At times when I press on the load cell voltage increases, sometimes decreases, sometimes stays the same. This load cell is brand new and I don’t understand what’s going on. I checked for resistance across the bridge and it looks consistent, 1k across excitation, 1k across signal, & 750ohms for other connections.
I have a known load of 1022g, which measures 0.142V on a mulitmeter, & no load measures 0.151V. I’ve used two tutorials code and followed the steps. The easiest being the 2nd, with LCD. The instructions are clear when the applications is launched place a ‘low load’ then press “Select” to set the voltage. Then put a different load and press “Select” to set the corresponding voltage. Everything looks fine up until here. Where I remove the load and place it back, then I might get an accurate reading or the reading will appear negative then too low or too high from the actual weight.
I haven’t adjusted the gain on this circuit and the Vref is as low as possible. I tried everything in-between but I can’t get this to be steady at all.
I don’t have a scale that can go to 20kg at the moment so my maximum calibration point is 0-2kg more or less.
The 20 Kg Micro Load Cell should have 5V excitation voltage (from its datasheet).
The Excitation + and Excitation - pins on the Strain connectors on the Shield are connected to 3.3V and GND of the Arduino board.
This means that your load cell is excited at 3.3V in your current setup.
Try to connect the red wire of your load cell to 5V pin on the Arduino.
I’ve given 5V excitation as suggested and the behaviour is similar. I’m expecting for this setup to be able to read changes of 200g, is this unrealistic?
When I press down onto the load cell, am I expected to see a drop in voltage? I was under the impression that the load cells output is as follows, Vout = RatedOutput (1mV/V) * mass(g)/capacity(g) * Vexcitation (V)
Then the amplifier is, Vout = Vdiff * Gain + Vref, where Vdiff is Vout of the load cell.
If I used a 5V excitation at capacity then I’d end up with…
Vout = 5mV, for the load cell
Vout = 2.5V + Vref, for the output of the shield to the analog input.
In a perfect world I’d read Vout = Vref at no load, right?
You are correct : If you use 5V excitation and with a 20Kg load, you should have 5mV output form the load cell. Have you tested it ?
Please also note that these load cells are not in the industrial grade. Therefore, they might be off-calibration and provide inaccurate results. Did you test another load cell with the Shield ?
Theoretically, yes, without a load, you should have Vout(from the analog pin A0 on the shield)=Vref (a maximum of 3.3V depending on your RefVoltage potentiometer position).
However, this will also depend on the load cell that you are using.
Off calibration is okay, as I can adjust accordingly. This load cell is bouncing and I’m not sure why, see the image in the following link for details. imagebin.ca/v/31ANwGj9du5x
I’ve changed from USB power, to 9VDC via the Uno, and 5VDC from a ACDC PSU and there isn’t much of a difference. The image shown is a ‘good’ capture where there isn’t much noise, but more often that not I see 400mVpp swings for some reason. When using a voltage divider circuit monitoring the analog in is consistent. I’m certain that my Arduino is fine, but I can’t say the same for the load cell.
