Another common way to use a 555 timer is in astable mode. The term astable simply means that the 555 has no stable state: Just as it gets settled into one state (say, the output at pin 3 high), it switches to the opposite state (output low). Then it switches back to the first state, and so on, ad infinitum.

This mode is also called oscillator mode, because it uses the 555 as an oscillator, which creates a square wave signal.

To understand how this circuit works, first notice that the trigger pin (pin 2) is connected directly to C1. In the monostable circuit, the timer was triggered by a switch that short-circuited the voltage applied to pin 2. In the astable circuit, the timer is triggered when the capacitor discharges — once the voltage across the capacitor drops to one-third of the supply voltage, pin 2 triggers the timer to start another cycle.

Examine how this timing cycle works, step by step, starting with the output at pin 3 in the high condition.

1. With the output high, the discharge pin (pin 7) is open, forcing current through resistors R2 and C1.

This causes the capacitor to charge at a rate that depends on the combined value of R1 and R2 and the value of C1.

2. As the capacitor charges, the voltage at pins 2 and 6 increases.

3. The threshold circuitry within the 555 causes the output voltage at pin 3 to go low.

This happens when the voltage at pin 6 (the threshold pin) reaches two-thirds of the supply voltage, t

4. When the output at pin 3 goes low, the discharge pin (pin 7) is connected to ground within the 555.

This allows C1 to discharge. This discharge occurs through R2, so the value of R2 as well as the value of the capacitor determines the rate at which the capacitor discharges.

5. As the capacitor discharges, the voltage at pins 2 and 6 decreases.

6. The trigger circuitry inside the 555 causes the output at pin 3 to go high.

But not until the voltage at pin 2 (the trigger pin) drops to one third of the supply voltage.

7. When the output at pin 3 goes high, the discharge pin (pin 7) is opened, and the cycle starts over again.