The figures below are showing two different light sensor circuits. The circuit shown in figure 1 is a very simple light sensor circuit that will activate an LED when the LDR in the circuit receives light. The circuit shown in figure 2 is using a relay at its output and when light falls on the LDR it will activate the relay
switch and hence any AC and DC appliance connected with the relay will become activated.
Working of the circuit is simple, when the photoresistor or LDR receives light it will switch on the transistor 2N2222, when transistor become ON it will activate the relay switch and any equipment connected with the relay will become activated. The relay value should be used according to the operating voltage. It is also important to always check that how many amperes your relay is able to handle and how much ampere your appliance will use. Let say if your appliance working ampere is 8 and you are using 10 ampere relay then it will run smoothly but if your appliance current consumption is upto 10A then you have to use higher current handling relay switch. The ampere of relay and operating voltage is always written on the relay.