photodiode boost

Discussion in 'The Projects Forum' started by jack mcleod, Apr 12, 2011.

  1. jack mcleod

    Thread Starter Member

    Sep 4, 2007
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    The photodiode (glued over a red indicator LED) outputs 1 micro A & 4.8 mV
    when the indicator LED is activated. A second red LED needs to duplicate the indicator LED's action. A circuit to do this please. Supply is 12V.
    471 op, 2N2222, 2N3819 Fet are handy.
     
  2. retched

    AAC Fanatic!

    Dec 5, 2009
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    I am a bit confused.

    You want a circuit to light 2 red leds from a 12v source? AC or DC?

    And you are going to place a photo diode over one of the LEDs and hope to get 1uA 4.8mv ?

    Is there anything else you want to tell us? please?
     
    jack mcleod likes this.
  3. jack mcleod

    Thread Starter Member

    Sep 4, 2007
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    A wirless transmitter is 30' up a tree when activated an "indicator LED" comes on but I
    cant see the "indicator LED" from the ground which is required. So I glued a photodiode
    over the indicator LED to monitor the indicator LED's output. The photodiode has two wires running to the base of the tree. The output on the wires is 1 micro A & 4.8 mV when the indicator LED is on.
    There is not enough current or voltage to have the second LED at the base of the tree
    follow the indicator's (up the tree) activity. What circuitry at the base of the tree would allow the second LED (red) to activate & deactivate? At the base of the tree there is 12V DC to power the needed circuitry. Sorry about the lack of explanation hope this helps.
     
  4. retched

    AAC Fanatic!

    Dec 5, 2009
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    Why not just remove the existing LED, and replace it with 2 wires that run to the base of the tree, then re-connect the LED to the wires at the bottom?

    That or you are going to need an opamp and additional power supplies to boost the signal.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    LEDs have to be connected so that current flows through them the proper way. If you connect them backwards, they won't light up.
     
  6. jack mcleod

    Thread Starter Member

    Sep 4, 2007
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    0
    The circuit's output needs to drive one red LED. The max. signal input is 4.8 mV &
    1 micro A, dc. How do I boost the input enough to turn the LED on with the given input
    and have it go off when there is no input?
     
  7. jack mcleod

    Thread Starter Member

    Sep 4, 2007
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    The opamp circuit is what I'm needing - power for it is not a problem.
    The transmitter's LED is required as is I cant just shift the indicator LED.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Jack,
    Why can't you simply remove the existing LED from the board, insert the two wires, and re-connect the LED at the end of the two wires?

    If you can't do that, what voltages do you have available, and are they regulated?
    A remote opamp circuit probably won't work very well as electrical noise would give false indications.
     
  9. jack mcleod

    Thread Starter Member

    Sep 4, 2007
    21
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    The circuits output needs to drive one red LED. The max. signal input is 4.8 mV & 1 micro A, dc. How do I boost this enough to drive the red LED? Supply
    power is not a problem.
     
  10. retched

    AAC Fanatic!

    Dec 5, 2009
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    Is this a school assignment?
     
  11. jack mcleod

    Thread Starter Member

    Sep 4, 2007
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    As only one red LED plus the circuit is being driven 2-4 1.5 V cells would do the job. (There is 120V at the base of the tree and I could build a regulated
    supply - probably over kill.) The circuit is only connected via the photodiode which forms an optocoupler - little if any noise. The circuit components FET,
    BJT, opamp or IC is not a concern. Putting the circuit together is not an
    issue knowing what to put together is. Both LED's just activate when a
    momentary SW is activated and go off when deactivated. I apologise for
    handling this poorly - hope you'll hang with me.
     
  12. jack mcleod

    Thread Starter Member

    Sep 4, 2007
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    Not a school assignment - age 69. The transmitter is used at a different location for part of the year and its indicator light is required.
     
  13. jack mcleod

    Thread Starter Member

    Sep 4, 2007
    21
    0
    The transmitter is relocated for part of the year and needs to have its
    indicator light. The circuit powers the LED & amplifyer so probably 2-4
    1.5V cells would do the job. The only connection is the photodiode, basically
    an optocoupler - little noise. The component's used and there arrangemnt is the mystery to me. Purchasing them and putting them together is not.
    PS At 69 school is a distant dream. Sorry I'm not very good at this but I can read a catalogue. FET, BJT, opamp, IC willing to use anything to make this
    happen. Thanks, Jack
     
  14. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    How was this 4.8mV signal measured? Was diode reversed biased with a load resistor and voltage applied? A photo transistor,photo darlington transistor, or light dependent resistor might give a better signal. At base of tree about all that is needed is a comparator, a few resistors, V supply- even batteries, depending how long LED is on, & the LED.
     
  15. retched

    AAC Fanatic!

    Dec 5, 2009
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    I would really consider de-soldering the LED and replacing it with some female headers. then you can easily insert an LED when you need it on the board, and insert the extension cord and LED when you need to see it at the base of the tree.

    Less components to buy, and a pretty simple fix.
     
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