DMS load cell with measuring amplifier HX711AD

fig. 1: 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

Table 1: Requirements for the Unit

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

Figure 1: Structure and deformation of a load cell. Source: https://learn.sparkfun.com/tutorials/getting-started-with-load-cells

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

Figure 2: Wheatstone bridge configuration. Source: https://learn.sparkfun.com/tutorials/getting-started-with-load-cells

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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

Figure 3: HX711 module and pin configuration. Source: https://justdoelectronics.com/load-cell-hx711-module/

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)

Figure 4: Source: https://cdn.sparkfun.com/datasheets/Sensors/ForceFlex/hx711_english.pdf

Related Links

• HX711 based weight scale

SVN-Repository

https://svn.hshl.de/svn/HSHL_Projekte/trunk/Arduino_Sensorsammlung

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