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Questions &/or issues related to products in the Flex / Force category should be posted in this forum. includes Force Sensitive Resistors, Load Cells & Amplifiers, Pressure Sensors, etc
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By keebler411
I followed the industrial IoT tutorial and have a scale working with the NAU7802 chip. I used a combinator board to connect the for load sensors. However, I can't replicate the setup to another 4 sensors. I've soldered up 2 sets of sensors and get the same results:

1) If I leave A+/A- connections the same as with the scale that works I get a really large number (shy of 24 bit max value) and it doesn't change much unless a huge force is applied

2) If I reverse A+/A- I get a no load reading in the same range as the working scale

3) If I now apply gradual pressure with say my foot I see the reading decrease until it underflows and I get a large number like before

4) Within the range of no load to just before underflow I can calibrate the scale and get a slope that is about the same magnitude as the working scale (good), but obviously negative because I reversed the connections.

I've carefully checked the connections and had other people check them too. Any ideas?
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By TS-Chris
Hi keebler411.

What load sensors are you using and how do you have them wired up to the NAU7802 breakout? Can you provide some photos showing your connections? If you could also send some photos of the top and bottom of your board that would be helpful too.
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By keebler411
I said in another thread switching to a similar load sensor purchased from Aliexpress (as opposed to Sparkfun) seems to have solved these problems. But it could be something else I did as I've been working with custom PCBs I've designed that combine the nau7802 and combinator circuit.

What I didn't say in the original post is that the 4 sensors were picked at random from a set of 16. So I got a little more methodical in the testing and two things became clear:

1) I tested the 16 load sensors from Sparkfun in combination with each other. First I found 4 that worked together in the normal configuration. I then swapped out one sensor in the working configuration for one of the other 12 sensors and retested. Three of the remaining 12 sensors could not be made to work with any combination of known good sensors. The remaining 9 sensors worked as expected.

2) The polarity of the Sparkfun sensors is opposite that of the ones I purchased from Aliexpress. But this is not unexpected. So far I have not encountered any problems with the Aliexpress purchased lead sensors.