Arduino + RPi, grounding and measuring resistance

Projects General Guidance
1

Hi

I'm using an Arduino connected to (and powered by) a raspberry pi via USB in a circuit to measure resistance, a bit like a galvanic skin response system. Arduino circuit attached.

It's been working fine with the programming, but now when I remove the monitor from the HDMI to use the Pi headless, the values I'm measuring have changed. When it operates normally, the circuit has to be completed by a human body between two points to output the measured values, but now it outputs a value just making contact with one point (the one connected to the A0 Arduino pin). This doesn't happen when the Pi is connected to the monitor - it normally measures nearly zero when you touch either of the two points separately, and only outputs values when they are connected. I have read other people with a similar problem on the RPi forum, and that it is to do with poor grounding in the Pi, and that the connected monitor would ground it properly via the HDMI cable. However I have seen no solutions so far.

Does anyone know how to properly ground the Pi or the Arduino/circuit and solve this issue? Any advice greatly appreciated!

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2

I've tried connecting a ground pin from the Pi to one of the two points in the Arduino circuit, the 'ground' point, which is connected to the Arduino pin, through a 100k resistor (this is the only value that worked), but this has radically reduced the range of values I can get (and has affected the lower threshold at which it starts to output a value).

You need to connect the Pi ground to the Arduino ground using a wire, not a resistor.

If doing this renders some portion of your Arduino/Pi/sensor collection inoperable, there is a problem with the other wiring.

3

The RPi and the Arduino already have a common ground via the USB connection. I suspect the difference is that the HDMI monitor also connected this to earth ground via its power cord.

One could reproduce this by running a wire from RPi ground to the earth connection on a power outlet, but that's a serious safety hazard for medical instrumentation and is something that should be explicitly avoided.