LDR
An LDR (light-dependent resistor, or photoresistor) has a resistance that falls as the light on it increases. Community circuits place it in a voltage divider that drives a transistor or relay, giving automatic night lights, street lights that switch on at dusk, light-triggered fans and simple light meters. For example, "Dark Activated LED Light" uses an LDR and a transistor to turn an LED on when the light level drops.
