The fully functional light detector turns an LED on when it detects a lower intensity of light. It can be configured to be highly sensitive and is capable of detecting the slight lowering of the sun's intensity as it is partially covered by passing clouds. A direct application of the circuit would be to turn on a night light as your room light goes out or to keep a porch light on during the night. The 330K Ohm Resistor and LED can be replaced with virtually anything to trigger a signal or an event. For example, a 9 Volts Relay would be used to make an absence-of-light activated switch. To make this circuit operate in reverse (the LED turns on when there is light), simply just exchange the positions of the Variable Resistor and the LDR.