Load Cell & HX711 Guide: Weight Measurement Projects

A load cell combined with the HX711 amplifier module gives you an accurate, affordable weight measurement system that you can build with Arduino in under an hour. From building a digital kitchen scale to monitoring beehive weight or tracking inventory in a smart warehouse, the load cell and HX711 combination is the go-to solution for precise weight sensing in maker and engineering projects. This guide covers everything from the strain gauge principle to a fully functional calibrated digital scale.

What Is a Load Cell?

A load cell is a force transducer — it converts mechanical force (weight) into an electrical signal. The core technology inside a load cell is the strain gauge: a thin metallic foil etched into a pattern that changes its electrical resistance when stretched or compressed. This change in resistance is tiny (typically a few ohms out of several hundred), which is why the HX711 24-bit amplifier is needed to read it accurately.

Inside a typical bar-type load cell, four strain gauges are arranged in a Wheatstone bridge configuration. When weight is applied, two gauges stretch (resistance increases) and two compress (resistance decreases), producing a differential voltage that is proportional to the applied weight. This differential arrangement cancels out temperature effects and common-mode noise, giving much better accuracy than a single strain gauge.

The output from a standard load cell is a differential voltage in the range of 1 to 3 millivolts per volt of excitation. For a 5V excitation, a 10 kg load cell produces a full-scale output of only 5 to 15 millivolts — far too small for a standard Arduino ADC to read accurately. The HX711 solves this with its 128x programmable gain and 24-bit resolution.

Types of Load Cells

Load cells come in several physical configurations, each suited to different mounting and application requirements:

• Bar type (beam type): The most common type for hobby projects. A flat rectangular aluminium bar bends slightly under load. Available in 1 kg to 50 kg ratings. Easy to mount with bolts. Used in kitchen scales, postal scales, and simple platform scales.
• Disc type (button type): Flat, circular, very low profile. Used in compression-only applications like force measurement under a press or in mattresses.
• S-type: Can measure both tension and compression. Used in hanging scales, crane load monitoring, and material testing. More expensive than bar type.
• Platform load cells: Single-point load cells designed for off-centre loading in platform scales. Allow the platform to flex without affecting accuracy.

For most Arduino projects — kitchen scale, postage scale, smart inventory — the bar type load cell at 1 kg, 5 kg, or 10 kg rating is the right choice and costs very little.

The HX711 Amplifier Module

The HX711 is a 24-bit analog-to-digital converter (ADC) designed specifically for weigh scale applications. Key features:

• 24-bit resolution: 16 million measurement steps, giving sub-gram accuracy with a typical load cell
• Programmable gain: 128x or 64x on channel A (for load cells), 32x on channel B
• Sample rate: 10 or 80 samples per second (selectable via the RATE pin)
• Interface: Simple two-wire serial (Clock + Data) — works with any GPIO pins, no I2C or SPI library needed
• Power-down mode: Ultra-low power consumption when not measuring
• Operating voltage: 2.6V to 5.5V

The HX711 module available from Zbotic.in includes onboard filtering capacitors, a RATE selection point, and colour-coded screw terminals for the load cell Wheatstone bridge connections, making wiring straightforward.

Wiring Load Cell to HX711 to Arduino

A standard bar-type load cell has four wires colour-coded as follows:

Then connect the HX711 module to the Arduino:

Calibration Process

After wiring, you need to calibrate the system with a known weight. Install the HX711 library by Bogdan Necula from the Arduino Library Manager (search for HX711). Then run this calibration sketch:

#include <HX711.h>

const int DT_PIN  = 4;
const int SCK_PIN = 5;
HX711 scale;

void setup() {
  Serial.begin(57600);
  scale.begin(DT_PIN, SCK_PIN);
  Serial.println("HX711 calibration utility");
  Serial.println("Remove all weight from scale");
  Serial.println("Press any key to tare");
  while (!Serial.available());
  Serial.read();
  scale.tare();
  Serial.println("Tare complete. Place known weight on scale.");
  Serial.println("Enter weight in grams in Serial Monitor, then press Enter:");
}

void loop() {
  if (Serial.available() > 0) {
    float knownWeight = Serial.parseFloat();
    if (knownWeight > 0) {
      float reading = scale.get_units(10);
      float calibFactor = reading / knownWeight;
      Serial.print("Calibration factor: ");
      Serial.println(calibFactor);
      Serial.println("Use this value in your main sketch as CALIBRATION_FACTOR");
    }
  }
  delay(500);
}

Note the calibration factor value. A typical 10 kg load cell might give a calibration factor around 420 to 460. This value varies between individual load cells and mounting configurations, so always calibrate your specific setup.

