Monostable mode: in this mode, the 555 functions as a "one-shot".
Applications include timers, missing pulse detection, bouncefree
switches, touch switches, frequency divider, capacitance measurement,
pulse-width modulation (PWM) etc

A Monostable Circuit
produces one
pulse of a set length in response to a trigger input such as a push
button. The output of the circuit stays in the low state until there is
a trigger input, hence the name "monostable" meaning "one stable state".
his type of circuit is ideal for use in a "push to operate" system for a
model displayed at exhibitions. A visitor can push a button to start a
model's mechanism moving, and the mechanism will automatically switch
off after a set time.

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.

555 timer equation


Where, t is in seconds, R is in Ω’s and C in Farads.