I made quite a number of electronic circuits and also modifications to some existing ones.
I modified a small wireless microphone and made the transmitter transmitted signal that can be received from as far as 3 to 4 kilometres. The original power could only reach 500 metres.
During my Honours project, I made a voltage squaring circuit with op-amp and transistors.
Most recently, I made a little circuit that descrambled the signal of Cable Vision. I have dismantled the circuit as I think it is illegal. That was fun anyway to know how to hack an analog scrambling system.