Detecting light with an LED

It is possible to use a led as a light sensor as well as a light emitter. Essentially a reverse biased led will act as a capacititor, if it is then disconnected the charge will drain at a rate roughly proportional to the light hitting it.

We can use this with a microcontroller - utilising the multi state ability of the ports. alt text

The resistor should be about 100 ohms, I have only used this with red leds - it may work with others.

Use the following sequence:

Set Port A output high
set Port B output low   // this makes sure the led is discharged
wait 1mS
set Port A output low
set Port B output high  // reverse bias and charge
wait 1mS
set Port B as input     // Port B is high impedance input
time how long for Port B to read low

The length of time will be dependant on the amount of light hitting the led.

There are several examples of this on the web - I will list them here as I find them again:

LED senses and displays ambient-light intensity

Red LEDs function as light sensors

Multi-Touch Sensing through LED Matrix Displays - very cool video

LEDs As Sensors

Very Low-Cost Sensing and Communication Using Bidirectional LEDs

Here is a good video describing how LEDs can be used as a light sensor Also Forest M.Mims III has wrote about simple projects using LEDs as touch sensors as well.

Also, I have read that SMD LEDs work better as light sensors than through hole LEDs...

Here is a circuit that uses a LED to detect light from an educational kit : LED light detector

LED 0 allows a very small , light-influenced current to pass through because its polarity is reversed.Make sure it is green , yellow or transparent or it may not work (red). The transistors amplify this current just enough to drive a normal polarized LED (LED 1).You can adjust the sensibility by adding more transistors , thus having a greater amplification , responding to a lower illumination level.