Inductive coupling in digital inputs

Hi, I’m using 8 inputs of a UNO set to mode INPUT_PULLUP. I need to read contacts that are three meters away. I’d like to use some old telephone cord with four conductors each. I capacitive signal parallel conductorsas planning on using each conductor as an input and run one common ground for all eight inputs. Because the conductors are close and run parallel I’m nervous that current in a closed path will induce current in an open path and I’ll get an erroneous signal. I don’t know what current to expect from the arduino and whether it’s strong enough to create that problem. Opinions anyone?


There is at least one piece of information missing. However, even with that missing information, the math is annoying.

Have you considered running the experiment?

You get inductive coupling when current flows, if you are feeding something into an Arduino input the input impedance is so high that there is bugger all current flowing, which translates into bugger all coupling.

Considering the effect of a pull up resistor there will be some coupling but if all connections have this then it will swamp any induced voltage especially if they are strong pull ups. I would use about 1K.

There will be capacitive coupling of course, without any load resistors the Arduino inputs are high impedance so that would be an issue.

If the bandwidth is low you can simply add some capacitors on the Arduino inputs to suppress unwanted signals (I think this is the case here - 10nF or so would be enough)

You can also debounce the inputs in software, which you probably need to do anyway (you mentioned contacts), and this would also prevent cross-talk glitches being registered. Then you don't care about glitches.

Hi, If you are worried, use 10k pullup instead of the internal pullup, or 4K7, this will decrease the input impedance and help if you have capacitive coupling between wires.

Tom...... :)