How do you use LEDs as light detectors?

I recall seeing LEDs as light detectors and they were driven by a PIC. Can't find the article anymore, does anyone recall where the article can be found?

As far as I know, an ordinary LED can function in the same way as a photodiode. It will only work moderately well however, and a phototransistor or photodode will outperform it. Also, it will still be most sensitive to near infrared light and less so to greens etc.

As an interesting aside, all semiconductors are light sensitive, that is why ICs are in black plastic. I once had a problem with an op amp that was housed in white ceramic; the ceramic was translucent and I had to put black tape on it to keep the light out. Light would change the bias on the op amp.

There was an old germanium transistor called the OC71, and a far more expensive photo-transistor called the OCP71 - if you got an OC71 and scaped the paint off (it was encapsulated in glass painted black), it worked exactly the same as an OCP71.

I found the article & PDF. Interesting stuff... Going to try and use it in a demo project.

https://www.merl.com/publications/TR2003-035/

There is a Design Idea in this folder entitled Stealth-mode LED controls itself.

i once tried to make a phototransistor outof the bc107(metal cap) , i filed off its top part and measured its resistance , ... it worked well ..

i dunno about photodiode or phototransistor properities of leds, but I have experienced photovoltaic properities as well as light-dependent capacitance, which is what the merl article talks about ... reverse charge the junction with 5v, and then sample the cathode ... if the led is dark you'll read a high (or if you use A2D, you'll read around 3 volts) ... if the led is light, you'll read a low (or 0 to about 1.8v)

if you want to experience it with a PIC, use the ST inputs, versus TTL ... the TTL inputs typically take too more voltage to register as "on", so you need a bright light

https://projects.dimension-x.net/technology-and-projects/ledsensors/