[Doug] pulls a signal from the 9V AC power before it hits the bridge rectifier, using a 100K resistor and a zener ... the C code pertaining to this timing circuit. It’s written for PIC but ...

If it’s DC, the easiest way is to use a shunt resistor and measure the voltage across it, and for AC you could use a current ... an amplification circuit, and the ability to be “zero adjusted ...