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Unwanted pulses

Discussion in 'Electronic Design' started by Gary, Nov 18, 2003.

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  1. Gary

    Gary Guest

    I am trying to steer the output of an infrared controller to one of eight
    devices. For the steering circuit I've used a 74htc138. Each output is
    connected to a pnp transistor (mpsa56) through a 5k resistor. The
    transistor is used to supply 5V to the ir emitter. All 8 emitters are tied
    together and into an npn transistor (pn2222a) that generates the pulses.

    One of the outputs of the 138 is always selected providing 5v to the
    appropriate emitter. Measuring the connection between the mpsa56 and ir
    emitter I get 5v for the selected emitter and zero for the other 7. However
    on a scope, I can also see the ir pulses at this connection on all of them.
    I've used visible ir emitters and they do not light but the devices, on the
    unselected emitters, can sometimes detect the pulses and react.

    How can I isolate the other seven emitters?

  2. Ejalogic12

    Ejalogic12 Guest

    I suspect that your power source may have poor isolation. Try putting a larger
    value capacitor from ground to V+
  3. Uns Lider

    Uns Lider Guest

    It might help to change the circuit topology a little. What's the frequency
    of your signal and how much current does each IR-emitter use?
    Another thing to consider is the light may be leaking from one IR-emitter
    to places where you don't want it...

    -- uns
  4. Luhan Monat

    Luhan Monat Guest

    Uns Lider wrote:

    Yea, watch that IR. I found it going right through a piece of black
    plastic electricians tape.
  5. Uns Lider

    Uns Lider Guest

    Wow! I found that interesting so I just did a little experiment. I put a
    square of black vinyl electrical tape over the IR LED on a remote control
    and pointed it at a video camera. The IR emissions were clearly visible
    even through the tape. Then I proceeded to turn on my CD player with the
    remote control -- electrical tape still in place! After I added a second
    layer of tape, the IR was no longer visible through the video camera, but
    I was still able to turn the CD player on if I held the remote within
    about an inch of the player's sensor.

    I then repeated the test with some special black tape used for
    light-proofing in darkrooms. With one layer of that tape, the IR was
    still just barely visible through the video camera, and the remote could
    turn on the CD player at a distance of several inches. Two layers of that
    tape prevented the remote from working at any distance.

    -- uns
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