Astable

Astable mode is one of three primary modes in which the 555 Timer can be run (with the others being Monostable and Bistable modes). The timer acts as an oscillator, emitting a voltage pulse on pin 3 whose length and frequency are determined by the two resistors and capacitor wired to it.

Operation


Pin 3 (the 555's output pin) will emit a positive, square pulse slightly less than input voltage with frequency:

$$f=\frac{1}{C_1 (R_1+R_2) ln(2)}$$

The length of the output "on" cycle (high voltage) is $$t= ln(2)C_1 R_1$$ while the length of the output "off" cycle (low voltage) is $$t= ln(2)C_1 R_2$$

This allows for setting of the on and off times independently. In the traditional astable wiring configuration, the diode is omitted, and the output "on" time is equal to $$t= ln(2)C_1(R_1+R_2)$$, meaning it will always be longer than the "off" duration.

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.

Uses
The 555 timer in astable mode is useful whenever oscillatory behavior is desirable.

Reference

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