Electronic – transmitting 5v signal over a long cable


I'm seeking for help here as I need a reliable answer to this. I need to get an input signal (low frequency 5v digital pulse), to a micro-controller from a (proximity) sensor situated in a distance from the control board.

I'll itemize the important points.

  • Max Tx distance : 50 m
  • Max digital pulse frequency : 10 Hz
  • Voltage range of the sensor : 5 to 30 v (it outputs the same voltage as supplied)
  • Max input to micro-controller : 5 v

For a simple, similar application, this is what I've done before; the sensor is supplied with 12 v. At the other end, the pulse (which now is 0-12v) is fed to the micro-controller through a 7805 regulator. That worked fine, but someone told me that method is not nice and not suited for reliable applications. I also feel that's ugly but i don't expect to mess around a lot with hardware, building separate circuitry etc… Can someone propose any better solution (or agree with mine :D).

I prefer a lot if i don't have to build any circuitry at all. If not possible, at least very simple one!(simple in the sense of hardware complexity. a circuit that don't need a PCB, just two wires here and there. That's why I love 7805 solution). However (unfortunately) the highest priority has to be given for reliability.

Best Answer

A recommended approach would be to use an optocoupler followed by a comparator (eg. LM339), or better, an integrated part such as the Fairchild Semi FODM8071 logic gate output optocoupler.

The reason the optocoupler is recommended:

There is likely to be a ground potential difference over a 50 meter cable, also the possibility of picking up EMI over the long cable. The optocoupler eliminates any ground loop / potential mismatch concerns, as well as any need to precisely match the sensor's supply voltage to the microcontroller's.

The use of the opto will allow a higher voltage to be used for the sensor circuit, reducing EMI noise sensitivity.

An added benefit of the specific Fairchild part suggested above is its high noise immunity. This will result in a more stable signal acquisition, important given the distances involved.

FODM8071 is a 5-pin leaded SMT part, so using it is essentially like not having to build any additional circuit - you could wire the part and its few support discrete components up deadbug style, if you like, or put them together on a proto-board PCB.