555 Timer Oscillator


There is a good set of tutorials and information online about the 555 Timer:

Download a pdf of the circuits below.

Astable mode


Our first example with the 555 creates a timing pulse. The frequency of the pulse can be controlled with the capacitor and resistor values. This is often called an astable multivibrator.

Essentially, the 555 is connected to itself (pins 2 and 6), which makes the circuit constantly re-trigger itself. It is called astable, because it continuosuly switches from one state to another.

It is used to blink lights, create sounds and control other circuits that require a timing clock.

If you want to calculate the exact frequency and duty cycle of the pulse, you can use one of the various online calculators:

Monostable mode (one-shot mode)


The 555 can also be configured in a monostable mode (aka one-shot mode). In this mode you can send a trigger to the trigger pin (with a momentary switch, for example) and the output will go HIGH (turn on) for a specific amount of time. After the time has passed, the output goes LOW (turns off) again.

In this mode the timing is controlled with one resistor and one capacitor (R1 and C1 in the image above).

What can you do with the 555?