The circuit is simple, as it uses only one transistor and a relay that can be MC2RC1 or MC2RC2, depending on the supply voltage is 6 or 12 V. With 12 V the circuit will have greater sensitivity, which is set in potentiometer P1. This is a light detector, in which the relay is activated with the incidence of light in the photosensor. The original photosensor is a TIL78, but any equivalent should work satisfactorily, including, as we have experienced, a 2N3055 without the protective helmet. The diode in parallel with the relay serves as protection for the transistor, preventing the high voltage generated in the switching from damaging it. The TlL78 phototransistor has a resistance greater than 1 M in the dark and less than 50 k in lighting with a 100 W incandescent lamp at 3 meters distance. The circuit will also operate satisfactorily if, instead of the phototransistor, an LDR is used. Simply change the 100 k to 1 M potentiometer with a 47k pot.
