What would we do with electronics, sooner or later we begin to require simple light sensor. You can certainly purchase such sensor on well-known Web stores where there are hundreds of them, but this is not a fun. In addition to spending extra money, delivery takes a time, and we don't want to wait.
LED's are an alternative for light sensors. As most of the semiconductors they generate voltage when lit. More or less, but produce. This property has been used in the switch that turns white LED off when illumination exceeds a certain level. How it works you can see in the picture.
As I said, all LED's produce voltage. From widespread, this is best obtained from Green LED's. If you're wondering, the practical comparison you can look here.
Most LEDs produce voltage in range 0,5v - 1.8V. The only disadvantage - is their high internal resistance which on average from 100 to 1000 kΩ. The latter shortcoming doesn't matter if you use MOSFETs at the input. They do not consume current, and managed only by a voltage.
August 28, 2016 SKootS