Measuring a current without ground

Discussion in 'General Electronics Chat' started by yoseph1998, Aug 28, 2016.

  1. yoseph1998

    Thread Starter New Member

    Aug 28, 2016
    I am working on a wireless scoring system for foil fencing. If you do not know what fencing is, here is a video or the wikipedia page.
    In foil the point at the end of the blade closes the circuit when it is not pressed and opens the circuit when it is. Since foil fencing requires you hit in specific spots of the body, when the point is pressed the on the opponent's lame it passes a current to opponent and the machine on his side needs to detect the current.

    Normally a fencing scoring system is connected to a central box where if the current is passed from one opponents blade to the other's lame it can complete a circuit and the central box can detect that. Since I am trying to make a wireless scoring system the issue is how can I measure the current from one machine on a different machine.

    If that didn't make sense, I'll explain it in a different way. I have two batteries and two micro controllers connect to the two batteries separately. One micro controller is outputting current and the second micro controller is trying to read that current, but it can't because the current from the first micro controller is never completing a circuit with it's battery.

    Here is a picture:

    I need to detect current through one wire. Would this work? I don't imagine so, I thought maybe if I used Alternating Current. But I don't know enough about Alternating Current to say it'll work.
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    I guess you need both parties to send transmit data separately and process them on a single score board
  3. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
    Without the ground there won't be any DC current.
    However, each competitor has capacitance to ground and you might be able to use that. If, on each competitor, you measure the capacitance of the lame, then when contact is made, that capacitance will increase and then the microcontroller doing the measuring can transmit that contact has been made.

    I don't know how reliable this would be. You would need to measure how much the capacitance varies whne contact is made and how much it varies as the competitor moves around.
    ErnieM likes this.
  4. BobTPH

    Senior Member

    Jun 5, 2013
    Given that there is a capacitance to ground on both sides, a high enough frequency signal should theoretically be able to go over one wire and the two capacitors to ground. I have no idea whether this would work practically or not, or what the design parameters would be.

  5. Zofz

    New Member

    Aug 29, 2016
    Two ideas:
    1. capacitance sensor - but it can be easy to cheat - any metallic surface would trigger.
    2. transmitter - receiver (also capacitive):
    the best is to have multiple areas of the shield/armor driven by different frequencies, so the virtual ground would be created, as the sum.
    (and you get more info)
    Receiver could be a Schmitt gate, with precisely controlled sensitivity and input resistance. Input is AC-coupled, biased to the center (between threshold voltages)
    Do not forget to make good ESD protection, input and output. The protection should use own voltage reference/clamp - Zener diode, because the ESD is high and uC power is low in this case.
    You need to program some kind of software demodulator, e.g. quadrature, assuming LO will be -1, Hi +1, the best is one for each frequency.
  6. JWHassler

    Active Member

    Sep 25, 2013