The output frequency bursts are conveyed into the mains supply cables. The receiver section counts the bursts coming from mains supply and shows the counting on three 7-segment LED displays. Q1 is the driver of the power output transistor Q2, coupled to the mains supply by L1 and C7,C8. Pulses are filtered by C5, squared by IC1B, divided by 10 in IC2B and sent for the final count at the clock input of IC5. R16 is connected to the Dot anode of D7 to permanently light the decimal point.
Linearity is very good.
Avoid touching the circuits when plugged and enclose them in plastic boxes.