Infrared Detector

Discussion in 'General Electronics Chat' started by campeck, Dec 4, 2009.

  1. campeck

    Thread Starter Active Member

    Sep 5, 2009
    194
    3
    With a lack of the 38khz IR detectors until later this week I decided to implement this circuit that is online here.
    http://www.societyofrobots.com/schematics_infraredemitdet.shtml

    Since my MCU needs 1.4V to detect high I am using a LM324N 4in1 Opamp
    for Two IR eyes.

    With the setup shown below It detects a hand at about 2 feet.
    I messed around with different resistor values before settling on this temporary setup. Can anyone see any obvious improvements?

    A little Youtube action!
    http://www.youtube.com/user/campeck944#p/a/u/0/bNixK6Ftxw0
     
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Your photo-transistor is an emitter-follower with no voltage gain. It is designed to drive a low impedance.
    But the input impedance of an opamp is very high.
    The photo-transistor should be common-emitter that has voltage gain.

    You are using a lousy old LM324 that was designed for low power. Since it is low power then its high frequency response is very poor. It has full output to only 2kHz. Most opamps go up to at least 100kHz.
    Use an opamp that works properly at the frequency you want. An MC34071 single, MC34072 dual or MC34074 quad opamp has full output to 100kHz, work from a single supply as low as 3V and have inputs and an output that go down to ground.

    Your circuit should use a coupling capacitor between the photo-transistor and the opamp so that the opamp does not amplify the widely varying DC from the photo-transistor. Then the input of the opamp needs a bias resistor.
     
  3. campeck

    Thread Starter Active Member

    Sep 5, 2009
    194
    3
    Where would I get a common emitter phototransistor. And are you gathering that it's common emitter from the schematic? Because I just used a transistor symbol modified.

    Well The IR led isn't blinking so is this still a problem?

    Hmm. Are you recommending this for a flashing IR led detector? Where I would use the opamp to allow a certain frequency through?

    If not, could you suggest some values and why?
     
  4. campeck

    Thread Starter Active Member

    Sep 5, 2009
    194
    3
    oh ok. Since I mentioned the 38khz sensors It makes sense to assume that thats what I wanted.

    But no. the IR led's are just on.
    But I am interesting in building my own 38khz filter now.
     
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