Comment by tinco

A transistor effectively is an AND gate. If there is a charge on both the source and the gate, then charge can move to drain. So if you charge up the capacitor and you connect it to the source of another transistor then you can check whether it is still charged by putting a charge to the gate and see if there is charge on the drain.

And you are right, that charge on the drain can then be used both to drive some logic and to activate the recharging of the capacitor that was just discharged.

By the way I am being handwavy about "charge" moving about, if you really want to learn the electronics it is more correct to call it a voltage relative to some ground that the charge always moves towards.