Sending square wave from LEDs to Photodiode

Discussion in 'The Projects Forum' started by axton, Jun 26, 2013.

  1. axton

    Thread Starter New Member

    Jun 26, 2013
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    0
    Currently in the middle of a project and need help with some photodiode response time issues. I have two LEDs alternatingly lighting up with the following signal: {LED1 turns on for 250 us, both LEDs are off for 250 us, LED2 turns on for 250 us, both LEDs are off for 250 us}. I have the two LEDs set to this timing (1 kHz) as confirmed by an oscilloscope; however, the photdetector (s1337-33br datasheet --> http://www.alldatasheet.com/datasheet-pdf/pdf/212335/HAMAMATSU/S1337-33BR.html ) is unable to respond quickly enough to differentiate the two signals without overlap. Whenever a pulse is generated and an LED goes off the photodetector slowly falls back to baseline at the same rate it takes the next pulse to be generated no matter the frequency. I need the gap between the two LEDs for background subtraction and can't seem to figure out how to get my signal to the photodetector. Any suggestions would be helpful as I am completely out of ideas at the moment.
     
  2. wayneh

    Expert

    Sep 9, 2010
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    Are we to guess at your schematic?
     
  3. axton

    Thread Starter New Member

    Jun 26, 2013
    2
    0
    My apologies, here is the schematic.
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,024
    3,236
    That is a hookup diagram. Do you have a schematic of the circuits?
     
  5. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    What is your load resistor?
     
  6. tshuck

    Well-Known Member

    Oct 18, 2012
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    I would imagine that a pull-up resistor should be used on the output of the photodiode...
     
  7. DickCappels

    Moderator

    Aug 21, 2008
    2,656
    632
    Actually a shunt resistor is what's needed. The photodiode and the scope probe have capacitance. If you have a high speed opamp, you can run the diode into the summing node of an inverting amplifier an use the amplifier's gain to make the diode its running into a short.

    I would also speed things up to reverse bias the photodiode and capacitively couple it to the summing non on the inverting amplifier because the revers bias reduces the junction capacitance of the diode.

    The circuit below might give you some ideas if you want to go the active rouse, but omit the 22 uf ca[acotpr amd don't use a shielded cable -keep your wires short.

    [​IMG][​IMG]
     
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