DMS load cell with measuring amplifier HX711AD
| Autor: | Onyesi John Abiagam |
| Sprache: | DE EN |
Introduction
A weighing scale is a measurement system used to determine the mass of an object by converting mechanical force into a readable value.
This system is based on a DMS load cell in combination with the HX711AD measuring amplifier. The applied force is converted into an electrical signal, which is processed and displayed as a weight value.
The signal is acquired using a microcontroller-based system and processed using Simulink. Calibration, signal filtering, stability detection, and tare functionality are implemented to ensure accurate and stable measurements.
Requirements
| Req. | Description | Priority |
|---|---|---|
| 1 | The mass must be determined using the HX711AD load cell via Arduino and Simulink. | 1 |
| 2 | The measuring range must be determined. | 1 |
| 3 | The measurement uncertainty (1σ) must be determined for the measuring range and displayed as a confidence interval. | 2 |
| 4 | The on/off button starts the system. The sensor must first be calibrated and display 0 g. |
1 |
| 5 | The weight must be referenced for the measuring range. | 1 |
| 6 | The measured values must be filtered/smoothed over time. | 1 |
| 7 | A beep must indicate when the measured value is stable/constant. | 1 |
| 8 | The sensor system must display the weight in g on a display. | 1 |
| 9 | The tara button resets the current weight to 0 g (recalibration). |
1 |
Required Components
| Component | Function |
|---|---|
| DMS Load Cell (1 kg) | Converts mechanical force into a small electrical signal |
| HX711AD Measuring Amplifier | Amplifies the tiny load-cell signal and converts it into a 24-bit digital value |
| Microcontroller (Arduino Uno R3) | Central control unit that reads sensor data, processes signals, and controls peripherals such as the display, pushbutton, and buzzer |
| LCD Display (I2C) | Displays the measured weight |
| Pushbuttons | Provide user input for system control functions such as tare (zeroing) and power operation |
| Buzzer | Provides feedback sound when the weight becomes stable |
Working principle
This section describes the complete working principle of the system, including force sensing with the DMS load cell, signal amplification and digitization using the HX711AD, and data processing by the microcontroller. Additionally, it covers user interaction via pushbuttons and the output of measurement results through the display and feedback sound using the buzzer.
DMS Load Cell
Introduction
A DMS load cell converts mechanical force into an electrical signal. It consists of:
- a deformable metal body
- strain gauges attached to the surface
- resistance changes when stretched or compressed
Working Principle
When force is applied:
- the metal body deforms
- strain gauges change resistance
- gauges are wired in a Wheatstone bridge
- deformation → bridge becomes unbalanced
- output: small differential voltage
---
HX711AD Measuring Amplifier
Introduction
The HX711AD processes the small differential signal from the load cell. It includes:
- a programmable gain amplifier (PGA)
- a 24‑bit sigma‑delta ADC
Working Principle
Signal processing steps:
- load cell outputs a millivolt‑level differential signal (A+, A−)
- PGA amplifies the signal
- 24‑bit ADC converts it to digital
- microcontroller reads data via:
- DOUT (data)
- SCK (clock)
Technical Overview
- The system consists of a DMS load cell, an HX711AD measuring amplifier, a microcontroller, an LCD, a buzzer, and two push buttons (power and tare).
- The load cell converts the applied force into an electrical signal, which is amplified and digitized by the HX711AD.
- The digital data is read and processed by the microcontroller.
- The measured weight is displayed on the LCD.
- A buzzer indicates when a stable measurement is reached.
- The power button controls the system state, and the tare button resets the measured value to 0 g.
Pin Assignment
3.1 Pin Assignment
This section describes the electrical connections between the load cell, the HX711AD,the buzzer,LCD and push buttons and the microcontroller.
Load Cell to HX711
| Load Cell Wire | Function | HX711 Pin | Description |
|---|---|---|---|
| Red | Excitation + (E+) | E+ | Supplies voltage to the bridge |
| Black | Excitation − (E−) | E− | Ground reference |
| White | Signal + (S+) | A+ | Positive input signal |
| Green | Signal − (S−) | A− | Negative input signal |
| Yellow (optional) | Shield | GND | EMI protection |
HX711 to Microcontroller
| HX711 Pin | Function | Microcontroller Pin | Description |
|---|---|---|---|
| VCC | Power Supply | 5V / 3.3V | Power input |
| GND | Ground | GND | Common reference |
| DOUT | Data Output | Digital Input | Sends measurement data |
| PD_SCK | Clock Input | Digital Output | Controls data read |
Microcontroller Peripherals
| Component | Function | Microcontroller Pin | Description |
|---|---|---|---|
| LCD | Display output | I2C (SDA/SCL) | Displays measured weight |
| Buzzer | Acoustic feedback | Digital Output | Indicates stable measurement |
| Push Button (Power) | System control | Interrupt Pin (e.g., D2) | Starts or stops the system |
| Push Button (Tare) | Reset function | Interrupt Pin (e.g., D3) | Resets measured value to 0 g |
Measurement method
- The raw digital value from the HX711 is converted into a weight value using calibration parameters.
Weight = (Raw value − Offset) × Scale factor
- To reduce noise, a recursive average filter is applied:
y[n] = α · x[n] + (1 − α) · y[n−1]
where:
- x[n] is the current measurement
- y[n] is the filtered value
- α is the filter coefficient (0 < α < 1)
- The stability of the measurement is evaluated using the standard deviation:
σ = sqrt( (1/N) · Σ (x_i − μ)^2 )
where:
- μ is the mean value of the samples
- N is the number of samples
- A measurement is considered stable when the standard deviation falls below a defined threshold.
- The final result is expressed as:
Weight = μ ± σ
Where:
- μ: mean value
- σ: standard deviation (measurement uncertainty)
Measuring Circuit
Components
- DMS load cell ( 1Kg)
- HX711AD measuring amplifier
- Microcontroller (Arduino Uno R3)
- LCD (I2C)
- Buzzer
- Push button (power)
- Push button (tare)
- Breadboard
- Jumper wires
Circuit Description
- The load cell is connected to the HX711AD measuring amplifier using a Wheatstone bridge configuration.
- The HX711AD is powered by the microcontroller and shares a common ground with all components.
- The HX711AD transmits digital measurement data to the microcontroller via the DOUT and PD_SCK pins.
- The LCD is connected to the microcontroller via the I2C interface (SDA and SCL).
- A buzzer is connected to a digital output pin to provide feedback sound.
- Two push buttons are connected to interrupt-capable pins:
- Power button: controls the system state
- Tare button: resets the measured value to 0 g
Software
Arduino IDE
Simulink
The DMS load cell with measuring amplifier HX711AD was further modelled in Simulink to simulate the signal acquisition, filtering, and standard deviation-based stability detection of the weighing system.
Measurement
Video
Datasheets
Related Links
SVN-Repository
https://svn.hshl.de/svn/HSHL_Projekte/trunk/Arduino_Sensorsammlung
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