Using an LED as Light Detector

This isn't the best way to use an LED as a detector (or a photo diode).

You would do much better to measure the photo current using a transimpedance amplifier.

https://ultimateelectronicsbook.com/op-amp-transimpedance-amplifier/

This will be much less affected by external noise and independent of the input impedance of the meter.

But the real question is - why bother - photo diodes are quite cheap.

(You could buy 700 for £8.61 from Aliexpress = £0.0123 each

https://www.aliexpress.com/item/1005006783218523.html

)

So what's gained by using an LED as a bad photo detector ?

MK

So what's gained by using an LED as a bad photo detector ? 
Well I have already gained something far more valuable then being able to detect light, thanks to your response, I have learned that my method of testing this is questionable,
there is something called a transimpedance amplifier, I will following your link and looking that up in the near future (first the honey do list). 

By the way I also have some photo diodes and now I believe my next step in tinkering with this may be to compare test and compare light detection results of cheap  LED's to cheap photo diodes.  Thanks for the suggestion.

Also, I forgot to mention this in my initial response, but one of the uses Mr. Mimms found for this was he was able to use a fiber optic fiber to transmit the light emited by the led then place an led as an receiver on the other end.  ie: by then modulating the light signal a message could be sent, then turn off the sending led and the receiver can send an response signal back in the same manner.  

I got to thinking about this and looking for articles on LEDs optimised for photo detection, so you might make a low cost plastic fibre bi-drectional.

Then I found this:

https://physicsopenlab.org/2020/02/27/using-led-as-a-single-photon-detector/

Which might suggest a way of doing it - SPADing with a cheap LED might be quite fun - I'll just need to get hold of a single photon pulser !

There are plastic fibres that work with red LEDs.

MK

SPDIF / Toslink audio uses red LEDs over plastic fiber

Had to go look up what SPDIF/Toslink is, but that is interesting. According to what I read by using a fiber optic connection to transmit sound it eliminates Electro Magnetic interference. Now I need to get some fiber optic cable, that sounds like fun to tinker with.

Might be wrong, but don't the audio TOSlink ones usually have drivers integrated, to make them easy to interface to logic. Certainly, the Cliff Electronics ones that I've recently bought to use with an FPGA do.

Something like this one is just a LED (you need to look carefully at the datasheets to make sure, because many in this range of parts also have drivers):

https://uk.farnell.com/broadcom-limited/hfbr-1521z/fiber-optic-transmitter-5mbd/dp/1173128

That's designed to work with 1mm plastic fibre (there's a separate plastic item that clamps around the fibre end to guide it into the tx/rx part). The range is fairly limited (10 to 20 metres, or so, depending on how hard you drive the LED, the bandwidth you're trying for, and what you've got at the receiving end) - that's ulitmately down to the lossiness of the plastic fibre, not inherent to these particular parts.

Plastic fibre communications can work up to 100 m, although as you point out, Toslink systems aren't generally designed to go that far.

Per your suggestion, I created a transimpedance amplifier, using a 741 op amp.  This morning I am (well was, until a need for more coffee struck) experimenting with that circuit and trying to understand it better prior to re-running my experiment with all 5 LED's. I also intend to add a photo diode into the mix and see how that compares.  By the way, thank you for your input.