LED As Photosensor ???

We all know that LEDs are wonderful devices which emit light in beautiful colors. What is not known by all is that LEDs (and many other semiconductors) also work the opposite way, that is, they generate electrical energy when they receive light. This energy is small compared to that of dedicated solar cells and is not useful for powering a device or charging a battery but is enough for using the LED as a photosensor. It could be possible to implement a transmitter-receiver using the same device for both functions. This instructable describes a very simple circuit for demonstrating the LED's application as a photosensor.

Here, 3 series connected red LEDs are generating over 30mV under normal room light. The positive (+) pole appears at the anode end, and the negative (-) at the cathode end.

Here I am covering the LEDs with my hand, and voltage drops.

This is a very simple circuit with the only purpose of demonstrating the use of LEDs as sensors. 50K potentiometer biases the LEDs to the threshold voltage of the 2N7000 FET transistor. Adjust for LED off when light is present.

3 LEDs are series connected and generate a very small voltage when receiving light. Positive is at the anode end and negative at the cathode end. Since the anode end voltage is fixed by the potentiometer, when receiving light the cathode end voltage goes negative and cuts-off the FET.

Adjust the threshold voltage with the 50K pot for FET off when light on.

LEDs can be any color.

2N7000 FET drives the output LED