The 555 Astable Circuit

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Electronic timers are central to school projects. You will find as you develop your circuits that the timer circuit can be adapted to suit many purposes. There are several reliable timers but the 555 timer is the most common. Whether you are putting together an alarm or a circuit to activate a computer, a timer is the common component. The 555 timer IC (integrated circuit) is very stable, relatively cheap and reliable. It may be used as monostable or astable.

Astable means that the 555 can operate repeatedly, it will switch on, then off, then on, then off, continually. The 555 is sometimes called an oscillator.


This is a typical 555 astable circuit that drives an LED. It is known as a LED flasher as the LED flashes on and off. The number of flashes per minute can be altered by turning the variable resistor. Remember the 555 is activated by current at pin two and the output is through pin three. Altering the variable resistor alters the time between ‘pulses’ at pin three. The pulse at pin three switches the transistor which allows the LED to come on. The LED flashes on and off because with this astable circuit the pulses from pin three are repeated until the power is switched off completely.


This 555 circuit is very similar to the one above and it is called a ‘pulse generator’. Circuits like this are often used to produce a pulse or signal that will start a second circuit. This can be seen in a simple alarm. Our alarm consists of two circuits, one is a 555 pulse generator and the other detects the pulse. If the pulse is removed a buzzer on the second circuit sounds. This type of circuit could be useful on a door. When the door is shut the alarm is turned on. The first 555 circuit generates a pulse (positioned on the door frame) and the second circuit detects the pulse and is positioned on the door itself. If the door is opened the connection between the two circuits is broken. The second circuit can not detect a pulse and so the buzzer sounds.