Digital Load Cell Guide: Understanding Code and Calibration

Digital load cells are essential components for measuring force and weight in various applications. Understanding the code and calibration processes is crucial for accurate data collection and analysis. This guide provides an in-depth look at the code behind a digital load cell used in science projects, along with tips for modifying and optimizing it for your specific needs. Whether you're a teacher or a student, you'll gain insights into how these sensors work and how to use them effectively.

The Importance of Code Extensions

When working with digital load cells, you'll quickly notice that certain blocks of code are not part of the standard MicroCode library. These blocks, often displayed in a unique color

, are associated with specific extensions necessary for the load cell to function correctly. Without these extensions, the code will not be able to communicate with the load cell and interpret its data.

These extensions contain predefined functions and variables that handle the specific communication protocols and data formats used by the digital load cell. This abstraction allows you to focus on the higher-level logic of your project without needing to delve into the low-level technical details of the sensor. Understanding how to incorporate and utilize these extensions is crucial for effectively integrating a digital load cell into your projects. Key components in your digital load cell setup include:

• HX711 Load Cell Amplifier: This amplifier is crucial for amplifying the small signals from the load cell so they can be read by the micro:bit. The HX711 is a precision 24-bit analog-to-digital converter (ADC) specifically designed for weigh scales and industrial control applications to interface directly with a bridge sensor.
• Micro:bit Board: Acts as the main control unit, running the code to read signals from the HX711 and performing necessary calculations or displaying the results. The micro:bit’s processing power and connectivity make it an excellent choice for educational science projects involving load cells.
• Load Cell: Converts force into a measurable electrical signal. Different types of load cells exist, including S-type, cantilever, and button load cells, each suited for different application requirements.
• Connecting Wires: Used to connect the load cell amplifier to the micro:bit, ensuring data transfer and power supply.

Keywords: Digital load cell, micro:bit, code extensions, HX711 amplifier, science projects

Setting Up the Code Environment

Before diving into the code itself, it's essential to set up your coding environment properly. In MicroCode, this means adding the necessary extensions to your project. These extensions provide the specific blocks needed to interact with the digital load cell.

To add an extension, navigate to the extensions menu in MicroCode and search for the appropriate extension for your load cell. Once added, you'll find new blocks available in the toolbox, typically categorized under a specific name related to the load cell or its manufacturer.

These blocks allow you to set scale pins, read scale data, and perform other essential functions.

The basic steps to setting up the code environment include:

1. Opening MicroCode: Navigate to the MicroCode editor in your web browser.
2. Adding the Extension: Click on the 'Extensions' tab in the editor, then search and add the extension for the digital load cell. This extension usually contains custom blocks for interacting with the load cell.
3. Defining Scale Pins: Use the 'set scale pins' block to define which pins on the micro:bit are connected to the load cell. This step is crucial for establishing communication between the micro:bit and the load cell amplifier.

It's also worth noting that not all extensions are available in the default extension store. Some may require manual import from a URL provided by the manufacturer or community. This process involves copying the URL into the extension search bar and importing the extension from that location.

Keywords: MicroCode, coding environment, extensions, adding extensions, set scale pins

Initial Configuration: On Start Block

The 'on start' block in MicroCode is where you place code that needs to run once when the micro:bit is powered on

. For digital load cells, this block is used for initial configuration and setup.

Typical tasks performed in the 'on start' block include:

• Displaying an Icon: Show an icon to indicate that the setup process has started. This provides visual feedback that the micro:bit is functioning correctly.
• Setting Scale Pins: Define the pins on the micro:bit that are connected to the load cell. This step is essential for establishing communication between the micro:bit and the load cell amplifier.
• Zeroing the Scale: Read the scale with no load to establish a baseline. This helps to eliminate any initial offset and improve accuracy.

The 'set scale pins' block requires you to specify the pins for the clock and data lines of the load cell. Consult your load cell documentation to determine the correct pin assignments. Once the pins are set, the 'read scale with zero load' block is used to establish a zero point. This process is akin to taring a traditional scale.

        Example On Start configuration:
        show icon [check mark]
        set scale pins (SCK: Pin A, DT: Pin B)
        read scale with zero load

Keywords: on start block, initial configuration, set scale pins, zeroing the scale, MicroCode

Continuous Mass Reading: The Forever Loop

The 'forever' loop in MicroCode allows you to run a set of instructions continuously

. For digital load cells, this loop is used to read the mass from the load cell and update a variable with the current value.

Inside the 'forever' loop, you'll typically find the following:

• Reading the Scale: Use the 'read scale in grams' block to get the current mass reading from the load cell.
• Setting a Variable: Assign the mass reading to a variable, such as 'mass'. This allows you to use the mass value in other parts of your code.

        Example Forever Loop:
        set mass to read scale in grams

The 'read scale in grams' block reads the raw data from the load cell and converts it into a mass value in grams. This conversion is handled by the extension and takes into account the calibration settings of the load cell. Assigning this value to a variable allows you to display the mass on the micro:bit screen, perform calculations, or use it in other parts of your project.

Keywords: forever loop, continuous reading, read scale in grams, setting variables, mass reading

Button-Triggered Actions

The micro:bit has two buttons, A and B, which can be used to trigger specific actions in your code

. For digital load cells, these buttons can be used to display mass, display force, or perform other functions.

Common button-triggered actions include:

• Displaying Mass: When button A is pressed, display the current mass reading on the micro:bit screen.
• Displaying Force: When button B is pressed, calculate and display the force acting on the load cell.
• Calibrating the Load Cell: When both buttons A and B are pressed, initiate the calibration process.

