This circuit is simple form but can be useful various such as The alarm circuit, an more electronic control circuits such as ON-OFF electrical equipment, the flash light control circuit or etc. following with do you want.
When has a light on the photocell (LDR1) The resistance will reduce. The bias voltage is supplied to the transistor up, just enough to make transistors and relays it to work.
The VR1 is a base circuit of the transistor that controls the sensitivity of the circuit.
If the offer is less so. The sensitivity of the circuit is low, It provides very valuable. The high sensitivity.
To operate the device in the PCB properly. In Figure 1 soldered properly. Try entering a power source in the circuit. then adjusted the VR1 to sensitivity by do you want.
You can buy parts in this projects below.
DC 9V 9 V 5PIN 250VAC 28VDC Mini Power Relay SPDT