Breadboard Workshop 1

These projects use a 555 astable circuit to make LEDs flash in various ways.

Projects: Flashing LED | Speed Control | Level Crossing | Cat Eyes | Alarm Box Light | parts

Also see: Breadboard | Circuit Diagrams | 555 Astable

Next page: Breadboard Workshop 2

Getting started

This first Breadboard Workshop shows several ways to use a 555 Astable circuit to flash LEDs. These projects are great if you are new to electronics and building circuits on breadboard for the first time.

No soldering is required and all the parts can be used again afterwards.

Although no experience is necessary it will be helpful to read the introductory page on breadboards to understand how the connections are arranged, here's a reminder:

Connections on breadboard


Make wire links on the breadboard using standard (1/0.6mm) single core wire (stranded wire is not suitable).

Resistors can be connected either way round but some parts, including LEDs and some capacitors, must be connected the right way round.

Identify resistors using the resistor colour code.

For the 6V Battery use 4xAA alkaline cells in a battery box, ideally with an on/off switch for convenience.

IC pins are numbered like this:

IC pin numbers

Project: Flashing LED

This first project is the example circuit from the breadboard page so you may have built it already.

With the values shown, the LED flashes about once per second but you can easily change this by using a different value for resistor R2.

For faster flashes make R2 a smaller value, for example try 22kΩ (red, red, orange colour code).

For slower flashes make R2 a larger value, for example try 220kΩ (red, red, yellow colour code).

You can choose any value you like for R2 (up to 1MΩ) but R1 must not be less than 1kΩ to avoid damaging the 555 timer IC.

Read the circuit diagram too

As you build each project keep checking the circuit diagram as well as the breadboard layout. That should help you understand the significance of each connection.

Your long term aim should be to build projects directly from a circuit diagram without a breadboard layout to help you.

Breadboard Layout for Flashing LED:

Flashing LED breadboard layout

Circuit Diagram:

Flashing LED circuit diagram

Project: Flashing LED with speed control

Before you start: make sure the battery is switched off or disconnected.

If you are continuing after building the first Flashing LED project the only component to remove is resistor R2 between 555 pins 2 and 7.

Connect the 100kΩ variable resistor 'speed control' using two wire links to pins 6 and 7 of the 555 timer IC.

Switch on or connect the battery, then turn the knob on the variable resistor to change the LED flashing speed.

Variable resistors

A variable resistor allows you to set any resistance within its range (0 to 100kΩ in this case). The small type used here is called a preset. This one has a finger knob to turn but most presets just have a slot for a small screwdriver because they are intended to be set when a circuit is built and then left alone.

Breadboard Layout for Flashing LED with speed control:

Flashing LED with speed control breadboard layout

Circuit Diagram:

Flashing LED with speed control circuit diagram

Project: Level Crossing Lights

Before you start building this project switch off (or disconnect) the battery.

If you are continuing after building the previous project (Flashing LED with speed control) there are no components to remove this time.

Add a second red LED and 330Ω resistor as shown below.

The second LED and its resistor are connected between the positive supply +Vs and the 555 output so this LED lights when the 555 output is 'low' (0V). The original LED lights when the 555 output is 'high' (+Vs) so the two LEDs will light alternately just like the red lights at a level crossing (in the UK).

Adjust the variable resistor speed control so the flash rate looks realistic (each LED on for ½ second).

Sinking and sourcing current

This project shows how the 555 timer's output (pin 3) can both sink and source current. Sinking current is when it flows into the output (LED and resistor between +Vs and output). Sourcing current is when it flows out of the output (LED and resistor between the output and 0V).

For more information see ICs.

Breadboard Layout for Level Crossing Lights:

Level Crossing Lights breadboard layout

Circuit Diagram:

Level Crossing Lights circuit diagram

Project: Cat Eyes

Remember to switch off (or disconnect) the battery before you start.

If you are continuing after building the previous project (Level Crossing Lights):

Add 2 green LEDs as shown and move one of the 330Ω resistors so each LED is connected. Finally, adjust the speed control so the green 'eyes' appear to blink: most of the time they should be lit but briefly going off to blink.

Unequal on/off times

This project shows how the 555 timer can be used to produce unequal on and off times for the LEDs. The LEDs need to be on most of the time with brief off periods for the 'blink', this is achieved when R2 (determined by variable resistor) is smaller than R1 (47kΩ).

Read more: Mark/Space times

Breadboard Layout for Cat Eyes:

Cat Eyes breadboard layout

Circuit Diagram:

Cat Eyes circuit diagram

Project: Alarm Box Light

Remember to switch off (or disconnect) the battery before you start.

If you are continuing after building the previous Cat Eyes project:

The LED should briefly flash every 5 seconds, like an indicator light showing that an alarm system is active.

Brief flashes

For this project the LED is mostly off and just flashes briefly when the 555 timer output (pin 3) is 'low' (0V). If the LED was connected between the output and 0V it would be on most of the time and briefly turn off, like the cat eyes of the previous project. Read more: Mark/Space times

Breadboard Layout for Alarm Box Light:

Alarm Box Light breadboard layout

Circuit Diagram:

Alarm Box Light circuit diagram

Build a similar project by soldering: Dummy Alarm on stripboard

Parts Required for Breadboard Workshop 1

If you wish to use a 9V PP3 battery:

Change the 330Ω resistors for the LEDs to 470Ω. You will also need a battery clip.

Parts from Rapid Electronics

Most parts are supplied in packs but the extras are likely to be useful for other projects and it is good to have spares. Also see advice for building up a starter kit of parts.

Next page: Breadboard Workshop 2