1. The problem statement, all variables and given/known data How does the stereo light in an analog FM radio turn on when you tune into a stereo station? 3. The attempt at a solution I know that radio tuners work on the concept of resonance. Current and voltage levels spike up at and near the resonant frequency of a circuit. My initial assumption would be that there is some kind of switching mechanism that when the voltage or current hits a certain level (at resonance), the switch is flipped and the light is turned on. In newer radios, I'm assuming the switching would be done by a transistor, but what was used in the older analog radios to turn on the stereo light?