While searching for something else, I stumbled upon a discussion with limited info on using and LED as a light sensor, I had never heard of this before, and had to test this out. Using a super bright flashlight, I was able to get the LED to OUTPUT about 1.9V, and about .6V with an other identical LED (High brighness water clear white LEDs) facing it at close range. And basically 0V when blocked out. I made the following sensor with 2 identical LEDs in it for my testing (The LEDs are built into the horseshoe, and face eachother, and are blocked/unblocked when the wheel mounted on a bolt at the bottom of the pic is spun through the sensor using a cordless drill. The output current of the LED as a sensor is next to nothing however, so it is VERY sensitive compared to using an LDR/Photoresistor.

I got the output to be usable using an lm358 Op-Amp, but is not great. Is there some other way to activate something using the LED's output? Like a common relay? I have no specific use for this project, it is just an exercise on something I found very interesting. Thanks in advance for any ideas!

 

Here's the original article:

http://web.archive.org/web/20101224...eeklyIssues/2004-06-25/backscatter/index.html

I expect that some of Forrest Mims' books since this article include updated experiments on the subject.

 

I did some experiments described here, post #9

 

Here's the original article:

http://web.archive.org/web/20101224...eeklyIssues/2004-06-25/backscatter/index.html

I expect that some of Forrest Mims' books since this article include updated experiments on the subject.

I'll look over that, thanks! I wonder what amount of current is required to run the light/input side of a 4N25? Maybe that is a usable interface?

 

If speed is not the issue, I would suggest a "photodiode amplifier".
Not sure if it works on LED though.

http://www.robotroom.com/ReversedLED.html