How to get weight data from glass electronic bathroom scale sensors?

Question

I am doing a small project with a bathroom scale but I run into some problems. I am using an Arduino Uno V3, HX711 module amp and a scale.

Scale:

HX711 amplifier:

I disassembled the scale to get to sensor wires and I am a bit confused. These sensors have three wires each. So they are half bridge sensors. The scale is using 4 sensors so when the scale is measuring it is measuring with a full bridge. And these 3 wires are RED, BLACK, BLUE. I don't know what is black (GND or Positive) and blue (same guess). Four red wires are given names E+, E-, S+ and S-. I will provide a picture of the disassembled scale.

Two blue wires are soldered together on each side. I guess red wires are signal (S+, S-, E+ and E-) but what are Blue and Black? Then I soldered the wires to these wires to get sensor readings but I don't know what is what.

The sensor is looking like this:

I tried:

1. Two different libraries for this project both named HX711 (for this module)
2. Changing A+ for A- on module

// Hx711.DOUT - pin #A1
// Hx711.SCK - pin #A0

#include "hx711.h"

Hx711 scale(A1, A0);

void setup() {

  Serial.begin(9600);

}

void loop() {

  Serial.print(scale.getGram(), 1);
  Serial.println(" g");

  delay(200);
}

with no success. I get 0.0g on the serial monitor when calm, and the same when load is on.

My questions are: How to know which wire is GND, +5V and which is signal? How to connect 4 of these sensors to read data? How to use HX711 libraries available and HX711 module? And, most important, how to read data from this scale on serial?

Answer 1

Your four half bridge load cell sensors can connect into a full wheatstone bridge as in https://electronics.stackexchange.com/a/199470/30711

If your sensors are like this 50kg load cell from SparkFun's https://www.sparkfun.com/products/10245 or Ebay's http://www.ebay.com/itm/4pcs-Body-Load-Scale-Weighing-Sensor-Resistance-Strain-Half-bridge-Sensors-50kg-/251873576571 they might have a compression and tension gage both on the top surface. The Ebay site has a diagram like:

... which indicates a positive strain gauge on the red-white, and a negative strain on the red-black. (note that the coloring order in this diagram does not match the coloring order in this picture. I have a similar gauge with blue-red-black colors, and the positive strain gauge is the right pair, negative on the left.) The gauged surface on the center bar between the face-to-face coupled 'E's in the sensor should act like a parallel bar and has portions under compression and under tension, rather than purely under tension. In cross-section, the gauged bar in the center is sort of the cross-piece in a Z-shaped spring. In this case, the strains oppose each other, and, if manufactured well, the reduction of resistance in the negative strain portion will offset the increase in resistance in the positive strain portion and the total white-black resistance should be constant. One still needs to set up the bridge so that the voltage dividers move in opposite directions with added load, and 4 devices wired in a white-to-white and black-to-black loop should work as above.

If you wire four of these up carefully by flipping them around so the stress sensitive portions unbalance the bridge constructively, you can use all four sensors without any extra resistors.

Basically, two diagonally-opposite sides of a wheatstone bridge are each formed by the compression elements of two gauges wired in series, while the remaining two sides of the bridge are each formed from two tension elements from two cells. With load on all the sensors, the compression resistances are reduced, while the tension resistances are increased and it pulls the bridge out of balance.

To get this, wire all four sensors in a big ring with maximum resistance, matching colors and initially ignoring the red center tap wires. (This is the function of the soldered-together blues and blacks are in your scale.) Choose two opposite (red) center taps as E+ and E-, and the remaining two (red) center taps as S+,S-. Put the excitation voltage on the E+/E- from the diagram above and read a force-sensitive voltage difference across S+/S-, and this is what you feed into your HX711 as A+ and A- (Ignore B+/B- as a second, unused channel.)

Here's a schematic with gauges 1-4 as G1 G2, G3, G4 per the above specs, applying an excitation on the G1 and G3 reds, and reading the signals off the G2 and G4 reds. The G4 gauge is loaded a bit with some positive strain increasing the G4+ resistance, and some negative strain reducing the G4- resistance. Ideally, loading G4 with 25kg would produce 0.5mV/V times its 2.5V excitation voltage, producing 1.250mV across Sig+/Sig-, and stretching R8 to be 1001 ohms and compressing R7 to 999 ohms as shown. (The schematic/simulator thing on electronics.stackexchange.com is pretty cool.)

^{simulate this circuit – Schematic created using CircuitLab}

Instead of the "White" wires in the diagram and like on my gauges, consider it the "blue" wires from your scale's sensors.

With only two of these half-bridge sensors, one should not match the end colors and hook white-to-black (blue-to-black) and black-to white, (black-to-blue) imposing an excitation voltage from between these two junctions, and read the differences across the reds, as increased load pulls one side high and the other side low. This will look like the plain 4-resistor wheatstone bridges in the common datasheets, rather than the 4-half-bridge/eight-resistor scheme above.

Answer 2

1. The HX711 provides power to a bridge and amplifies the bridge's differential voltage. The scale's original circuit board (which drives an LCD readout) also provides power to a bridge. You are unlikely to get any useful readings while both circuits are applying power to the bridge.

(You could cut the existing wires and attach connectors to allow changing back and forth between the original board and your Arduino circuit.)

2. Some scales like the one shown use a 9 V battery to run the electronics. That voltage, if used, is likely to be incompatible with connecting an Arduino (typically powered by 3.3 V or 5 V) or an HX711 (powered with VDD in the range 2.7 V to 5.5 V) at the same time as the original board.

Answer 3

Text mainly from my comments with some additions - with image added.

The sensors are quite possibly configured as in Fig1 in the data sheet.
If so there should be +5 and 0V across two points, and weight signal between two others.

Use a DMM to Check for Vdd (probably 5V if pV battery, ~= 3V with Lithium battery). Find two leads to bridge with ~= Vdd on. Other two probably at Vdd/2 above ground. It is likely there are 4 wire effectively so each of the red wires on one side may be conncted tpo one on the other side - an Ohm-meter will tell you. Connect meter on LOW V range between other two and see if reading varies with weight change. Report back.

As jwpat7 says - you may need to repower sensors off HX711 supply to get correct readings.

HX711 looks nice. Under $1/module in modest volume on Alibaba and under $US/0.50 / IC in hundreds.

What Fig1? I know where is GND and VDD on HX711 module, and where goes the signal wires(A-,A+,B-,B+). That is clear. I dont know which colors of wires from sensors is GND VDD and Signal. I will do this. I will put back the power on the scale (3v from battery) and I will try to measure wires to determine what is what (i will put some load on the scale). I have a multimer so i will try to determine which wires are signal, and which wires are GND and VDD. I will report back. Tnx Russell. –

See fig of HX711 Data sheet for larger view of this image.

Answer 4

The sensor you have in your scale are a simple voltage dividers. Each consist of one resistor and one extensiometer, both connected in series. One wire (E+) is VDD another (E-) is GND and the third is divider output(S). You can not directly use it with HX711 which needs a bridge sensors not just dividers. You can try to set a 1/2vdd on S- and connect the signal from the sensor to S+. In case you get a negative weight, simply swap these connections. Your assumption that your scale uses full bridge, because it has four sensors, is completly wrong. You can read the signals from the scale sensors using analog inputs in you arduino (without HX711) but you should amplify it before to get reasonable resolution. Good luck! Greg