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By Lewis
Hi everyone,

I have a couple of OpenScale circuits, and I am trying to use them to measure the axial strain through a rod when it is compressed with a known force. I have strain gauges that are fixed to the rod in a full-bridge configuration, and connected to the board via the 'Load Sensor Combinator' ports. I am attempting to calculate strain using the raw value that the board outputs. I recognise that this value is related to the voltage across the wheatstone bridge, and hence the strain being detected by the strain gauges. However, when a load is applied, the raw value does not change with the load. It fluctuates randomly with what I assume is noise, but other than that they are remaining static.

I thought that this may be due to placing the strain gauges into the wrong ports (LL, UL, UR, LR), resulting in an always balanced bridge. However I played around with these configurations and nothing worked.

I also looked through the hook-up guides for OpenScale and the HX711 amplifier board, as well as the 'getting started with load cells'. There, I noticed that the strain gauges used in those guides have a centertap, which is like a halfway lead between the positive and negative wires. however, the strain gauges I am using do not have this; they have three wires, but two of them are shorted together at the gauge. I have tried three different ways of plugging them into the board:
1) Just plug in the positive and negative wires, leaving the centertap port open.
2) Plug in all three wires directly.
3) Plug in the positive and negative wires directly, but put a resistor (corresponding to half the value of the gauge's resistance) between the center wire and the centertap port. This was to try and simulate the half-way resistance of the centertap.

All of three of these still result in the same problem of no response to load. I have checked that the wires are plugged in correctly, and are not touching. I know that the strain gauges themselves aren't the issue because I've checked them using some lab equipment, and they give me the right values. I'm pretty much at my wits end trying to figure this out. Can anyone help me? It would be very much appreciated.

By Lewis
If anyone is wondering, I managed to find a work around.

Essentially I was trying to use the on-board Load Cell Combinator as a Wheatstone Bridge, however, there were issues with this in that the output wouldn't change at all. I email SparkFun and it sounds like the Combinator isn't designed for use with regular strain gauges, which is what I was trying to use. To get around this, I just constructed my own Wheatstone bridge outside of the circuit using a bit of strip-board and some screw headers, and then connected this into the Load Cell input of OpenScale. This allowed me to use the board to measure loading with strain gauges mounted to the object of interest, without having to use the load sensors that SparkFun recommends.

Hope that maybe helps anyone with a similar issue.