Notes

Three resistors, one DC voltage, one LDR (light dependent resistor), one three terminal Op-Amp, and one LED (light emitting diode) make up this circuit. A light detector, often known as a light sensor, is a device or circuit that senses how much light it receives. When it falls on the LDR, the light remains off, and when it stops falling on the LDR, the LED turns on. The amount of resistance reduced from the LDR must be at least 50% or higher for the LED to light up in this circuit; any number below this will result in the LED not lighting up. In the picture it does light up because the resistor is set to 70%, this allows the LED to get enough voltage to light up as the LDR produces less resistance.
Light detector circuits are mainly used in a variety of applications, such as automated appliance start-up when light is detected, security systems, and so on. This circuit may be used in an alarm system to activate an alarm when the intensity of light increases. It is also suitable for use in cupboards and closets. The light will switch on automatically when the door is opened. As the LDR detects less and less light, the resistance drops, lowering the voltage drop, which continues to rise until light, in this case an LED, turns on, which always happens when the LDR is over 50%, the point at which the resistor will allow enough power to turn the LED on