The 555 Timer integrated circuit is quite possibly one of the most revolutionary pieces of technology ever seen. Introduced in 1971 by Signetics, it is still in common usage today in an incredible variety of equipment, and just about all major semiconductor manufacturing companies produce a version of the chip.

The popularity and longevity of the 555 is due to the fact that it is simple to use (only 8 pins), cheap to produce and buy and incredibly versatile. It can be used as a "one-shot" monostable timing device, where it will act as a switch causing some other item to be switched on or off for a programmable length of time. It can also be used as an astable multivibrator, whereby it sends out a regular sequence of on-off pulses at a specified frequency. With the use of these two options a great variety of timing and counting functions can be produced.

Pin Specification

       Ground -+ 1   8 +- Vcc
               |       |
      Trigger -+ 2   7 +- Discharge
               |       |
       Output -+ 3   6 +- Threshold
               |       |
        Reset -+ 4   5 +- Control Voltage

In monostable mode, the IC should be connected with a resistor R between pins 6 and 8, a capacitor C between 1 and 7, and pins 6 and 7 should be directly connected. Pins 4 and 5 should be left unconnected. When a trigger pulse is applied to pin 2 the 555 will generate an output pulse on pin 3 of duration approximately equal to:

T = 1.1 x R x C (in seconds)

The lower limit is around 10 microseconds, the upper limit can be considered infinity (in practice bounded only by limits of C and R).

In astable operation the pins should be connected as follows: A resistor R1 between pins 7 and 8, a second resistor R2 between 6 and 7, a capacitor C between 6 and 1, and pin 2 should be directly connected to pin 6. This is probably far easier to understand if you draw the relevant connections on a diagram!

No trigger pulse is required in this operating mode as the IC wlil begin generating pulses as soon as power is applied. The pulse frequency in Hertz is calculated with:

f = 1 / (0.693 x C x (R1 + 2 x R2))