Electronic Weight Scale Shield For OLEDUINO-v2

This compact shield is based on the HX711 ADC, specifically designed to interface load cells and strain gauge sensors. It can be paired with the OLEDUINO-V2, enabling the creation of a miniature electronic weighing scale.

The HX711 load cell amplifier shield allows users to accurately measure data from load cells and strain gauges. At the core of this design is the HX711 chip—a precision 24-bit analog-to-digital converter (ADC) developed for weigh scale and industrial control applications. It provides a direct interface to bridge sensors, simplifying system design.

The device features an input multiplexer that selects between Channel A and Channel B differential inputs, feeding into a low-noise programmable gain amplifier (PGA). Channel A supports programmable gains of 128 or 64, corresponding to full-scale differential input voltages of ±20 mV or ±40 mV, respectively, when powered with a 5V analog supply (AVDD). Channel B offers a fixed gain of 32.

An integrated power supply regulator eliminates the need for an external analog regulator, while the on-chip oscillator removes the requirement for external clock components. Additionally, built-in power-on-reset circuitry simplifies digital interface initialization.

No internal register programming is required, as all control and communication with the HX711 are handled directly through the header connector, making the shield easy to use and integrate into projects.

Application Example

Features

• Operation supply voltage range: 5V (2.6 ~ 5.5V)
• On-chip active low noise PGA with selectable gain of 32, 64 and 128
• On Board Power LED
• Header Connectors for Inputs (Sensor)
• On-chip power supply regulator for load-cell Sensor
• ADC analog power supply
• On-chip oscillator requiring no external component with optional external crystal
• On-chip power-on-reset
• Simple digital control and serial interface: pin-driven controls, no programming needed
• Selectable 10SPS or 80SPS output data rate (Jumper J1)
• Simultaneous 50 and 60Hz supply rejection
• Two selectable differential input channels
• Current consumption including on-chip analog power supply regulator: normal operation < 1.5mA, power down < 1uA
• PCB Dimensions 25.4 x 25.4 mm

Compatible with OLEDUINO v.2

https://www.electronics-lab.com/project/oleduino-v-2/

HX711 Details

RATE Jumper (J1)

Jumper J1 provided select the data rate for the HX711. If jumper is solder to High, then the data rate is 80SPS If the jumper selects L, then the data rate is 10 SPS.

Analog Inputs

Channel A differential input is designed to interface directly with a bridge sensor’s differential output. It can be programmed with a gain of 128 or 64. The large gains are needed to accommodate the small output signal from the sensor. When 5V supply is used at the AVDD pin, these gains correspond to a full-scale differential input voltage of ±20mV or ±40mV respectively.  Channel B differential input has a fixed gain of 32. The full-scale input voltage range is ±80mV, when 5V supply is used at the AVDD pin.

Output Data Rate and Format

When using the on-chip oscillator, output data rate is typically 10 (RATE=0) or 80SPS (RATE=1).  When using external clock or crystal, output data rate is directly proportional to the clock or crystal frequency. Using 11.0592MHz clock or crystal results in an accurate 10 (RTE=0) or 80SPS (RATE=1) output data rate.  The output 24 bits of data is in 2’s complement format. When input differential signal goes out of the 24-bit range, the output data will be saturated at 800000h (MIN) or 7FFFFFh (MAX), until the input signal comes back to the input range.

Power Supply Options

Digital power supply (DVDD) should be the same power supply as the MCU power supply.  When using internal analog supply regulator, the dropout voltage of the regulator depends on the external transistor used. The output voltage is equal to VAVDD=VBG*(R1+R2)/ R1 (Fig. 1). This voltage should be designed with a minimum of 100mV below VSUP voltage.

Serial Interface Pin PD_SCK and DOUT are used for data retrieval, input selection, gain selection and power down controls.  When output data is not ready for retrieval, digital output pin DOUT is high. Serial clock input PD_SCK should be low. When DOUT goes to low, it indicates data is ready for retrieval. By applying 25~27 positive clock pulses at the PD_SCK pin, data is shifted out from the DOUT output pin. Each PD_SCK pulse shifts out one bit, starting with the MSB bit first, until all 24 bits are shifted out. The 25th pulse at PD_SCK input will pull DOUT pin back to high.

Arduino Code

This Arduino code is provided for testing purposes and uses pins D2 and D3 for interfacing. It enables users to build a mini weight scale system. The code can be customized, and calibration values can be adjusted based on the characteristics of the sensor.

Code Credits: https://www.hackster.io/jack100

Connections

• CN1 Load Cell Sensor: Pin 1 = AV/+Voltage Red Wire, Pin 2 = Sensor A- Green Wire, Pin 3 = Sensor A+ White Wire, Pin 4 = Sensor B-, Pin 5 = Sensor B+, Pin 6 = GND Black Wire
• D1: Power LED
• J1: Jumper = Rate Jumper , High=80 SPS, Low=10 SPS
• HX711 D=Out = Arduino D3, HX711 SCK = Arduino D2

Schematic

Parts List

Connections

Gerber View

Photos

HX711 Datasheet