. A clap will produce a waveform that increases the voltage on pins 2 and 6 and the 555 is turned OFF. The 100u slows down the change in voltage from the 555 so the clapping will be able to change the state of the chip. The circuit works on 6v to 9v.

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TOGGLE 555 - ON/OFF
This circuit will turn the output ON when a light shines on the LDR connected to pin 2 and OFF when a light shines on the LDR connected to pin 6.

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TOGGLE A RELAY
This circuit will toggle a relay each time the switch is pressed. The action cannot be repeated until the 10u charges or discharges via the 100k.

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KNIGHT RIDER
In the Knight Rider circuit, the 555 is wired as an oscillator. It can be adjusted to give the desired speed for the display. The output of the 555 is directly connected to the input of a Johnson Counter (CD 4017). The input of the counter is called the CLOCK line.
The 10 outputs Q₀ to Q₉ become active, one at a time, on the rising edge of the waveform from the 555. Each output can deliver about 20mA but a LED should not be connected to the output without a current-limiting resistor (330R in the circuit above).
The first 6 outputs of the chip are connected directly to the 6 LEDs and these "move" across the display. The next 4 outputs move the effect in the opposite direction and the cycle repeats. The animation above shows how the effect appears on the display.
Using six 3mm LEDs, the display can be placed in the front of a model car to give a very realistic effect. The same outputs can be taken to driver transistors to produce a larger version of the display.

The Knight Rider circuit is available as a kit for less than $15.00 plus postage as Kitt Scanner.

Here is a simple Knight Rider circuit using resistors to drive the LEDs. This circuit consumes 22mA while only delivering 7mA to each LED. The outputs are "fighting" each other via the 100R resistors (except outputs Q0 and Q5).

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FLASHING INDICATORS
This is a request from Daniel, one of our subscribers.
He needed to flash "turn indicators" using a 555 and a single 20 amp relay. Here is our suggestion. The timing resistor needs to be selected for the appropriate flash-rate.

Flashing the "TURN INDICATORS"

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TOUCH SWITCH and TOUCH ON-OFF
The Touch Switch circuit will detect stray voltages produced by mains voltages and electrostatic build-up in a room. In the first circuit, pin 2 must see a LOW for the circuit to activate. If sufficient static voltage is detected by the plate, the chip will change state. If not, you will need to touch the plate and the 0v rail. In the second circuit, two touch plates are provided and the resistance of your finger changes the voltage on pin 2 or 6 to toggle the 555.

The circuit can be made 100 times more sensitive by adding a transistor to the front-end as shown in the diagram below:

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SIREN 100dB
This is a very loud siren and if two or more piezo's are located in a room, the burglar does not know where the sound is coming from.
A robber will not stay anywhere with an ear-piercing sound as he cannot hear if someone is approaching.
It's the best deterrent you can get. The "F" contact on the piezo is "feedback" and is not needed in this circuit.

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POLICE SIREN
The Police Siren circuit uses two 555's to produce an up-down wailing sound. The first 555 is wired as a low-frequency oscillator to control the VOLTAGE CONTROL pin 5 of the second 555. The voltage shift on pin 5 causes the frequency of the second oscillator to rise and fall.

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HEE HAW SIREN
Build the circuit and listen. Change the resistors and capacitors to get all sorts of different results.

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HEE HAW SIREN with one 555
The flashing LED is used to provide the timing and also to alter the 555 to produce two different tones.

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RAIN ALARM
This circuit consumes no current until moisture is detected on the rain plate.

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PWM CONTROLLER
See also: PWM
This controller will deliver up to 30 amps and control the motor from 5% to 95%.

