Load Sensors

Understanding Load Sensors: How They Work and How to Use Them

Have you ever wondered how digital weighing scales work? Or how industrial machines can detect how much pressure they’re applying? The answer lies in a clever little device called a load sensor—a small but powerful component that converts force into measurable electrical signals.

In this article, we’ll explore what load sensors are, how they work, and how you can use one in your own electronics projects.

1. What is a Load Sensor?

A load sensor, often referred to as a load cell, is a type of transducer. Its job is to convert mechanical force (like weight or pressure) into an electrical signal. This electrical signal is usually very small and needs to be amplified before it can be read by a microcontroller or data acquisition system.

These sensors are found in a wide range of devices:

• Electronic kitchen scales
• Industrial weighing systems
• Smart luggage
• Robotic grippers
• Structural monitoring systems

2. How Do Load Sensors Work?

Most load sensors are based on a technology called the strain gauge. A strain gauge is a special wire pattern that changes its electrical resistance when it stretches or compresses. When a force is applied to the sensor, it causes a tiny deformation. This change is then translated into a change in resistance.

But here’s the challenge: the change in resistance is extremely small. That’s why strain gauges are arranged in a Wheatstone bridge configuration—this setup helps detect those tiny changes more accurately and gives a small voltage output that corresponds to the force applied.

A strain gauge implanted in a metal beam

3. Common Types of Load Sensors

There are several types of load sensors, each designed for different mechanical applications:

Single-Point Load Cell

Also referred to as a bending beam load cell, this is one of the most common load cell types used in DIY electronics and small-scale automation. It is a bar-type strain gauge load cell, often found on online marketplaces like AliExpress.

Key Features:

• Material: Aluminum alloy
• Wiring: 4-wire configuration (typically red, black, green, and white)
• Capacity: Ranges from 100g to around 200kg, depending on the model
• Structure: Features a drilled hole near the center with strain gauges glued around this region—typically arranged in a full Wheatstone bridge configuration

What Is It Used For?

This type of sensor is typically found in:

• Electronic scales (kitchen, postal, or jewelry)
• Weighing platforms and compact balance systems
• Robotics, such as force feedback in grippers
• DIY projects with microcontrollers like Arduino or ESP32

Why the Hole in the Middle?

The hole is more than just a manufacturing feature—it’s a mechanical stress concentrator. When a load is applied to the sensor:

• The beam bends slightly, with the highest strain occurring near the drilled hole.
• The strain gauges detect these minute deformations.
• The resulting change in resistance is converted into a voltage signal through the Wheatstone bridge, enabling precise measurement of the applied load.

S-Type Load Cell

Shaped like an “S”, these are used for both tension and compression forces.

Button Load Cell

Compact and good for applications where space is limited.

4. The Wheatstone Bridge Explained

To understand how a load sensor produces reliable electrical readings from mechanical force, we need to explore a key component inside it: the Wheatstone bridge.

What Is a Wheatstone Bridge?

A Wheatstone bridge is an electrical circuit used to measure very small changes in resistance. It consists of four resistors arranged in a diamond shape—two on the left, and two on the right. In the case of a strain gauge-based load cell, these resistors are actually the strain gauges themselves.

Here's how it works:

• When no force is applied, the bridge is balanced, and the output voltage is nearly zero.
• When a load is applied, the resistance of some strain gauges changes slightly.
• This unbalances the bridge and causes a small voltage difference to appear across the output terminals.

This voltage is proportional to the amount of strain, which in turn corresponds to the applied force.

Why Use a Wheatstone Bridge?

The Wheatstone bridge provides several advantages:

• High Sensitivity: It detects very tiny changes in resistance—perfect for strain gauges.
• Noise Rejection: It cancels out common voltage noise and temperature variations across the sensor.
• Linear Output: Most importantly, it gives a nearly linear relationship between applied load and voltage output, which makes calibration easier and readings more accurate.

Most load sensors use a full-bridge configuration, meaning all four resistors are active strain gauges. Some cheaper or compact models may use a half-bridge or quarter-bridge, but full bridges offer the best performance and stability.

5. Signal Amplification: The Role of HX711

Load sensors produce a very low voltage signal (in millivolts). To read this signal with an Arduino or any other microcontroller, you need an amplifier and ADC (Analog to Digital Converter). A common and inexpensive module for this job is the HX711.

The HX711 amplifies the signal from the load cell and converts it into digital form, making it easy to interface with Arduino, ESP32, or Raspberry Pi.

6. How to Scale Your Load for Arduino Projects

Once you’ve connected your load sensor and the HX711 module to your Arduino, you’ll need to calibrate the readings to display actual weight values. This is done using the scale.set_scale(...) function.

Understanding scale.set_scale(...)

In the Arduino HX711 library:

Copy
1scale.set_scale(float scale_factor);

This function calibrates the sensor by applying a scale factor that converts raw ADC readings (unitless numbers) into real-world weight units like grams or kilograms.

