How do you use LEDs as light detectors?

[William At MyBlueRoom]

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?

 

[Dr.EM]

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.

 

[Russlk]

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.

 

[Nigel Goodwin]

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.

 

[William At MyBlueRoom]

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

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

 

[Roff]

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

 

[akg]

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 ..

 

[justDIY]

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/