Need assistance why trigger Voltage doesn't go 0-Volts

Discussion in 'The Projects Forum' started by nizee, May 22, 2008.

  1. nizee

    Thread Starter Member

    Apr 29, 2008
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    0
    I'm building a video sensor that will sense video output from my TV (composite video output) and then trigger a 5V out when video is sense.


    I've build the attachment circuit and test it out. I get the output to trigger 5V out when video sensor a video from the TV, but when i OFF the TV, i'm getting voltage that jump from 0.3V to 1.6V at the trigger output. That 1.6V could trigger a false output to another device.

    Can anyone let me know what i did wrong on the circuit?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    1,728
    TV's have standby voltages that are on even when the set is off. It may be that your composite video output from your TV has a floating DC level on it.

    If you place a small (say, 470pF to 4.7nF) ceramic capacitor in series between R3 and the composite video input, that will block the DC level, but let the effects of the AC signal pass.

    You might also need a diode in series so that you just get the positive peaks of the signal.
     
  3. nizee

    Thread Starter Member

    Apr 29, 2008
    12
    0
    Thanks for the reply.

    I have 4.7nF ceramic (but measure with meter is 5.4nF - is it fine?) and a few different type of diode (1N4001, 1N4002, 1N4148, ). Which should i use for this application? The diode will be inbetween the ceramic cap and composite input, correct?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Sure, give that ceramic a try.

    The 1N4148 might work OK. The 1N400x series are too slow for RF signals. Try it with just the cap first.
     
  5. nizee

    Thread Starter Member

    Apr 29, 2008
    12
    0
    Thank you Sir. Appreciate the advice.

    I'll try without the diode first this weekend and see how it goes.
     
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