Here is a very useful project of an automatic street light or lamp circuit which can be used to activate your outdoor lights like garden lamps, night lights etc. automatically when the sun set and turn off automatically when the sun rises. The circuit is quite sensitive and versatile and can be used
to activate any 220 volt AC device at night and turn off in day all you have to do is simply connect the device with the load point. You can also connect a 220V plug socket with the load point to connect different electronic devices but make sure the relay points can able to handle the provided load. The circuit is simple using two 2N3904 transistors and few other components. The 50K VR (Variable Resistor) is used to adjust the circuit to activate the relay on the required darkness. The circuit can be operated with 6 volts, 9 volts and 12 volts but it is important to use the same voltage relay also.
Working of the circuit
The circuit is actually a dark detecting circuit or we can also say it dark / light sensor circuit. The two transistor used in the circuit are working as switches. When light falls on the LDR its resistance decreases and the voltage at the base of Q1 will increase which will switch on the Q1 and the Q2 will remain switch off. When there is no light on the surface of the LDR its resistor increases, which will switch off the transistor Q1 and the voltage at the base of Q2 increases and hence the transistor Q2 will switch on and activates the relay switch.