The LM555 Timer

The 555 timer is a popular circuit used for timing purposes. It can be used in:

• Monostable mode in which the output is a high for a time determined by the external circuit.

• Astable mode in which the output changes continually.

The 555 timer is a small 8-pin integrated circuit, and we can add resistors and capacitors to an external circuit to make it act as a monostable or an astable.

The pin arrangement (pin-out) is shown below:

555 timer in Monostable Mode
The 555 timer in monostable mode is constructed using the circuit shown in this diagram.

555 timer in Astable Mode

The 555 timer can be wired up to produce a train of pulses by ensuring that the circuit is astable, which means that it is not in a stable state. We can make astable circuits from other components, but the 555 timer gives a train of digital pulses. The diagram shows the circuit.