Building a Digital Weighing Scale

With the calibration factor in hand, here is the complete digital scale sketch including an LCD display (optional — you can use Serial Monitor instead):

#include <HX711.h>

const int DT_PIN  = 4;
const int SCK_PIN = 5;
const float CALIBRATION_FACTOR = 440.0; // Replace with your calibrated value
const int TARE_BUTTON = 2; // Button connected between pin 2 and GND

HX711 scale;

void setup() {
  Serial.begin(57600);
  pinMode(TARE_BUTTON, INPUT_PULLUP);
  scale.begin(DT_PIN, SCK_PIN);
  scale.set_scale(CALIBRATION_FACTOR);
  scale.tare(); // Zero at startup
  Serial.println("Digital Scale Ready");
  Serial.println("Reading   Weight");
}

void loop() {
  // Handle tare button
  if (digitalRead(TARE_BUTTON) == LOW) {
    scale.tare();
    Serial.println("Scale zeroed (tared)");
    delay(500);
    return;
  }

  if (scale.is_ready()) {
    float weight = scale.get_units(5); // Average of 5 readings
    if (weight < 0) weight = 0.0;     // Ignore negative noise

    Serial.print("Weight: ");
    Serial.print(weight, 1);
    Serial.println(" g");
  }
  delay(500);
}

Tare Function

The tare function zeros the scale with a container or packaging on it, so you measure only the contents. In the sketch above, pressing the tare button calls scale.tare(), which takes 10 readings and sets their average as the new zero point. This is identical to how commercial kitchen scales work.

For automatic tare on startup (common in postal and inventory scales), call scale.tare() in the setup() function after a short delay to ensure the load cell has settled. The tare offset is stored in the HX711 library object and persists until power is removed or tare is called again.

Multiple Load Cells for Platform Scale

For a weighing platform (like a body weight scale or a luggage scale), you typically use four load cells — one at each corner of the platform. Each load cell handles one quarter of the total load. The four load cells are wired in parallel: all E+ wires together, all E- wires together, all A+ wires together, and all A- wires together. The HX711 then reads the combined output as if it were a single load cell.

This works because the four Wheatstone bridges in parallel produce the average output, and the Arduino reads the total platform weight directly. Use load cells of the same model and rating for all four corners for the best accuracy.

Accuracy Tips

• Mechanical mounting matters most: The load cell must be mounted rigidly on one end and have the platform attached to the other end with no side forces. Side loading reduces accuracy significantly.
• Take multiple readings: scale.get_units(10) averages 10 readings. More averages reduce noise at the cost of response speed.
• Temperature effects: Load cells drift slightly with temperature. For high-accuracy applications, re-tare before each measurement session.
• Overload protection: Never exceed the rated capacity. Even a brief overload can permanently deform the strain gauge and ruin the load cell.
• Vibration: Use software averaging and add rubber feet to the scale platform to dampen vibration from nearby machinery.
• Settling time: After placing weight, wait for the reading to stabilise (about 0.5 to 1 second with 10 SPS mode) before recording.

Real-World Applications

The load cell and HX711 combination is used in a surprisingly wide range of real-world projects in India and globally:

• Smart inventory management: Small bins with load cells can alert when stock drops below a threshold. Used in electronics stores, pharmacies, and manufacturing lines.
• Beehive monitoring: Beekeepers monitor hive weight remotely — a hive gaining 1 to 2 kg per day indicates an active honey flow. Load cells under the hive connected to an ESP32 send daily weight logs via MQTT.
• Portion control in food industry: Restaurant kitchens and food processing units use load cell based systems for consistent portioning, saving ingredients and ensuring quality.
• LPG cylinder monitoring: Many Indian households have adopted load cell systems to monitor LPG cylinder weight and receive WhatsApp alerts when refill is needed.
• Postal and courier scales: Small businesses shipping parcels use DIY Arduino scales for quick weight measurement before courier bookings.

Frequently Asked Questions

Q: What is the accuracy of a typical Arduino load cell setup?

With careful calibration and mechanical mounting, you can achieve accuracy of plus or minus 0.5 to 1 gram on a 10 kg load cell. The HX711 24-bit ADC is capable of better accuracy in theory, but mechanical factors like overload, off-centre loading, and temperature usually limit real-world accuracy to about 0.1% of full scale.

Q: Can I use any load cell with the HX711?

Most standard strain gauge load cells with a Wheatstone bridge (four wires, typically red, black, white, green) work with the HX711. Check that the load cell excitation voltage matches the HX711 supply (2.6V to 5.5V) and that the output sensitivity (mV/V) is within the HX711 input range.

Q: Why does my scale reading drift over time?

Drift is usually caused by temperature changes (the load cell and HX711 both have temperature coefficients), creep in the load cell metal under sustained load, or electrical noise. Re-tare before each use session and ensure the supply voltage to the HX711 is stable (use the Arduino 5V pin, not a noisy regulator).

Q: How do I get faster readings from the HX711?

Connect the RATE pin of the HX711 to VCC to switch from 10 SPS to 80 SPS mode. With less averaging (get_units(1) instead of get_units(10)), you can get updates every 12 ms. Note that faster rates mean more noise, so apply a moving average filter in software.

Q: Can I use the HX711 with ESP32 or Raspberry Pi?

Yes — the HX711 library is available for both ESP32 (Arduino framework) and Raspberry Pi (Python). The wiring is the same. Note that on ESP32, any GPIO pins work for DT and SCK, and the 3.3V supply is sufficient for the HX711.