An led not only lights when power is applied, but also generates a tiny voltage when light is applied. A PICAXE microcontroller can be used to monitor this voltage. I will show you how to make a electronic candle that will flicker an led when it gets dark.
The candle part of the circuit is a piece of perf board with a green led, 220 ohm resistor, diode,capacitor and a M08 picaxe. I used molex connectors to make the perf board detachable from "power stalk" made from twisted 18 gauge enameled wire. You can use insulated solid wire such as building wire for the stalk. You can also use a yellow or red led. White and blue leds don't work well as light sensors. The diode protects from reverse polarity and allows using alkaline cells by dropping the voltage to the picaxe a bit. Nicad cells work well with picaxes.