So I have some flow sensors, which have a 3 wire interface.

The three wires are 5V, signal, and ground. Signal is pulse output that varies between 5V and ground at a maximum frequency of about 200 Hz.

The wires are a bit short (30cm) and I would like to extend them to 1 to 3 meters.

I've bought some really nice 4 core cabling, with 2 twisted pairs, shielding, and center wire strands.

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I'd like to know if there is anything I need to consider when going from the 3 wire signal interface to the 4 wire twisted pairs.

My plan is to make one wire from each pair ground (both the blue and green wires), the one wire from the first pair (red wire) 5V and the other wire from the other pair (yellow wire) the signal. So that each pair has a ground wire, and when the signal is activated the ground return current can follow the path of least resistance.

Is there anything I need to consider with the shielding and center strands? What would be the best way to connect them, considering that the previous interface had no shielding at all.

Can I connect the shielding directly to PCB ground, or should I connect it via a resistor (~10k), or via a ferrite bead?


  • 2
    \$\begingroup\$ Yes, you can do it as you have described (or in any other combinations). The signal frequency is relatively low, so you shouldn't have much problems with it. You can connect shielding directly to PCB ground. \$\endgroup\$ – cyclone125 Mar 12 '19 at 14:27

Since you already have return paths for both signal (green) and power (blue) inside the cable, you want to make sure that the shield does NOT carry any current. This would only unbalance your balanced pairs. Connect the shield to the circuit at only one end (the end that has the more solid earth connection), using it strictly as an electrostatic (E-field) shield. At the other end, the shield can connect to the enclosure, but there should be no connection between the enclosure and the circuit at that end.

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