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Difference between reverse bias and forward bias for a photodiode

  1. Mar 11, 2017 #1
    I'm creating a project where the motors have to be triggered by a photodiode. A light provides a steady stream of current to the photodiode and when that incident light is scattered, the motors move. But in reverse bias a photodiode needs to have a dark environment to provide 'infinite resistance' and only allows current to pass if light is shone upon it, opposite of what I need accomplished. I don't have the schematics of the apparatus yet. Does forward bias of a phototdiode perform the task of a reverse bias photiodiode oppositely?
    Last edited by a moderator: Mar 12, 2017
  2. jcsd
  3. Mar 11, 2017 #2


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    Staff: Mentor

    Since this is more of a concept question than a homework problem, I'm moving the thread to the Electrical Engineering technical forum where the details of how a photodiode operates can be discussed.
  4. Mar 12, 2017 #3


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    Gold Member

    Photo diodes pass very small currents and you might be better using a Light Dependent Resistor.
  5. Mar 12, 2017 #4
    Would an infrared LED work (for the corresponding photodiode)?
  6. Mar 12, 2017 #5


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    Photo diodes pass currents of microamps, so that an amplifier will be needed to drive the motor. On the other hand, an LDR might just drive the motor directly, or with very little gain. I think LEDs are OK when used as photo diodes.
  7. Mar 12, 2017 #6
    Could you assist me with the code (ardunio)?
  8. Mar 12, 2017 #7


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    Staff: Mentor

    This is not really correct. If you read more about photodiodes (like on wikipedia), you should get a better idea of how they work.

    Basically, you usually use a photodiode in reverse bias. When there is no light incident on the photodiode, very little current flows in the reverse direction (just a leakage or "dark" current). When light illuminates the reverse-biased photodiode, a reverse "photocurrent" flows, and this can be amplified to provide the signal that you want to use in your light detection circuit. Something like this:

  9. Mar 12, 2017 #8
    How can I use an LDR to trigger the servo? If the LDR detects a decrease of light how can I use that to make the servo perform its task?
  10. Mar 13, 2017 #9


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    What do you mean by scattered? Do you mean blocked, bright or dim?

    If you need to turn on motor current when it gets dark, and reverse motor current when it gets light then you can change the motor switching circuit or software to “invert the signal polarity” of the output. It has nothing to do with the bias of the photodiode that is used to detect the light.

    A diode is forward biassed when current flows through the diode normally, with a voltage drop of about 0.6 volts for a silicon diode. That is also called photo-voltaic mode and is used for solar power panels.
    A reverse biassed diode does not conduct normally, except it has a very small leakage current. But if light shines on the reverse biassed diode junction the light causes a higher current to flow. That is also called photo-conductive mode and is used for optical data communications.

    Before we can give you specific advice on a simple reliable circuit solution, we need a better idea of what changes the light levels and how big the motors are. Do your motors need to gradually open and close something to maintain the light level, or are you making something that must operate a door based on some light level.

    Your mention of programming an Arduino suggests you will have some programmable logic between the light sensor and the motor. That makes the bias of the photodiode signal quite independent of the motor power.
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