Abstract: Two transistors are arranged as a Darlington amplifier to produce a constant current through an output resistor coupled to the emitter of the second transistor. AC power is supplied to a full wave bridge which converts the AC power from the source to a DC signal. A first zener diode and the base to emitter voltage drops of the transistors control the voltage across the output resistor. As the input voltage to the bridge is increased above the voltage between the emitter of the second transistor and ground, the extra voltage will be dropped across the collector to emitter of the second transistor. The same current will flow through the output resistor regardless of any input voltage in a defined range. By adding a resistor and diode and detecting the voltage between the resistor and diode with respect to the circuit the zero crossings can be detected.