Nowadays, the precise measurement of weight and strength It is key in both industrial applications and domestic and research projects. If you have ever wondered What is a strain gauge sensor? and how it complements the HX711 module If you're looking for a way to create high-precision digital scales, you've come to the right place. Here, we'll break down every aspect, from theory to practice, so you understand every detail and learn how to implement your own weighing system.
In recent times the use of has become popular strain gauges along with modules such as the HX711 in the creation of homemade scales, home automation products, and even school experiments. Best of all, these components are affordable, compatible with platforms like Arduino, and allow for reliable measurements at a professional level If they're calibrated correctly. Let's take a step-by-step look at everything, from basic operation to integration and programming.
What is a strain gauge and how does it work?
La strain gauge is a piezoresistive sensor which transforms a deformation (whether by pressure, traction or compression) into a variation in electrical resistance, which allows us to indirectly measure forces or weight applied to a surface. This property is fundamental for load sensing in modern load cells..
In its most basic design, a feeler gauge consists of a very fine metallic filament placed inside an adhesive film. When the structure to which it is glued is deformed, the filament stretches or compresses, changing its resistance. This change, although small, is proportional to the force exerted. and can be analyzed electrically.
The piezoresistive effect The gauge's resistance increases when stretched, and decreases when compressed. This allows physical effort to be transformed into a precisely measurable electrical signal.
These gauges usually have standard resistance values ​​such as 120Ω, 350Ω or 1000Ω, and the changes caused by deformations are minimal: for example, a variation of just 0.12 Ω over 120 Ω for significant loads. However, without a suitable amplification and measurement system, these small variations would be difficult to detect.
Load cell: the application of strain gauges
An load cell It is a transducer that uses strain gauges to convert mechanical forces into electrical signalsThe operating principle is simple: one or more gauges are attached to a metal structure designed to deform predictably under load. When weight is applied, the structure deforms, as do the gauges, and an electrical signal proportional to the stress is generated.
There are different types of load cells (hydraulic, pneumatic, strain gauge, etc.), although in electronics and embedded systems the most common is the strain gauge. Its consistent design allows for a high precision and stability.
Cells vary in size, shape, capacity, and mechanical arrangement, from small ones weighing grams to industrial versions weighing tons.
Internally, most load cells mount 1, 2 or 4 strain gauges forming a Wheatstone bridge., an electrical circuit sensitive to small variations in resistance.
Wheatstone Bridge: The Secret of Sensitivity
El Wheatstone Bridge is a circuit of four resistors arranged in a square, in which one or more may be strain gauges. When there is no load, the circuit is in equilibrium and there is no potential difference between its outputs. When the gauges are deformed, this equilibrium is broken. and detectable voltage differences appear which reflect the applied load.
This configuration multiplies the sensitivity to changes in resistance and allows the detection of tiny variations, which would be impossible to capture otherwise.
In home or laboratory scales, it's common to use four gauges forming a complete Wheatstone bridge, as is the case with many bathroom scales and weighing platforms. This helps reduce temperature errors and improve linearity and accuracy.
Why do we need HX711 module?
Although the Wheatstone bridge amplifies voltage variations, the changes are extremely small (on the order of microvolts). Microcontrollers like Arduino can barely detect them, let alone make precise measurements.
El HX711 module It is a chip specifically designed for weighing systems with strain gauge load cells, facilitating accurate readings.
El HX711 works like instrumentation amplifier y 24-bit analog-to-digital converter (ADC)Its main function is to:
- Receive the differential signal from the Wheatstone bridge.
- Amplify it for easy reading.
- Transform it into a high-resolution digital signal so that the microcontroller can process it.
In addition, the HX711 features a simple digital interface with only 2 pins (Clock and Data), similar to the I2C bus, which simplifies connection and communication with software.
Features of the HX711 module
El HX711 It stands out for its high sensitivity and low cost. Some of its main features are:
- 24-bit precision to detect minimal weight changes.
- Integrated and programmable signal amplification (usually x128 or x64).
- Two independent analog input channels.
- Digital interface with 2 pins (Serial Data and Serial Clock).
- Feed 2,6V to 5,5V, compatible with Arduino and other microcontrollers.
- Low energy consumption.
