Light-operated switch for controlling artificial lighting system - uses timer chip hysteresis, is simple, cheap and draws low current

摘要

The circuit uses the built-in hysteresis of a commercially available IC, e.g. a TIMER IC 555, to ensure that switching only occurs after major changes in ambient light level. The sensor is a light-dependent resistor which forms part of a resistive divider chain. The IC has internal reference voltages equal to 1/3 and 2/3 of the D.C. supply. - When the ambient level becomes high after a period of darkness, the sensor output voltage exceeds 2/3 of the supply and the IC de-energises the relay. It will not re-energise unless the sensor output falls below 1/3 of the supply. A capacitor protects the circuit against supply transients. Quiescent current is 3mA, and the IC can supply 200mA to the relay.(0/1)