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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
#208290
Hi, I'm a student. I ordered 2 set of 3-wire Load Sensor - 50kg (SEN-10245) and SparkFun Qwiic Scale - NAU7802 (SEN-15242).
At first time I used Mbed, but it doesn't work. Then I carefully read the datasheet and learn from the example again
to confirm how to control and power up it. But it still doesn't work. So, I decide to comeback to the beginning.
By borrowed the Arduino UNO from my friend, because the example code is compatible to Arduino. It should be easy but it isn't.
I connected SDA to A4, SCL to A5, 3.3 and GND from UNO to ADC NAU7802. Then connect 1 load sensor to ADC.
I connected RED, BLACK, WHITE from sensor to RED, BLACK, WHITE on ADC respectively. Then run the example code.
Ex1. BasicReadings: I got negative value about -900 or -1200
the reading value doesn't change when I press the sensor
Ex3. BasicReadings: same result (of course lol)

And after i read these: "This device is optimized to accept differential input signals, but can also measure single-ended signals.
When measuring single-ended signals with respect to ground, connect the negative input (VIN1N or VIN2N) to ground
and connect the input signal to the positive input (VIN1P or VIN2P). Note that when this device is configured this way,
only half of the converter full-scale range is used, since only positive digital output codes are produced."

-I tried to jump GRN or WHT of ADC to GND then I got reading value about -8388607.

I spent many days to solve it but still cann't. Has anyone has any advice? Please feel free to teach me.
Thanks in advance ,

P.S. In my experiment, I would like to use 6 load sensor to measure the force at each point separately. but at fist, I would like to test with only 1 load sensor.
#208305
HI ThaelasuttTug.

You're using a 3 wire load sensor and you need to be using a 4 wire load cell with this board.
It is possible to combine two or four 3 wire load sensors together to create a single 4 wire load cell but if you're just starting out, I'd suggest starting with a 4 wire load cell like one of the load cells below and using the directions found in our hookup guide.
#208322
TS-Chris wrote: Fri Oct 04, 2019 10:33 am HI ThaelasuttTug.

You're using a 3 wire load sensor and you need to be using a 4 wire load cell with this board.
It is possible to combine two or four 3 wire load sensors together to create a single 4 wire load cell but if you're just starting out, I'd suggest starting with a 4 wire load cell like one of the load cells below and using the directions found in our hookup guide.
Dear TS-Chris,

Hi, thank you so much for your quickly response.
At first, I'll try to combine two of 3 wire load sensors together as you advised.
But when I measure the resistance of the sensor between BLACK and WHITE I got 2K ohm and RED and BLACK or WHITE is 1K ohm.
That means I have to jump BLACK(-) and WHITE(+) of 2 sensor together, right?

I has a little question to confirm, in this case of using 2 sensor. How it should works?
Do I need to press the 2 sensor at the same force or it will work separately but shares value?

Thanks in advance,
#208447
At first, I'll try to combine two of 3 wire load sensors together as you advised.
But when I measure the resistance of the sensor between BLACK and WHITE I got 2K ohm and RED and BLACK or WHITE is 1K ohm. That means I have to jump BLACK(-) and WHITE(+) of 2 sensor together, right?
Yep! The picture below should help in connecting the two sensors together to form a full bridge.
Capture.JPG
I has a little question to confirm, in this case of using 2 sensor. How it should works?
Do I need to press the 2 sensor at the same force or it will work separately but shares value?
Both sensors share the load but weight on either one will work. In other words, the sum of the weight on both sensors is what gets passed onto the amplifier. 10 pounds on one and 90 pounds on the other will be read as 100 pounds. 0 pounds on one and 100 on the other still gets read as 100 pounds.
You do not have the required permissions to view the files attached to this post.
#208482
Thank you so much, I got it.
Now I'm using the example code2 and the circuit above, but i still have the problem.
1. the readings are decreasing when I press on the scale.
Then I try reversing the white and green wires and I still got the same and this time it is a negative value too.
And no matter how many time I calibrate, the weight is always be inf or nan. (it just occurred 2 days ago)
2. I paste a mass on a bar at each point (as shown in the photo) the reading value is not same!!
https://drive.google.com/open?id=1KIL7q ... Poam9ZwAfO
at point 1 442,500
point 2 440,600
point 3 436,200
at no load 445,800

it's quite strange, right?

Thanks in advance,
#208508
Thanks for sending a picture, that really helps figure out what the issue might be.

I think I see the problem and it has to do with how you have your sensors mounted.
The way these sensors work is there's a strain gauge glued to the metal T shaped part in the center of the sensor. What that does is measure how much the metal deforms when a force is applied to it and from that you can get a reading.

It looks like you have your sensors mounted flat on some aluminum bars and the sensors don't have room to deform. That's what's causing the odd readings you're seeing and the inability to calibrate.

What you need is a hole in your bars that allow the center portion of the sensor to bend down into allowing the metal to deform.
file-3.jpg
Here's an example on how I've mounted a sensor to a piece of MDF.
Top view:
file.jpg
Bottom view:
(note the hole for the sensor to deform into)
file-2.jpg
If you change your setup to include a hole, these should work OK then.
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#208532
Dear TS-Chris
Thank you so much for clearly explanation. I got much knowledge from you, sir.

Oh...look like I had misunderstanding about the structure of this sensor too much.
Now I already change the experiment setup as your recommend. The readings are still decreasing ,
switch A+ and A- or V+ to V- the reading becomes negative.
Then I borrow the 4-wire load cell (bar shape) from my friend, It can works properly.
The reason I have to use this sensor because its shape is slim and easy to install to my experiment.

Next step. I would like to record the data from 2 or 3 set (one set included 1 ADC+ 2 Load cell).
But from ADC's data sheet, it has only one register address.
From my understanding, I think I can't control all of it in the same time. Is it right?

Thanks in advance,
Best regard,
#208704
Try swapping just the red and black wires and see if that gets your readings to go the right direction.

For multiple NAU7802's you'd need to split your I2C bus with something like our Qwiic Mux board to do this.
The Qwiic Mux acts like a 8 way switch that lets you select different outputs. Each NAU7802 goes to their own channel on the Qwiic Mux, then your code selects which NAU7802 to communicate with.

So, you'd select channel one on the Qwiic Mux and take a reading from the first NAU7802. Then change to the second channel and take a reading and so on.
#208984
Dear TS-Chris

I swapped the red and black and got the negative reading and it still decreasing.
Then I swapped it back and try to reverse some code in the library file. Now it look fine (in my opinion).
But if you have any suggestion please feel flee to teach me.
Thank you so much,

Beast regard,