The module usually comes with two rows of pins: one to connect to the load cell and one for the microcontroller.
These pins are usually labeled as: E+, E-, A+, A-, VCC, GND, DT, SCKTypical load cell cables are:
- Red: Positive excitation (E+ / VCC)
- Black: Negative excitation (E- / GND)
- White: Negative output (A-)
- Green: Positive output (A+)
Connection types and load cell variants
Standard load cell wiring is four-wire, although colors may vary depending on the manufacturer. Some inexpensive or recycled versions have three wires, while more advanced ones include a fifth yellow or blue wire for shielding or grounding.
In home projects, the most common load cells are 5 kg or 20 kg, although there are versions of up to 50 kg or more for industrial applications.
To connect several cells, such as in bathroom scales, a combiner module or manual connection, which requires electrical knowledge. It's important to note the direction of the arrow on the cell for correct measurement, ensuring that the central part remains free and the deformation is optimal.
Assembling a digital scale: materials and connections
To build a digital scale With strain gauges and HX711, you will need:
- A microcontroller (Arduino UNO, Nano, Mega, ESP8266, etc.).
- At least one load cell (1kg, 5kg, 20kg… as needed).
- An HX711 module.
- A rigid surface for the platform.
- Cables, connectors and screws.
Optionally, you can add:
- LCD screen or display to show the weight.
- Buttons for tare and mode.
- Supports or plates for the structure.
- Connectivity components such as WiFi or Bluetooth with ESP8266/ESP32.
The connections are simple:
- Connect the cell wires to the HX711 pins: Red to E+, Black to E-, White to A-, Green to A+.
- VCC and GND of the HX711 to the 5V and GND of the microcontroller.
- DT and SCK of HX711 to digital pins (example 3 and 2).
- Mount the cell on the structure, ensuring that only the central area remains free for correct measurement.
Programming with Arduino and calibrating the scale
To read the data, the Bogde's HX711 bookstore, available in the Arduino IDE Library Manager. Some key features include:
- begin(pinData, pinClock): start the module.
- task(s): sets zero weight in the tare function.
- set_scale(scale): defines the factor that converts readings into weight units.
- read() / read_average(n): get raw or average readings.
- get_value(n): returns the reading without tare weight.
- get_units(n): offers the weight adjusted with the scale and tare.
Calibration consists of placing a known weight, taking the reading, and calculating the scale factor: scale = reading / actual weight. It is then introduced into the code to adjust for future readings.
It is recommended to take multiple measurements and adjust the scale factor on the serial monitor to obtain accurate and stable results.
Sample programs for digital scale with HX711 and Arduino
A simple example showing weight on serial monitor would be:
#include "HX711.h" #define CALIBRATION 20780.0 // Replace with your own value byte pinData = 3; byte pinClk = 2; HX711 balance; void setup() { Serial.begin(9600); balance.begin(pinData, pinClk); balance.set_scale(CALIBRATION); balance.tare(); } void loop() { Serial.print("Current weight: "); Serial.print(balance.get_units(10), 1); Serial.println(" kg"); delay(500); }
The system can be enhanced by adding an LCD display, buttons, or storing the scale in EEPROM for quick and accurate calibrations, achieving a more professional experience.
Possible problems and practical advice
1. Variations in wire colors: Check the connections using the datasheet or by measuring resistances. Generally, the pair with the highest resistance corresponds to the excitation (+/-).
2. Incoherent readings: Swap the A+ and A- output leads if the measurements appear reversed or erratic.
3. Mechanical stability: Make sure you secure the cell correctly and that only the central part of the structure supports weight to avoid errors.
4. Noise and interference: Use short, shielded cables if possible, and move the system away from sources of electrical noise.
5. Temperature variations: Gauges are sensitive to thermal changes; if possible, perform calibrations under stable conditions or use cells with 4 gauges.
Expansion and possible applications of the system
With your operating system, you can add features:
- Display weight on an LCD screen.
- Set alerts for weight limits.
- Connect it to the cloud via ESP8266/ESP32 for remote monitoring.
- Use it in experiments, ingredient measurement, automation, home automation, etc.
The HX711 integration facilitates educational projects, inventory control, commercial scales, gas cylinder control, and many more creative ideas.
