A Discover Circuits visitor
contacted me a while back. He needed a day/night switch which would work over
a 12v to 24v range from both AC and DC sources. He liked the idea of a solid
state switching circuit instead of the usual relay. He needed a switch which
could handle up to 5 Amps of current. I scratched my head a while and came up
with the circuit below.
The circuit uses a bridge
rectifier circuit to convert AC into DC. A 12v zener diode limits the
operating voltage of the circuit to 12v. A tiny Schmitt trigger inverter and a
phototransistor detects the ambient light intensity. If there is sufficient
light, the inverter disables a push-pull oscillator circuit. The oscillator,
with a differential output, drives a charge pump circuit which when activated,
provides sufficient gate-source voltage to a pair of n-channel FETs to turn them on
fully. The transistor switch can handle both AC and DC with a current in
excess of 5 Amps.