Led as light sensor [Arduino]
If you connect led in the opposit way he can act as light sensor:
positive leg to analog pin
negative leg to 5V
You can use that for some things, but he not sensetive as LDR,
ןf you use a for example a green led he not "see" well red light
If you use charlieplexing method you can control one led to light and as a light sensor.
The idea is like this:
Two wires connect the led to two analog pins.
To light the led you set the two analog pins as digital output and one high one low.
[original story: Let's Make Robots]