Not needed for the AQA exams.

This astable circuit oscillates at a fairly stable frequency. A 555 timer would perform better. For ultimate accuracy, a quartz crystal would be used to control the frequency.

330 R = Orange Orange Brown 1 K = Brown Black Red 10 K = Brown Black Orange 15 K = Brown Green Orange

When the left most transistor Q1 turns on, the potential at c1 drops from about five volts to close to zero. This causes an identical 5V potential decrease to happen on the other side of Cap1. This five volt drop causes the potential at b2 to drop to about -4.4V. This turns off Q2 causing the potential at c2 to rise to close to 5V. This further turns on Q1.

When the middle transistor Q2 turns on, the potential at c2 drops from about five volts to close to zero. This causes an identical 5V potential decrease to happen on the other side of Cap2. This five volt drop causes the potential at b1 to drop to about -4.4V. This turns off Q1 causing the potential at c1 to rise to close to 5V. This further turns on Q2.

Neither of these states is stable. Which ever transistor is turned off, it will eventually turn back on due to the current re-charging the capacitor and the base potential rising.

The LED driver circuit makes the astable output visible without significantly upsetting the astable circuit.