Internally, calling:

Copy
1float weight = scale.get_units();

Performs this calculation:

Copy
1weight = (raw_value - offset) / scale_factor;
2

Where:

• raw_value is the raw HX711 output
• offset is the zero point set by scale.tare()
• scale_factor is what translates the data into meaningful units

How to Calculate the Correct Scale Factor

Instead of guessing the scale factor by trial and error, you can calculate it using a known weight:

Steps:

• Zero the scale with no load:

Copy
1scale.tare(); // Set offset to zero

• Place a known weight (e.g., 500g) on the load cell.
• Get the raw unscaled value:

Copy
1long raw_value = scale.get_units(10); // average of 10 readings

• Calculate the scale factor:

Copy
1float scale_factor = raw_value / known_weight;
2Serial.print("Calculated Scale Factor: ");
3Serial.println(scale_factor);
4

• Apply the scale factor:

Copy
1scale.set_scale(scale_factor);

Now, every call to scale.get_units() will return values in your preferred unit.

7. Using a Load Sensor with Arduino (Advanced Version with EEPROM)

To make your calibration persistent even after the Arduino is reset, you can store the scale_factor in EEPROM and only calibrate on first use or when needed.

Components Needed:

• Load cell (e.g., 1 kg)
• HX711 amplifier module
• Arduino board (Uno, Nano, etc.)
• Push button (connected to D4 and GND)
• Jumper wires and breadboard

Wiring Overview:

• HX711 to Arduino:
• • VCC → 5V
  • GND → GND
  • DT → D2
  • SCK → D3
• Push Button:
• • One leg → D4
  • Other leg → GND
  • No resistor needed (uses INPUT_PULLUP)

Arduino Code:

Copy
1#include <HX711.h>
2#include <EEPROM.h>
3
4#define DT_PIN 2
5#define SCK_PIN 3
6#define BUTTON_PIN 4
7#define EEPROM_ADDR 0
8#define KNOWN_WEIGHT 500.0 // in grams
9
10HX711 scale;
11float scale_factor;
12
13void setup() {
14  Serial.begin(9600);
15  pinMode(BUTTON_PIN, INPUT_PULLUP); // internal pull-up
16  scale.begin(DT_PIN, SCK_PIN);
17
18  EEPROM.get(EEPROM_ADDR, scale_factor);
19
20  if (isnan(scale_factor) || scale_factor == 0.0 || scale_factor == -1.0) {
21    Serial.println("No saved scale factor found.");
22    Serial.println("Place a 500g weight on the scale.");
23    Serial.println("Then press the button to calibrate.");
24
25    while (digitalRead(BUTTON_PIN) == HIGH) {
26      delay(100);
27    }
28
29    Serial.println("Calibrating...");
30    scale.tare();
31    delay(1000);
32    long raw = scale.get_units(10);
33    scale_factor = raw / KNOWN_WEIGHT;
34
35    Serial.print("Calculated scale factor: ");
36    Serial.println(scale_factor, 6);
37
38    EEPROM.put(EEPROM_ADDR, scale_factor);
39    Serial.println("Scale factor saved to EEPROM.");
40  } else {
41    Serial.print("Loaded scale factor from EEPROM: ");
42    Serial.println(scale_factor, 6);
43  }
44
45  scale.set_scale(scale_factor);
46  scale.tare();
47  Serial.println("Scale is ready.");
48}
49
50void loop() {
51  static float lastWeight = 0.0;
52  float weight = scale.get_units(5);
53  float threshold = 0.1;
54
55  if (abs(weight - lastWeight) >= threshold) {
56    Serial.print("Weight: ");
57    Serial.print(weight, 2);
58    Serial.println(" g");
59    lastWeight = weight;
60  }
61
62  delay(500);
63}
64

Assignment for You

Once you've got your basic weighing system working, try these two enhancements:

1. Recalibration Mode on Startup
   Modify the code so that if the button is held down while the Arduino is starting, it triggers recalibration mode and calculates a new scale factor.
2. Tare Function in Weighing Mode
   Use the same push button to reset (tare) the scale during normal operation with a short press, just like commercial digital scales.

These upgrades will make your Arduino-based weighing system more powerful and user-friendly.

8. Real-Life Applications of Load Sensors

• Digital Weighing Scales: From kitchen scales to shipping scales.
• Smart Luggage: Detect weight without external scales.
• Industrial Automation: Monitoring press forces or filling weights.
• Robotics: Measuring grip force in robotic hands.
• Structural Health Monitoring: Detecting stress in bridges or buildings.

9. Practical Considerations

• Avoid Overloading: Every load sensor has a maximum capacity. Exceeding it can permanently damage the sensor.
• Stable Mounting: Load cells must be mounted securely and evenly to provide accurate results.
• Temperature Drift: Readings can vary slightly with temperature changes. Some load cells are temperature-compensated.
• Electrical Noise: Keep wires short and shielded to avoid interference in readings.

10. Conclusion

Load sensors are incredible tools that bring the physical world into your digital projects. Whether you’re designing a smart weighing system, a robotic gripper, or just experimenting with force measurement, understanding how these sensors work opens up a range of possibilities.

Try building your own digital scale using a load cell and HX711—it’s an educational and satisfying DIY project that gives you hands-on experience with force measurement.

Or you can explore other categories

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Microscopy