THE 555 INTEGRATED CIRCUIT

V. Ryan © 2007 - 2009

This integrated circuit is used for timing. Many circuits are composed of timers and the most common of them all is the 555 Integrated Circuit. It is used in many commercially manufactured items such as video recorders and timers.

You must understand the basic workings of this important IC. The 555 has eight pins (legs) but the function of two are very important. These are pin two and three. This chip is used in timing circuits.

The circuit below is a simplified version of a 555 circuit. It is a timer. When the switch is pressed a current / voltage goes into the IC through pin two (the input pin). The chip starts counting and when it has finished counting it ‘pulses’ a current or voltage from pin three (the output pin). This voltage from pin three switches a transistor and allows the LED to light.

The time from pressing the switch to the LED lighting could be anything from 1 second to twenty minutes.

A 555 will switch on a range of components not only LEDs. For example, it can switch on a relay which then allows a second circuit to work.

The circuit shown opposite is a simple version of a real 555 circuit. The real circuit includes resistors and capacitors. (See following sheets).

 The 555 integrated circuit acts as a counter. When activated by current entering in through pin 2, it starts to count for a certain amount of time. When it has finished counting it emits current from pin 3. This usually activates other components such as LEDs or relays. When the push switch is pressed it allows current to flow into pin 2, starting the timing sequence. The combination of the resistor and capacitor determines the length of the timing sequence. In general terms, if the resistor and capacitor values are high the timing sequence is long. If the values are low the timing sequence is short. The transistor acts as a very sensitive switch. When current enters the base electricity can flow from the collector to the emitter. this allows the LED to light. The LED lights when current flows through it. In this circuit it indicates when pin three is emitting current.