Square wave over DC power line

Discussion in 'General Electronics Chat' started by emoney123, Jun 4, 2012.

  1. emoney123

    Thread Starter New Member

    Jun 4, 2012
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    I have a project that requires a limited number of wires for power and communication. I am looking for a circuit that will carry a square wave (pulse) from a function generator over a DC power line (the pulse is input to a mico that triggers an interrupt). I would like to disturb the power line as little as possible but still be able to detect the individual pulses before they are sent to the micro. Any help would be very appreciated.
    Thanks
     
  2. #12

    Expert

    Nov 30, 2010
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    One definition of a DC power line is that it is clean, filtered, and low impedance. It has the ability to absorb square waves. The first thought process is that you must introduce an impedance into the DC line so the power supply doesn't attenuate the square wave into obscurity. After that, it is about how much resistance or impedance, and how much shall I clean the DC line after I get the pulse out on the other end.
     
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  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    You might want to look into a scheme called "Digital Command Control" which is actually a control method for model trains. It works just as you propose: a small pulse signal on a constant DC.

    I mention it as you may find useful prior art online to use as a springboard.
     
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  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Alternatively you could just modulate the DC voltage regulator, easy enough to do if the regulator is a LM317 etc.
     
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  5. MrChips

    Moderator

    Oct 2, 2009
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    This is possible and has been done before. As others have said, the supply line must have enough pliancy in order to superimpose another signal.

    Carrier current radio is used to broadcast a radio signal throughout a building or campus using the AC lines.

    For a DC supply line you have to loosen up the filtering and modulate the supply line by a few millivolts.
     
  6. WBahn

    Moderator

    Mar 31, 2012
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    Communication via high frequency signals superimposed on power lines is quite common. The X-10 home automation products that have been around for decades do exactly this. There are also high speed powerline models. The power utilities use it for sending command and telemetry data from one station to another.

    The signals are effectively blocked by transformers. So in X-10 you have a "house code" that you set because any house served from the same final stepdown transformer as yours will hear everything you transmit. Since security was not a big concern when the protocol was developed, this also means that if you detect X-10 signals from your neighbor that you can issue commands to their house's X-10 devices. I don't know to what degree they have enhanced the security of the protocols to guard against this. Since the data rate is really low (about 20bps), it is hard to imagine that they have done much.
     
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