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Complete electronics beginner here. I'm attempting to build a weighing scale with an Arduino.

I'm attempting to follow this tutorial, with the shield that they use.

enter image description here

I've arranged my load cells on the scale into a Wheatstone Bridge, and I can get some readings just by taking a voltage reading from the two cells marked V+ and V-. The board has terminals that are linked to a INA125p, and the pins should give an amplified reading.

Pin A1:0.96 
Pin A2:1.42 
Pin A3:5.00 
Pin A4:5.00 
Pin A5:5.00

Regardless of whether I have the scale wired in or not, I get these readings from the individual pins. The scales seem to be able to take power from the Arduino shield.

The scales that I have are different from the ones in the example. From what I can figure out, the yellow cables that they reference turn their particular scale on when weight is put on it.

I've put the wires where I take the reading into the 2 and 4 terminals on the circuit board that they have (where the tutorial references a differential signal), but again, it makes no change.

unsolder or cut the white wire that is referred to as G3 (Wheatstone bridge mass) on the original printed circuit, extend take it to terminal 1 on the new printed circuit;

unsolder or cut the white wire that is referred to as G2 (Wheatstone bridge positive) on the original printed circuit, extend take it to terminal 3 on the new printed circuit;

unsolder or cut the white wire that is referred to as G1 and G4 on the original printed circuit, extend take it to terminal 4 and 2 on the new printed circuit (differential signal on the Wheatstone bridge)

I've attempted lots of different combinations of cables and terminals on the new circuit, but it appears to make no difference to the readings.

My question is: Based on the tutorial and the drawing above, am I taking the readings from the correct place? Are there any things I'm obviously doing wrong with the circuit itself? If the circuit is correct, how would I go about trying to get correct readings from the scale when I add weight to it?

I apologise for the vague nature of this question, but I am a complete beginner trying to figure this out, and I am struggling to follow the tutorial. I'll try and provide any additional information that you need.

EDIT - I have a Salter SAL-3003 SSSVDR08 scale.

Each load cell has 3 cables, Red, Black and White. The red ones are the ones marked with the V+, V- etc. On the original board, they were wired in threes.

  • Re “The scales that I have are different from the ones in the example”, it may help if you edit your question to include the make/model of the scale you got parts from, plus a picture of the load cell and its circuit board connection points – James Waldby - jwpat7 Oct 21 '15 at 20:54
  • @jwpat7 I've added some information. The original circuit board was wired like this goo.gl/photos/epmSuzkkyNKfFSkL7 where R,W,B is red, black and white cables – jmo Oct 21 '15 at 21:02
  • Wiring 4 three-wire load cells would be done as in electronics.stackexchange.com/questions/102164/… Each half-bridge unit would be use as essentially 2 eighths of the cell, with their middle wires corresponding to the non black or white wires in your schematic. The raw "reading" out of the wheatstone bridge and as an input to the INA125 should be on the order of (5V*0.001 * load/capacity) – Dave X Dec 14 '15 at 21:11
  • See arduino.stackexchange.com/a/18698/6628 for some of the load-cell wiring. – Dave X Dec 15 '15 at 18:11
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Fixed (to some extent)! Faulty connection between the INA125p and the plint it was sitting in. I refitted it and i'm now getting varied readings with the scales, but that's to be expected with the noise the INA125p generates.

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