NE555 Light sensitive Switch

The use of the 555 timer 1C with an LDR provides a high performance light switch. An LDR is a Light Dependent Resistor and is a very low cost way of detecting light. The 555 is used with its trigger and thresholds tied together to provide a Schmitt trigger with a very low input current but which can drive a relay taking up to

200mA of current. The trigger is activated when the light level on the LDR falls below a predetermined level.

The 555 can supply current up to 200mA, so the relay type is not critical. Any with a coil resistance from 100-280 ohms would be suitable.