The 555 Timer integrated circuit
is quite possibly one of the most revolutionary pieces of technology ever seen. Introduced in 1971
, 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.
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))