Monostable

Monostable mode is one of three primary modes in which the 555 Timer can be run (with the others being Astable and Bistable modes). When triggered via pin 2, it emits a voltage pulse on pin 3 whose length is defined by the resistor and capacitor.

Operation


Pin 3 (the 555's output pin) will emit a positive pulse when pin 2 (the trigger pin) drops below 1/3 of the supply voltage being provided to pins 4 and 8. This pulse will last for

$$t=RC ln 3 \approx 1.1 RC$$

Simply select the appropriate values for R and C to achieve a pulse length of desired duration. The pulse emitted by pin 3 will be approximately equal to the supply voltage; otherwise, it will be zero (grounded).

Capacitor $$C_2$$ is used to attach pin 5 to ground. Pin 5 is known as the "control" pin; a voltage can be applied to it to control the sensitivity of the timer, but generally, it's tied off to ground with a small capacitor ($$~0.1 \mu F$$) to protect it and prevent it from interfering with expected operation.

If pin 4 (the reset pin) is pulled low to ground, any output pulse will be interrupted, and pin 3 will assume it's default ground state.

Uses
The 555 timer in monostable mode is a great way to condition input, i.e. debouncing a button.

Reference

 * http://en.wikipedia.org/wiki/555_timer_IC#Monostable_mode
 * Make: Electronics