I have been doing some experiments to get load cells working with Arduino.
This is a collection of my experiences.  Something about:

  • What is a load cell
  • How did I wire it up to an Arduino board
  • What crazy application did I use it for?
Dancing people in a rope in the hull of the illutron ship – Load cell senses the weight and control music and light.
  • Light was controlled by using diode boxes and Sonny Windstrupsgreat code
  • Music was controlled by using Pure Data to read the Arduino board and generate a midi-controller to use in Ableton Live.
  • Dancing: Allison Lorenzen and others. Mounting: Thomas Fabrik. Rigging: Karl Gillick. Interactive music: Bo Boye.

So.. What is a load cell anyway

A digital weight.

Usually a piece of steel containing one or more Strain Gauges arranged in a Wheatstone bridge.

How to get a load cell

The load cell I have been using is a commercial one. (SKANTRONICS FH SERIES – STAINLESS-STEEL Shear Beam Load Cell)

These are quite expensive if you are heading to buy one.

Break a digital weight

There is actually a load cell sitting inside any digital weight you can find. Paper weight, Bath weight.

Buy or find a cheap one, and take it apart. Then rehook the loadcell inside.

Its a little bar of steel with 4 wires coming out of it.

The 4 wires are pretty standard color coded.
Look here for a good reference:http://www.controlweigh.com/loadcell_colors.htm

You can play around with the gain to get the accuracy you want.

Connecting a load cell to an Arduino board

You need some sort of microvolt amplifier to read the minute change in volt over the bridge.
ina125P – An IC that does it all for you.

  • Very simple to use.
  • Cheap. Around 7 € at Farnell
  • Usable for many different instrumentation amplifications
This is the schematics for hooking it up

The resistor sets the gain. Look in the datasheet of the ina125p for details.

The connection as suggested for single supply operation in the datasheet
And this is how it looked at the breadboard
…And on a shield

Simple arduino example

This is a simplified example of how to convert your load cell analog readings to kilo/load.

// Arduino as load cell amplifier
// by Christian Liljedahl 
// christian.liljedahl.dk

// Load cells are linear. So once you have established two data pairs, you can interpolate the rest.

// Step 1: Upload this sketch to your arduino board

// You need two loads of well know weight. In this example A = 10 kg. B = 30 kg
// Put on load A 
// read the analog value showing (this is analogvalA)
// put on load B
// read the analog value B

// Enter you own analog values here
float loadA = 10; // kg
int analogvalA = 200; // analog reading taken with load A on the load cell

float loadB = 30; // kg 
int analogvalB = 600; // analog reading taken with load B on the load cell

// Upload the sketch again, and confirm, that the kilo-reading from the serial output now is correct, using your known loads

float analogValueAverage = 0;

// How often do we do readings?
long time = 0; // 
int timeBetweenReadings = 200; // We want a reading every 200 ms;

void setup() {

void loop() {
  int analogValue = analogRead(0);

  // running average - We smooth the readings a little bit
  analogValueAverage = 0.99*analogValueAverage + 0.01*analogValue;

  // Is it time to print? 
  if(millis() > time + timeBetweenReadings){
    float load = analogToLoad(analogValueAverage);

    Serial.print("analogValue: ");Serial.println(analogValueAverage);
    Serial.print("             load: ");Serial.println(load,5);
    time = millis();

float analogToLoad(float analogval){

  // using a custom map-function, because the standard arduino map function only uses int
  float load = mapfloat(analogval, analogvalA, analogvalB, loadA, loadB);
  return load;

float mapfloat(float x, float in_min, float in_max, float out_min, float out_max)
  return (x - in_min) * (out_max - out_min) / (in_max - in_min) + out_min;

DC offset

If you are having an offset in you load cell, you need to add a resistor to compensate. This circuit should show you how:

Other considerations

  • I am using the 5V of the Arduino board as exitation to the load cell, not the 5v-ref on the chip
  • Using a 1760 kg load cell I was able to sense 0.1 kg changes
  • You may have to swap the voltage applied to the bridge to get a decent range on the analog signal
  • Measure the output voltage (input to analog 0) while you change your load, and adjust the gain until you get a good sense
  • Analog input at the arduino board is used. Play around with the aref – analogreference – to get a good sense.
  • analogReference in my sketch was set to INTERNAL making it 1.1 V – Wich gave me a good range