IR LEDs and phototransistor questions

Discussion in 'General Electronics Chat' started by JingleJoe, Mar 1, 2013.

  1. JingleJoe

    Thread Starter Member

    Jul 23, 2011
    185
    10
    I was allways under the impression that you could use a reverse biased Infra-Red LED as an IR sensor or phototransistor, but is this true?
    I did some experiments recently which would seem to suggest not, or am I doing something totally wrong?


    Preface:
    I found some old IR LEDs in my box of LEDs and decided to make a receiver/transmitter, however two of the LEDs must be receivers or bare-bones phototransistors.
    They look just like an LED but black (to our puny human eyes anyway) but they seem to only pick up IR light rather than emit it, like the other similar LEDs do.
    I checked the voltage drop accross each component and sure enough the emitters have the standard 1 volt-ish voltage drop and the sensors/phototransistors seem to be open circuit- untill you shine IR on them.
    So do I definately have phototransistors?

    I found you could make them into inverting and non-inverting receivers by swapping thier position with the associated resistor... let me find a diagram... just like this:
    http://forum.allaboutcircuits.com/i...nsworld.comir_ultrasonicbasicirdetectemit.gif
    That's pretty much my circuit exactly.
     
  2. rstevenson

    New Member

    Apr 5, 2011
    21
    1
    I have used IR leds as a Led/phototransistor pair recently. It is possible!
     
  3. JingleJoe

    Thread Starter Member

    Jul 23, 2011
    185
    10
    What was your set up? Do you have a circuit diagram or was it the same as the above linked circuit?
     
  4. rstevenson

    New Member

    Apr 5, 2011
    21
    1
    It was different than above, I believe. Let me remember how i did it and ill draw up a schematic

    edit: Just tried this on my breadboard. It works, I dont know if there are any adverse effects on the led. I found if I put 5v on the "phototransistor" I would get an output voltage greater than 5v so i used 3.3v instead to get around 4.5v
     
    Last edited: Mar 1, 2013
  5. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    The best way to tell which is which is to use a digital camera, preferably a cheap one.

    If you test out the diode on your DMM with the DMM set on Diode test, the Digital Camera preview window will show IR as a bluish light being emitted in the camera preview display, while IR photo-transistors do not emit light no matter the polarity.

    I've seen both the IR LEDs and IR Photo-transistors with the dark lens, and only two leads, the color of the casing alone doesn't always identify which is the diode and which is a photo diode or phototransistor.

    A reverse biased LED does respond slightly if reverse biased and light of the same wavelength strikes it, however, the change is very tiny compared to the change in a photo-transistor output when illuminated by IR.
     
  6. JingleJoe

    Thread Starter Member

    Jul 23, 2011
    185
    10
    Thanks for the info chaps, I've got it working now :) I have no idea what I was doing wrong last night, it works beautifully now- I'm transmitting analogue signals with only some distortion around the bottom (where it goes below the diode voltage and gets chopped off).
    That diagram helped, thanks Mr Stevenson :) I think I simply had too low a resistor on my receiver, however it is still showing a small and visible voltage change with the 100k resistor I was using last night so I don't know what was happening there...
    Also I don't know how you got more than your supply voltage out of that circuit XD does the receiver itself have a photovoltaic effect? like a solar pannel? or have you discovered zero point energy!?!?! hahaha
     
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