I've got some questions about using LEDs as photodiodes. - Page 1
Forum section: electronics / projects

author: Ben321
author_rank: Frequent Contributor
timestamp: January 09, 2023, 05:41:35 am
content: I've read that you can use LEDs in reverse, so they act like photodiodes. Is this true? If so, what wavelength is its peak sensitivity? Does the wavelength of peak sensitivity when being used as a photodiode correspond to the wavelength of peak emission when being used as an LED?

author: moffy
author_rank: Super Contributor
timestamp: January 09, 2023, 06:25:04 am
content: Quote from: Ben321 on January 09, 2023, 05:41:35 amYes it is true, in fact all silicon devices(I know LEDs are not silicon) are photovoltaic. When transistors were in metal cans some people used to remove the top of the can and turn the transistor into a photo transistor. I don't think there is a peak wavelength as long as it is above the energy bandgap it is one electron for one photon.

author: Berni
author_rank: Super Contributor
timestamp: January 09, 2023, 06:48:47 am
content: Yep LEDs and solar panels are basically the same thing.You can try it out really easily by hooking up a LED to a multimeter set to volts. Shine a bright light into the LED and you will see the voltage climb up to about the diodes forward voltage. In the same way you can make a solar panel light up like a LED by pushing current trough it (Tho you need a camera to see it since it emits IR light)You will have to measure the exact wavelength response yourself, but it is certainly not only sensitive to the wavelength the LED emits when lit up. Most wavelengths above that should produce a current since the photon just has to have enough energy to get that electron over the bandgap (and this bandgap is what determines what color the LED emits). Otherwise the photon just gets turned into heat.This is the reason why solar panels are actually IR LEDs. They want to have a low bandgap down in the IR range so that they can capture as much of the suns spectrum as possible.

author: james_s
author_rank: Super Contributor
timestamp: January 09, 2023, 07:33:45 am
content: A simple google search comes up with a plethora of information on this topic. Using LEDs as photodiodes is a very old trick, the LED chip is sensitive to wavelengths equal to or shorter than that which it emits. Many LEDs have colored encapsulation which further narrows down the wavelengths it is sensitive to.

author: Ben321
author_rank: Frequent Contributor
timestamp: January 09, 2023, 09:17:38 pm
content: Quote from: Berni on January 09, 2023, 06:48:47 amSo you have been mentioning its operation in the photovoltaic mode. Do LEDs also behave like photo-diodes in the photo-conductive mode (apply a small voltage across the LED in the reverse-polarity direction, and then measure the current through it when light shines on it)?

author: james_s
author_rank: Super Contributor
timestamp: January 10, 2023, 12:32:28 am
content: Yes LEDs are photodiodes, both photoconductive mode and photovoltaic mode.

author: bidrohini
author_rank: Regular Contributor
timestamp: January 10, 2023, 03:25:25 am
content: Yeah. You can find this small adafruit video interesting. It's about using LEDs as photodiodeshttps://youtu.be/ZRzpSmJ5vfk

author: Berni
author_rank: Super Contributor
timestamp: January 10, 2023, 06:19:09 am
content: Quote from: Ben321 on January 09, 2023, 09:17:38 pmBoth of those are the result of the same effect.Just in photo-conductive mode you fix the voltage across the diode and sense the current flowing (they produce a current even at 0V bias, just that more bias gives you more sensitivity) while in photovoltaic mode you present a high impedance so that the current is pushing causes the voltage to rise instead. The issue with using the voltage mode is that once the voltage reaches the bandgap voltage it stops making current, so this puts a limit on the usable dynamic range. But it is the easiest one to demonstrate since pretty much any multimeter on volts put directly onto the diode terminals will make a voltage.You can also run it in current mode by placing a LED across a multimeter in uA modeAs an interesting use of this effect, here is someone who made a "touchscreen" out of a LED matrix display. It uses one row of LEDs to iluminate the finger while the next row is working in photodiode mode to detect the reflected light from the finger:https://cs.nyu.edu/~jhan/ledtouch/index.html

author: Doctorandus_P
author_rank: Super Contributor
timestamp: January 10, 2023, 11:23:16 am
content: I once tested and verified this and it does indeed work.Then I wanted to give a demonstration to some friends and it did not work...It turns out that some brands / types of LED's do this much better then other brands.So just take a led, measure the voltage with a DMM and shine a bright light into it.This can also have very undesirable side effects.I once read a story about a very low power design with a uC and a few LED's, and to save a few uA's the LED pins were switched to "input" when the LED's were off. This worked nicely in the lab, but in the field some units had a much to high current consumption and drained the battery. The cause was that the voltage generated by the LED was enough to get an input pin in it's "linear" region where both the top and bottom transistors conduct current.