What is a Load Cell?

A load cell is a transducer that converts force or pressure into an electrical signal. It consists of a spring element and strain gauges. When a force is applied to the load cell, the spring element deforms, causing the strain gauges to change their resistance. This change in resistance is proportional to the applied force and can be measured using a precision ADC like the HX711.

Load cells come in various types, including:

1. Beam load cells
2. S-type load cells
3. Button load cells
4. Single point load cells

Each type has its own characteristics and is suitable for different applications.

Load Cell Specifications

When selecting a load cell for your application, consider the following specifications:

Specification	Description
Capacity	The maximum load the cell can measure (e.g., 1kg, 5kg)
Rated Output	The electrical signal at full capacity (e.g., 1mV/V, 2mV/V)
Accuracy	The maximum error as a percentage of the rated output
Nonlinearity	The maximum deviation from a straight line
Hysteresis	The maximum difference between loading and unloading
Temperature Range	The operating temperature range of the load cell

The HX711 ADC

The HX711 is a specialized ADC designed to interface with load cells and bridge sensors. It has the following features:

• 24-bit resolution
• Selectable 10SPS or 80SPS output data rate
• Programmable gain amplifier (PGA) with selectable gain of 32, 64, or 128
• On-chip active low noise PGA
• On-chip power supply regulator for load-cell and ADC analog power supply
• Simple digital control and serial interface: pin-driven controls, no programming needed
• Simultaneous 50 and 60Hz supply rejection
• Current consumption including on-chip analog supply regulator:
• Normal operation < 1.5mA
• Power down < 1uA
• Operation supply voltage range: 2.6 ~ 5.5V
• Operation temperature range: -40 ~ +85°C

HX711 Pin Description

The HX711 has the following pins:

Pin	Name	Description
1	VSUP	Power supply (2.7~5.5V)
2	BASE	Regulate output voltage (NC when not used)
3	AVDD	Analog supply (2.6~5.5V)
4	VFB	Analog output (feedback to AVDD, NC when not used)
5	AGND	Ground for analog power supply
6	VBG	Analog output (NC when not used)
7	INA	Channel A negative input
8	INB	Channel B negative input
9	INC	Channel A positive input
10	IND	Channel B positive input
11	PD_SCK	Power down control (high active) and serial clock input
12	DOUT	Serial data output
13	XO	Crystal oscillator output (NC when not used)
14	XI	Crystal oscillator input (NC when not used)
15	RATE	Output data rate control (high for 80SPS, low for 10SPS)
16	DVDD	Digital supply (2.6~5.5V)

Connecting the HX711 to a Load Cell

To connect the HX711 to a load cell, follow these steps:

1. Connect the load cell’s E+ (excitation positive) wire to the HX711’s AVDD pin.
2. Connect the load cell’s E- (excitation negative) wire to the HX711’s AGND pin.
3. Connect the load cell’s S+ (signal positive) wire to the HX711’s INA pin.
4. Connect the load cell’s S- (signal negative) wire to the HX711’s INB pin.
5. If your load cell has additional wires (e.g., for temperature compensation), consult the load cell’s datasheet for proper connections.

Calibrating the Load Cell

Before using the load cell with the HX711, you must calibrate it to ensure accurate measurements. The calibration process involves applying known weights to the load cell and recording the corresponding ADC values. You can then use these values to calculate a calibration factor, which converts the ADC readings to weight measurements.

To calibrate the load cell:

1. Connect the HX711 and load cell as described in the previous section.
2. Place a known weight on the load cell.
3. Read the ADC value from the HX711.
4. Repeat steps 2 and 3 with different known weights.
5. Calculate the calibration factor using the following formula:
   calibration_factor = (known_weight_1 - known_weight_2) / (ADC_reading_1 - ADC_reading_2)

Once you have the calibration factor, you can use it to convert ADC readings to weight measurements:

weight = ADC_reading * calibration_factor

Using the HX711 with Arduino

To use the HX711 with an Arduino, you can use the HX711 Arduino library. Here’s a simple example sketch that demonstrates how to read weight measurements from a load cell:

#include "HX711.h"

const int LOADCELL_DOUT_PIN = 2;
const int LOADCELL_SCK_PIN = 3;

HX711 scale;

void setup() {
  Serial.begin(9600);
  scale.begin(LOADCELL_DOUT_PIN, LOADCELL_SCK_PIN);
  scale.set_scale(your_calibration_factor);
  scale.tare();
}

void loop() {
  if (scale.is_ready()) {
    float weight = scale.get_units();
    Serial.print("Weight: ");
    Serial.print(weight, 3);
    Serial.println(" kg");
  } else {
    Serial.println("HX711 not found.");
  }

  delay(500);
}

Make sure to replace your_calibration_factor with the calibration factor you calculated during the calibration process.

Troubleshooting

If you encounter issues while using the HX711 and load cell, consider the following:

1. Check your wiring connections to ensure they are correct and secure.
2. Verify that your load cell is compatible with the HX711 (check the load cell’s datasheet).
3. Ensure that your power supply is stable and provides enough current for the HX711 and load cell.
4. Recalibrate your load cell if measurements seem inaccurate.
5. Check for any physical damage to the load cell or HX711.

FAQs

1. Q: Can I use the HX711 with a 5V Arduino?
   A: Yes, the HX711 is compatible with 5V and 3.3V microcontrollers.

2. Q: How do I connect multiple load cells to one HX711?
   A: You can connect multiple load cells in parallel to one HX711. Make sure that the total capacity of the load cells does not exceed the HX711’s maximum input range.

3. Q: What is the maximum sampling rate of the HX711?
   A: The HX711 has two selectable output data rates: 10 samples per second (SPS) and 80 SPS.

4. Q: How can I improve the accuracy of my weight measurements?
   A: To improve accuracy, ensure that your load cell is properly calibrated, use a stable power supply, and consider using noise-reduction techniques like averaging multiple readings or applying a digital filter.

5. Q: Can I use the HX711 with a Raspberry Pi?
   A: Yes, you can use the HX711 with a Raspberry Pi using libraries like the HX711 Python Library.

In conclusion, the HX711 and load cells form a powerful combination for creating accurate and reliable weight measurement systems. By understanding the HX711’s features, properly connecting and calibrating your load cell, and following best practices, you can build a LoadCellGuide that meets your application’s requirements.