The 555 timer chip is an 8-pin device that can be operated either as a very accurate Monostable, Bistable or Astable Multivibrator: Analogue & Digital Electronics Report, SUTD, Singapore

• 555 Timer IC

The 555 timer chip is an 8-pin device that can be operated either as a very accurate Monostable, Bistable or Astable Multivibrator to produce a variety of applications such as one-shot or delay timers, pulse generation, LED and lamp flashers, alarms and tone generation, logic clocks, frequency division, power supplies, and converters, etc, a fact any circuit that requires some form of time control.

• Monostable 555 Timer

The Monostable 555 Timer circuit triggers on a negative-going pulse applied to pin 2 and this trigger pulse must be much shorter than the output pulse width allowing time for the timing capacitor to charge and then discharge fully. Once triggered, the 555 Monostable will remain in this “HIGH” unstable output state until the time period set up by the R1 x C1 network has elapsed. The amount of time that the output voltage remains “HIGH” or at a logic “1” level, is given by the following time constant equation.
T = 1.1 * R * C