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 |
Working principle
This section explains how the DMS load cell and the HX711AD convert an applied force into a digital signal.
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
Measurement
Video
Datasheets
Related Links
SVN-Repository
https://svn.hshl.de/svn/HSHL_Projekte/trunk/Arduino_Sensorsammlung
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