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SOLAR TRACKER
For Solar Tracker circuits using the 555, go to Duane Johnson's website:
http://www.redrok.com/electron.htm
where he has developed a number of Solar Trackers on his enormous website. It will take you an hour to go through the whole site, so here is a starting point:
http://www.redrok.com/electron.htm#led555

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HULDA CLARK ZAPPER
This is the circuit for Dr. Hulda Clark's Zapper, designed in 2003. The frequency is approximately 30kHz positive offset square wave. It has a red LED light that lights up when the unit is on. Perfect for regular zapping, extended zapping and other Hulda Clark related experiments.
This device is used to cure, treat and prevent any disease. It will cure anything. Simply hold the two probes (one in each hand) for 5-10 minutes then rest for 20 minutes, then repeat two more times. Do this each day and you will be cured. Here is the .pdf of her book: A Cure For All Diseases.   Website: http://clarktestimonials.com/ Hundreds of people have been cured of everything from herpes to AIDS.
On the other side of the coin is the claim that Dr Hulda Clark is a complete quack. Here is a website called: Quackwatch. The second diagram shows the two copper tubes and the circuit in a plastic box. I am still at a loss to see how any energy can transfer from this quack machine, through the skin (50k skin resistance and 9v supply) and zap a bug in your intestine. It's a bit like saying I will kill all the mice in a haystack by stabbing the stack with a needle.

From:   junaid.walker@gmail.comHulda
Hulda Clarke was persued by the FDA for false medical claims, and she escaped over the border into Mexico for a while. In her 70's she died of cancer, so obviously wasn't cured by her theories. In later life personal friends said she did admit that her frequencies 'didn't work', instead supporting that cures came about by "Blood electrolysis" (see https://www.electrobiotics.com/Blood-Electrification?chapter=10).
They explain that this can be possible since this stimulates the immune system which in turn is capable of 'zapping bugs'.

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TILT SWITCH
The output is LOW at start-up due to the capacitor on pin 4. When the mercury switch closes, the output goes HIGH and remains HIGH until the reset button is pressed. This circuit is called a LATCH. See Latch circuit and Memory Cell above.

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MOSQUITO REPELLER
This circuit produces a tone above the human audible range and this is supposed to keep the mosquitoes away. You need a piezo diaphragm that will respond to 15kHz and these are very difficult to find.

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DRIVING A RELAY
The 555 will activate a relay. When pins 2 and 6 are connected as an input, the chip requires only about 1uA to activate the output. This is equivalent to a gain of about 200,000,000 (200 million) and represents about 4 stages of amplification via transistors.
In the first circuit, the output will be opposite to the input. The relay can be connected "high" or "low" as show in the second diagram. One point to note: The input must be higher than 2/3V for the output to be low and below 1/3V for the output to be high. This is called HYSTERESIS and prevents any noise on the input creating "relay chatter."

THE DIODE
The diode across the relay is very important. It prevents spikes entering the 555 via pin3 but it also has an effect on the way the chip works.
In an experiment that activated the relay at 5Hz, the frequency changed to 2Hz when the diode was added. Somehow the noise entering the chip had an effect on the frequency.
The diode also improved the output waveform, as you would expect.

NEGATIVE LOGIC
An interesting point to remember.
In the first diagram above, the relay is connected so that it is active when the output is low. This is called NEGATIVE or NEGATIVE LOGIC. It has the same reasoning as -5 - (-5) = 0.
Or in English: "I am not NOT going."
When the input is low in the first diagram, the output is HIGH and the relay is OFF. The circuitry creates two reversals and makes it easy to see that when the input is LOW, the relay is OFF.

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SCHMITT TRIGGER (Hysteresis)
also called AN INVERTER
also called A SCHMITT INVERTER
This circuit is the same as Driving A Relay circuit above. It is the same circuit with a different name.
We have also animated the circuit to show how the output goes high or low according to the input level. The animation shows a wide gap between the input levels when the time when the output goes HIGH or LOW and this gap is called the HYSTERESIS GAP.
This circuit is called a SCHMITT TRIGGER and it is used in many building-blocks (using a different chip - such as 74c14) to prevent false triggering.
It prevents false triggering because as the input rises, the output does not change until the input voltage is fairly high. If the input voltage falls, the output does not change until the input falls about 30%. This means small fluctuations (noise) on the input do not have any effect on the output. The output is the INVERSE of the input - in other words the 555 is a SCHMITT INVERTER. The second diagram shows a Schmitt Trigger building block.

