This light sensitive circuit can operate a relay to switch on lamps or any AC loads when it senses darkness. It is ideal to use as switch less night lamps driver.
LDR is used as the light sensor. Its resistance is low about 100 ohms in bright light but increases to 10 meg or more in dark. Preset VR1 set the sensitivity of the LDR. During day time LDR conducts so that the gate of T1 will not get base bias. So that relay remains de energized. When the intensity of light reduces, LDR offers more resistance and more current passes to the base of T1 and it conducts. Relay then switch on the load. Adjust VR1 to trigger relay at the particular light level. LED indicates the activation of relay.