I am playing around with the following circuit: Falstad's simulator
What I understand is:
- First, the +5V terminal at the top starts "charging up" the 100F capacitor. While charging, the capacitor "lets electricity flow through", that's why the upper NPN transistor is open.
- The mentioned NPN which is open makes the LED light up.
- In the meanwhile, the 100F gets fully "charged up" and this acts like a gap in the circuit. The upper NPN reaches cut-off state and the LED goes out.
What I am not sure about:
- The 10F (lower) capacitor which should be full after this all opens the lower NPN.
- By opening the lower NPN, the 100F (upper) capacitor should empty its charge.
- After the 10F (lower) capacitor is empty, the lower NPN should be closed (which is not the case) and the whole process should "restart" with 1.
In the simulator, I can see that V(b) of the lower NPN transistor is between V(c) and V(e) (I do not understand why) - and maybe this is what causes 6. to NOT happen and the whole process to "fail".
Do I understand the situation well? Could you help me with some more explanation? Why isn't the process happening as I described in my points?
A redrawn, possibly cleaner variant here