In this post, we’ll look at how to build a 555 timer astable circuit. When used in this way, the 555 timer will provide a clock pulse signal output on pin 3 similar to that shown on the oscilloscope in the photo above.

The basic circuit that will pulse the output pin 3 on and off is shown below. We need to select the correct value for resistors R1 and R2, and capacitor C1. To help you workout the component values, there is an astable 555 timer calculator at the bottom of this post. This circuit diagram assumes that you want to flash an LED on and off using the 555 timer. If however, you wish to drive some other device, then you must select the appropriate way to drive your chose device, e.g. via a transistor.

Component values for R1, R2 and C1 need to be calculated to obtain the desired frequency output on pin 3. As a quick cheat to save having to run through the calculations, you can use the two calculators below to calculate the values of R1, R2 and C1 that fulfil your frequency requirement.

Armed with the required component values, you need to build your circuit. A good place to start is on a breadboard. As well as the off-the-shelf 555 timer ICs, you could also try building your own 555 timer out of discrete components. The photo at the top of the page shows the Sloth Byte Discrete 555 Timer kit that is available from this store and Tindie.

Frequency by components calculator

Play about with the calculator below to see what effect the different component values have of the frequency and duty cycle.



Frequency: 0.687 Hz
Time Period: 1.455 s
Duty Cycle: 52.38 %
Time High: 762.3 ms
Time Low: 693 ms


Components by frequency calculator

Frequency: Hz
Duty Cycle: %

Calculated component values.

C R1 R2 F Duty Cycle
2.2μF 1.5Ω 47kΩ 6.978Hz 50.00%
4.7μF 1.5Ω 22kΩ 6.978Hz 50.00%
2.2μF 4.7Ω 47kΩ 6.977Hz 50.00%
2.2μF 10Ω 47kΩ 6.977Hz 50.01%
4.7μF 4.7Ω 22kΩ 6.977Hz 50.01%
22μF 1.5Ω 4.7kΩ 6.977Hz 50.01%
4.7μF 10Ω 22kΩ 6.976Hz 50.01%
47μF 1.5Ω 2.2kΩ 6.975Hz 50.02%
100nF 1.5Ω 1000kΩ 7.215Hz 50.00%
100nF 4.7Ω 1000kΩ 7.215Hz 50.00%