Wiring circuit

 A very useful light activated switch circuit can be made by utilising a single 555 timer with its trigger and threshold inputs connected together.
The schmitt has a very low input current (1.5uA) and can directly drive a relay taking up to 200mA of current. The circuit shows a 555 schmitt being used to energise a relay when the light level on a photoconductive cell falls below a preset value; the relay energises when the voltage on pins 2 and 5 is greater than 2/3Vcc and de-energises when the voltage falls  below I/3Vcc. This gives a hysteresis of 1/3Vcc, The circuit can be used in many other similar applications where  a high input impedance and low out- put impedance are required with the minimum component count.