This circuit, which we found in an Argentine publication from the 1970s, uses a large high voltage LDR (some types can operate directly with the mains voltage). In this case, a relay for 110 V. must be used. R2 must be a wire potentiometer of 22k x 5 W and R1 a resistor of 5k6 x 2 W. The diode is 1N4004 and R3 has values between 2k2 and 10k depending on the relay. Relays of lower voltages, such as 48 V, can be used with the appropriate choice of this resistor. The secondary of the transformer is 10 mA or more. For the 110 V power grid, the transformer can be omitted.
