The circuit uses a unique bidirectional switch network made from two
n-channel MOSFETs, wired in opposite directions. The DC control voltage needed to
turn on the two FETs is developed by tapping into some power from the 24vac source through
two capacitors. A small Schmitt trigger inverter provides some hysteresis, so once
the lights turn on, the extra illumination does not turn the switch back off. With
the components shown, the circuit can easily handle 10 Amps of current.