SCHMITT TRIGGER
BUILDING BLOCK

THE 555 AS A SCHMITT TRIGGER
Here is another way to "see" a 555.
The 555 is drawn as a SCHMITT TRIGGER.
The 555 is actually a SCHMITT TRIGGER when pin 2 and 6 are connected together and pin 3 is take to the join of these pins.

Pin 2 detects 33% of rail voltage on the electrolytic to turn the 555 ON and pin 6 detects 66% of rail voltage on the 100u to turn the 555 OFF.
The IC can also drive a LOAD with about 200mA and it can either source or sink the 200mA, so the chip is quite versatile.

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MUSIC BOX
This circuit produces 10 different tones and by selecting suitable values to change the voltage on pin 5, the result can be quite pleasing. Note: the two unused outputs of the 4017 produce a tone equal to that produced by the 555 when pin 5 has no external control voltage.

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REACTION TIMER GAME
This is a game for two players.
Player 1 presses the START button. This resets the 4026 counter chip and starts the 555 oscillator.
The 555 produces 10 pulses per second and these are counted by the 4026 chip and displayed on the 7-Segment display.
The second player is required to press the STOP button. This freezes the display by activating the Clock Inhibit line of the 4026 (pin 2).
Two time-delay circuits are included. The first activates the 555 by charging a 10u electrolytic and at the same time delivering a (high) pulse to the 4026 chip to reset it. The second timer freezes the count on the display (by raising the voltage on pin 2) so it can be read.

Prototype produced by Bruce Pierson
see: Bruce Pierson website for PCB layout

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TRAFFIC LIGHTS
Here's a clever circuit using two 555's to produce a set of traffic lights for a model layout.
The animation shows the lighting sequence and this follows the Australian-standard. The red LED has an equal on-off period and when it is off, the first 555 delivers power to the second 555. This illuminates the Green LED and then the second 555 changes state to turn off the Green LED and turn on the Orange LED for a short period of time before the first 555 changes state to turn off the second 555 and turn on the red LED. A supply voltage of 9v to 12v is needed because the second 555 receives a supply of about 2v less than rail. This circuit also shows how to connect LEDs high and low to a 555 and also turn off the 555 by controlling the supply to pin 8. Connecting the LEDs high and low to pin 3 will not work and since pin 7 is in phase with pin 3, it can be used to advantage in this design.

Here is a further description of how the circuit works:
Both 555's are wired as oscillators in astable mode and will oscillate ALL THE TIME when they are turned ON. But the second 555 is not turned on all the time!
The first 555 turns on and the 100u is not charged. This makes output pin 3 HIGH and the red LED is not illuminated. However the output feeds the second 555 and it turns on.
Output pin 3 of the second 555 turns on the green LED and the second 100u charges to 2/3 rail voltage and causes the 555 to change states. The green LED goes off and the orange LED turns on.
The second 100u starts to discharge, but the first 100u is charging via a 100k and after the orange LED has been on for a short period of time, the first 555 changes state and pin 3 goes LOW.
This turns on the red LED and turns off the second 555.
The first 100u starts to discharge via the 100k and eventually it changes state to start the cycle again.
The secret of the timing is the long cycle-time of the first 555 due to the 100k and the short cycle due to the 47k on the second 555.
One problem is the timing for the first time the circuit is turned ON is different to the continued operation, because the 100u's have to charge from 0v.
The sequence was not perfect with 100u + 100u so Paul Mathew changed the first 100u to 220u and the sequence was perfect.
You can change the 100k to 220k instead of changing the electrolytic.

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4 WAY TRAFFIC LIGHTS
This circuit produces traffic lights for a "4-way" intersection. The seemingly complex wiring to illuminate the lights is shown to be very simple.