The astable configuration generates an ongoing stream of pulses of fixed length and frequency. It's called astable since it has no stable state, continually toggling back and forth.
This is very similar to the monostable circuit with two seemingly minor differences:
- There's a second resistor (R2) between discharge and the capacitor/threshold.
- Trigger is connected to threshold, rather than being an external input.
Because of these differences, the operation is quite different.
As before, C1 charges, but this time it's through the combination of R1 and R2. Since the trigger input is the voltage on C1, the 555 triggers when C1 discharges to 1/3Vcc, at which point the flipflop gets set and C1 starts charging again. This means that C1 never discharges to 0v so when it charges isn't not charging from 0v, it's changing from 1/3Vcc. The practical effect of this is that the charge time isn't a full (R1+R2)C. Instead it takes 0.693(R1+R2)C1 seconds. Why 0.693? Math.
When the discharge pin gets grounded in response to the capacitor/threshold voltage reaching 2/3Vcc, C1 discharges through R2. This takes some amount of time determined by R2 and C1. Once again C1 is discharging from 2/3Vcc to 1/3Vcc so the time will be 0.693(R2C1).
Here's an actual scope trace of the threshold (green) and output (yellow) pins.
This circuit is called astable because it doesn't have a stable state that it needs to be bumped out of like the monostable. Rather, it's continuously alternating between charging C1 from 1/3Vcc to 2/3Vcc and discharging it back to 1/3Vcc. The output is correspondingly high while C1 charges and low while it discharges.
- The time the output is high is 0.693(R1+R2)C1.
- The time the output is low 0.693(R2)C1.
- The total cycle time is the sum of these: 0.693C1(R1+2R2).
- This means that the frequency is the reciprocal of that: 1.44/((R1+2R2)C1).
Because the charge time depends on R1+R2 and the discharge depends on R2, the two times can not be equal, so you can't get a 50% duty cycle. This, however is seldom a real issue. In those cases where it is, you can achieve it by doing some tricks using the CMOS version of the 555.