A simple night light. Replace the potentiometer with a photoresistor, and adjust the resistors at the comparator for the desired trigger voltage. Aka, the bightness level that the device should turn on, and off.
Revised: 27k resistors. Pot simulates a photoresistor with with an approximate minimum resistance of 7k and an approximate maximum of 45k. Transistor is a 2N3904 and the opamp is a LT1006.
