light detection circuit

Discussion in 'The Projects Forum' started by Hyperian, Aug 14, 2008.

  1. Hyperian

    Thread Starter Member

    Apr 25, 2008
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    I'm using a photodiode in parallel with a resistor in series with a LED to detect when light hits the photodiode.
    The photodiode goes from 7kohms (with light) to 5Mohm (without light) and I'm using a resistor of 1.1Mohm.

    I am planning on replacing the resistor (since it's big) with a transistor. If i remember my classes right, i should use a NMOS. any advice on the setup?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    The bolded portion really doesn't make sense. A schematic would be invaluable here.
    That sounds reasonable.
    Why? You can get 1M resistors in 1/10 W, and even smaller if need be.

    Let's start with a schematic on what you have now. If the physical size of your existing resistor is a problem, that is very easily solved. Adding a variable like a transistor would mean that you'd have to add more resistors to control the transistor - so your project would escalate in cost and complexity while plummeting in reliability.
     
  3. Hyperian

    Thread Starter Member

    Apr 25, 2008
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    I was trying to avoid having to draw a schematic cause i would have to do it with paint but here it is
    [​IMG]
    Well i can get my hands on a NMOS easier than a specific type of resistor that my school might not provide.

    I was thinking that the gate of the NMOS would be connected to a lower value resistor which connects to the power supply to get a specific voltage on the gate to allow for the NMOS to act as a resistor. (i hope i'm right it's been a while)
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    OK, I see the problem.

    You need the photodiode and resistor in series to create a voltage divider network.
    If the photodiode is on the + side, when the photodiode is in a low resistance state (light), the output of the divider will be high; when the photodiode is in a high resistance state (dark), the output of the divider will be low. However, the amount of current output available will be very small; in bright light it will be roughly 5V/7kOhms, or about 0.7mA. This would be enough to bias on an NPN-type transistor, which could then sink the current from your LED; which would need a current-limiting resistor.

    See the attached.
     
  5. Hyperian

    Thread Starter Member

    Apr 25, 2008
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    interesting, but i'm trying to replace the 1.1Mohm resistor with a transistor biased in the linear region so it will have a resistance of around 1Mohm.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    It's easier to get your hands on resistors than it is to get your hands on transistors.

    Do you have a particular transistor in mind? Without knowing what the actual specifications of the transistor are, it will be exceedingly difficult to recommend a way to bias it.

    Meanwhile, it will require a pair of resistors to properly bias the transistor.

    Your call.
     
  7. Hyperian

    Thread Starter Member

    Apr 25, 2008
    13
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    mm i thought with BJTs you need 2 but with MOS you need 1? anyway i was thinking of something like this
    [​IMG]
    i think i saw something like this before but i cant simulate it in any of the circuit programs since i cant find the components i need, what circuit simulating program do you use?
     
  8. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    I have no idea what's going on here. Do you want the led to illuminate when light falls on the photodiode? What's the objective?
     
  9. Hyperian

    Thread Starter Member

    Apr 25, 2008
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    I want the LED to light up when there's light hitting the photodiode.
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    The photo-diode should be reverse-biased. It leaks a very small current when it is in light. It needs a very strong light to develop enough current for the transistor to light the LED.

    I corrected the circuit.
     
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