inside the led

LED as Light Detector

inside the ledDid you know that the LED can be used as a light detector?
In fact green LEDs are the best for this, but all LEDs will detect light and produce a voltage equal to the characteristic voltage-drop. The current they produce is very small, only the super and high bright one can produce a higher output due to the fact that their crystal is more efficient at converting light into electricity.


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    If you take a 2N3055 metal can and you saw the can’s top open then you have photovoltaic cell The larger the chip area the higher the output

  • Jim Keith

    Wow! Interesting…

  • joribo

    Yes, my OC612 from 1963 had (good luck) no gel, I just scraped off the black paint, is still in use as phototransistor.
    There is a nice instruction in Hagen Jakubaschk’s book how to convert Selenium rectifier plates to photo cells. Have saved four large plates in 1970. Will try asap.


    Make me think of what we did 45 years ago for phototransistors. The OC71 in the beginning had clear gel inside and we scraped the paint off and they were the same as the OCP71 in operation just a LOT cheaper . Then Mullard filled the glass casing with non clear gel. Then we stuck them into a tumble drier with some putty. This made the gel to collect on one side exposing the die Voilla!! A cheaper phototransistor!

  • joribo

    it seems to depend on the manufacturing process of the LED. I tried green LEDs and they produced around 3 V in sunlight. A friend of mine made the same test and got only a few millivolts. My LED’s were from ca 1980 and his LED’s were made around 2000. I sent him my LED’s and he confirmed the difference. Of course, all our LED’s had a clear plastic casing.

  • Jim Keith

    A few days ago, I experimented with a number of LEDs as light sensors. Among those with colored lens, green worked best. However, everything changes with a clear lens. The best by far was an ultrabright RED with a clear lens (HLMP3750A). Data from this device is as follows:

    Touching a 40W fluorescent lamp: 200mV
    1M away from fluorescent lamp: 20mV
    Touching 100W incandescent lamp: 1200mV
    1M away from incandescent lamp: 65mV
    At window on very cloudy day: 10mV
    Bright sunlight: ???

  • Dave

    Just wanted to mention that if you connect them in series they will produce more current and voltage…Work GREAT for tracking the sun with solar trackers.

  • Derrick

    I wonder how well this works in compassion as a photo sensor. I have been using photo sensors in my projects and they work great.

  • raman

    if we try to use that small current, we need extra circuitry which makes the cost higher.

  • unnikrishnan

    more helpfull to me

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