The twilight switch WZM-01/SOS is used to control lighting devices or other energy receivers depending on illuminous flux density. The system switches on the lighting at dusk and switches off at dawn. The switching on threshold can be smoothly adjusted by the user. The system is resistant to momentary changes of illuminous flux density, which guarantees reliability. External illuminous flux density sensor transmits the signal to the twilight switch WZM-01. Sensor connecting cable of 50 m maximum lenght enables the sensor to be put in a suitable place.