Been reading the manual on this. There are two light dependant transistors and a light inside the meter , the transistors are in series with the 120V AC relay coils in the control unit ( don't understand this because transistors need DC to operate ? Might be a bridge rectifier in there ) ) . The blue and red pointers allow light onto the transistors and switch them on or off as the case may be. The coil contacts will handle 5 Amps so can be used to switch other devices on/off
I am not sure about the 50 cycle side of things . There are two wire terminals on the back of the meter , I think this is where the 50 cycle is sampled . But how much 50 cycle Voltage is needed to deflect the pointer ? Mike
The sticker on the meter says:
RTG 400 - 0 - 350 MV
Term. RES, 1500 OHM