Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

LED indicator for composite video signal

  1. Dec 7, 2007 #1
    Sorry.. I know this is really simple..

    I have 3 composite video signals, and switches to select between them.

    The switches I have are SPCO, so I can't just use another pole for the switch voltage.

    So, I need a way of detecting the video, and this switching on and LED.

    At first I thought composite video was AC, so just made a transistor amp, decoupled the input.. but didn't work. Looking at the composite video on the scope I can see it is actually a DC voltage 0-0.5V... ah.

    So.. I have a circuit where I feed the video in through a 1K resistor into the collector.

    I have the LED and a 500R in series with the emitter (3904 transistor).

    This 'works' in that it lights the LED when a video signal is present, but only weekly (3mA).

    So, I need to increase the input voltage above 0.65 I suppose to bring it into saturation.

    But how do I do that without effecting the video camera ?

    I feel I need some sort of resistive voltage divider, but my attempts at building one alaways lead to the transistor being on all the time.. what am I doing wrong ?

    I know I could add another transitor stage and that would do what I want, but I don't really want to do that :-(
  2. jcsd
  3. Dec 7, 2007 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Composite video is neither AC nor DC; it is a complicated waveform. You should be able to detect the presence of a composite video signal by looking for any voltages above the sync threshold. A simple comparator will do the trick easily.

    - Warren
  4. Dec 7, 2007 #3


    User Avatar

    Staff: Mentor

  5. Dec 7, 2007 #4
    Actually a composite video signal has BOTH AC and DC components. It is complex, but to say that it is neither AC or DC is simply wrong.
  6. Dec 8, 2007 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    That's quibbling a bit, Averagesupernova...

    - Warren
  7. Dec 8, 2007 #6
    Yes it is quibbling. But in my beginning years of electronics if someone told me that something was neither AC or DC I would have been seriously confused. It is easy to understand from a beginners viewpoint that we can have a DC offset that is changing in time. Said signal would be a signal with both AC and DC components. Can you honestly tell me that calling said signal is neither AC or DC makes it easier to understand? There is already enough confusion happening on this forum we don't need any more. I troubleshot and designed NTSC video generators for about 6 years, there is probably not a whole lot about composite video you can tell me that I don't already know. Some composite video may be AC coupled at an output jack so a comparator may or may not work in this case. DC restore circuits clamp the front and back porch at a specific DC offset (or 0) to regain the DC component in video.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: LED indicator for composite video signal
  1. Composite Video Output (Replies: 4)