Infrared Tx & Rx circuit problem

Discussion in 'General Electronics Chat' started by caseybee, Aug 12, 2010.

  1. caseybee

    Thread Starter New Member

    Aug 12, 2010
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    I have little electronics knowledge and trying to build from examples. I have a pair of side-looking Tx & Rx devices (from Maplin parts CH10 & CH11) and am trying to use them in the attached circuit. I get no output on the LED. I presume resister values need adjusting. Any suggestions what to change and what to measure at different parts of the circuit.
     
  2. wannaBinventor

    Member

    Apr 8, 2010
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    By no output on the LED I assume you are talking about the indicator LED (the green one in the diagram).

    I'm not familiar at all with the part numbers you are giving. Google doesn't seem to be either.

    If that phototransistor has automatic gain control it's probably going to be turned off all the time because it's constantly seeing that signal from the IR transmitter LED. In my limited IR experience, you need to modulate (turn on and off) the signal at a frequency that fits the IR receiver or else it thinks its just all "noisy" ambient light. Despite this, when the phototransistor isn't conducting I would imagine that the voltage at the inverting input would be higher and as such the output would go low, turning on the indicator LED.

    I'd also make sure that your comparator has the ability to source and SINK current. If it can't sink current through its output pin, it won't matter what's happening on the inputs based on how you've got the indicator LED set up.

    Take the inverting input of the comparator and connect it to your positive rail and see if the indicator LED comes on. If it does, then you know your comparator is in fact able to sink current and the problem lies in getting that phototransistor the turn on so the voltage on the non-inverting input will go higher. If it doesn't, make sure everything is connected right (LED anode oriented in correct direction, etc) and then check the datasheet of your comparator.
     
  3. hgmjr

    Moderator

    Jan 28, 2005
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    A couple of quick things to try.

    1. Take a voltmeter and measure the voltage across the IRLED. The voltage should be on the order of 1.5 to 2 volts. If it is equal to your supply voltage then you either have the IRLED installed backwards or the IRLED has failed open-circuit.

    2. Take your cellphone camera or digital camera and point it at the IRLED. You should be able to confirm that it is emitting.

    Side-looking IRLEDs can be tough to align properly. What is the distance of separation between the emitter and the detector?

    hgmjr
     
  4. caseybee

    Thread Starter New Member

    Aug 12, 2010
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    Tested the transmitter with camera and get the little blue dot. So that bit works. My problem seems to be the change in voltage output of the receiver is too small (less than 1v). I think I will have to try and find a different type of Tx/Rx pair. Anyone got any suggestions?
     
  5. wannaBinventor

    Member

    Apr 8, 2010
    179
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    I'd guess that the phototransistor isn't becoming fully saturated due either a weak transmission or improper modulation of the transmitted IR signal. Did you ever find a datasheet?
     
  6. hgmjr

    Moderator

    Jan 28, 2005
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    Caseybee,

    What is the separation distance between your emitter and receiver?

    hgmjr
     
  7. Markd77

    Senior Member

    Sep 7, 2009
    2,803
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    Try replacing the 2 10K resistors with a potentiometer. Anything 20K to 1M should be fine. I agree that there may be other problems, but you should get some kind of result.
    Just had a look at Maplin and the description is:
    I think the 470K resistor can safely reduced to 1K or 2.2K and still not exceed the 20mA of the phototransistor. That should give much better results.
     
    Last edited: Aug 16, 2010
  8. DumboFixer

    Active Member

    Feb 10, 2009
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  9. hgmjr

    Moderator

    Jan 28, 2005
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    I believe that before we can get much further with this topic we are going to need the part numbers for the photo-emitter and the photo-detector.

    hgmjr
     
  10. marshallf3

    Well-Known Member

    Jul 26, 2010
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    True, the IR xmtr/rcvr pair I use has the receiver set to a specific frequency and it really only responds to signals in that narrow bandwidth. It also requires a specific pulse train of X amount of pulses folllowed by Y amount of delay before the pulse train can begin again.
     
  11. Markd77

    Senior Member

    Sep 7, 2009
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    Maplin in their infinite wisdom haven't supplied the manufacturer or part numbers but it is just a dumb phototransistor, no fancy gain control or frequency detection. Specifications are in my post above.
     
  12. marshallf3

    Well-Known Member

    Jul 26, 2010
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    They appear to be designed for rather short range communications as in reading an encoder strip.

    As to "not getting output" you can't see the IR beam however if the approximate normal forward bias current is flowing through the emitter device it's probably working. Radio Shack used to sell little IR detector cards and I'm sure someone still does.

    As to your circuit the 470K resistor is way too far to the extreme, it may not allow sufficient current for the detector to do much of anything. Any reason for this? Seems to me a 10K would be fine there as well.
     
  13. caseybee

    Thread Starter New Member

    Aug 12, 2010
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    I've changed the resitor to 1K and closed the gap to 5mm and now get about 1v difference. No much use to me as I was looking for a gap of at least 50mm (enough for a model train to get through). Never wasted so much time on something costing so little! I will look for some other way of train detection. Thanks for all your help.
     
  14. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Actually I'd keep the circuit and use better components for the emitter and receiver part. Those must intentionally be very low sensitivity for the environment they were designed to be used in.

    I can attest to the fact that these are very sensiive detectors:
    http://www.mouser.com/catalog/catalogUSD/642/180.pdf

    If you need to keep it infrared choose one that resonds in that area then get an emitter in the same range. These might be overkill due to a lot of output but it's easy enough to lower the sensitivity at either end.
    http://www.mouser.com/Search/ProductDetail.aspx?R=TSAL7600virtualkey61370000virtualkey782-TSAL7600
     
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