There are times when you would like to flash a LED through a thick plastic case without wires. The circuit below performs this feat. It can be powered from a supply ranging from 3 V to 6 V. It draws about 300 µA of average current at 5 V. It uses a low power op Amp as a pulse generator. A negative going pulse from that generator gates a power oscillator on for about 20 ms. That oscillator is connected to a series resonant circuit, which drives a small coil.
Feedback from the coil insures that the oscillator runs at the LC network’s resonant frequency. A similar coil, located some distance away, collects some of the magnetic field produced by the oscillator’s coil and uses it to drive a LED. A separation of about 1/2 inch is about the limit for this circuit.