        Example Button Actions:
        on button A pressed:
            show number mass
        on button B pressed:
            set force to mass * 0.0098
            show number force

The code for these actions typically involves using the 'on button pressed' block, along with blocks to display numbers, perform calculations, or initiate other processes. The 'set force to mass * 0.0098' line calculates the force in newtons based on the mass in grams. The conversion factor, 0.0098, is an approximation of the acceleration due to gravity (9.8 m/s^2 divided by 1000 to convert grams to kilograms).

Keywords: button-triggered actions, on button pressed, displaying mass, displaying force, calibrating the load cell

Calibration Process: Ensuring Accuracy

Calibration is a crucial step in ensuring the accuracy of your digital load cell. Calibration involves comparing the load cell's readings to known weights and adjusting the code to compensate for any errors

.

To calibrate the load cell, you'll need a set of known weights. The code typically includes a calibration routine that prompts you to place a known weight on the load cell and press a button. The code then reads the load cell's output and calculates a calibration factor to adjust future readings.

        Example Calibration Routine:
        on button A+B pressed:
            calibrate with 50g load
            show number mass

The 'calibrate with 50g load' block initiates the calibration process. This block reads the load cell's output when a 50-gram weight is placed on it and calculates a calibration factor to adjust future readings. Calibration may not be needed, but its a great function to have ready. Ensure your system doesn't suffer from drift or non-linearity. Drift refers to the slow deviation of the output signal over time when a constant load is applied, and non-linearity means that the output signal does not change linearly with the applied load.

It's important to note that calibration should be performed regularly to maintain accuracy. Factors such as temperature, humidity, and mechanical stress can affect the load cell's output over time.

Keywords: calibration process, ensuring accuracy, known weights, calibration routine, calibration factor

Zeroing the Scale: Eliminating Offset

Zeroing the scale is another important step in ensuring accuracy. Zeroing involves reading the load cell with no load applied and subtracting this value from future readings

. This eliminates any offset and ensures that the load cell reads zero when no weight is applied.

To zero the scale, you'll typically use a block such as 'read scale with zero load'. This block reads the load cell's output with no weight applied and stores this value in a variable. This value is then subtracted from future readings to compensate for any offset.

        Example Zeroing:
        on logo pressed:
            read scale with zero load

The 'on logo pressed' block allows you to trigger the zeroing process by pressing the micro:bit logo. The 'read scale with zero load' block then reads the load cell's output with no weight applied and stores this value in a variable. This value is then subtracted from future readings to compensate for any offset. You can change how long you need to press the logo from the dropdown menu if you accidentally press it too often!

Keywords: zeroing the scale, eliminating offset, read scale with zero load, micro:bit logo, accuracy

Handling Common Issues with Digital Load Cells

When working with digital load cells, you may encounter issues such as signal noise, drift, and non-linearity. Addressing these issues is crucial for obtaining accurate and reliable measurements.

One common problem is signal noise, which can be caused by electrical interference or poor connections. To minimize signal noise, ensure all connections are secure and properly shielded. You can also add a low-pass filter to the code to smooth out the signal and reduce the impact of noise.

Drift refers to the slow deviation of the output signal over time when a constant load is applied. Drift can be caused by temperature changes, mechanical stress, or aging of the load cell. To minimize drift, calibrate the load cell regularly and ensure it is operating within its specified temperature range.

Non-linearity means that the output signal does not change linearly with the applied load. Non-linearity can be caused by manufacturing imperfections or overloading the load cell. To minimize non-linearity, use a load cell that is appropriate for the expected range of loads and avoid exceeding its maximum capacity.

Here is a table summarizing common issues and solutions:

Keywords: signal noise, drift, non-linearity, troubleshooting, digital load cell

Customizing the Code for Advanced Functionality

The provided code serves as a starting point for your digital load cell projects. You can customize the code to add advanced functionality and tailor it to your specific needs.

Some possible customizations include:

• Adding a Display Filter: Implement a moving average filter to smooth out the mass readings and reduce noise. This can improve the stability and readability of the display.
• Implementing a Tare Function: Add a button-triggered tare function that allows you to zero the scale with a load applied. This can be useful for measuring the weight of a container without including its own weight.
• Adding a Data Logging Function: Implement a data logging function that records the mass readings over time. This can be useful for analyzing trends and patterns in the data.
• Integrating with Other Sensors: Combine the load cell with other sensors, such as temperature or humidity sensors, to create a more comprehensive measurement system.

To add these customizations, you'll need to use additional blocks and functions in MicroCode. For example, to implement a moving average filter, you can use the 'average' block to calculate the average of a set of mass readings. To implement a tare function, you can use the 'read scale with zero load' block to zero the scale with a load applied. To implement a data logging function, you can use the 'serial write line' block to send the mass readings to a computer or other device.

Keywords: customizing the code, advanced functionality, display filter, tare function, data logging

Step 1: Assembling the Hardware

First, ensure all necessary hardware components are available: a digital load cell, an HX711 amplifier module, a micro:bit board, and connecting wires. Connect the load cell to the HX711 amplifier, paying close attention to the wiring diagram provided with the load cell. Typically, this includes connecting the excitation voltage (VCC), ground (GND), and signal wires (A+ and A- or similar). Then, connect the HX711 amplifier to the micro:bit using connecting wires. The data (DT) and clock (SCK) pins on the HX711 should be connected to digital pins on the micro:bit (e.g., P0 and P1). This ensures that you are set up to measure mass or force accurately.

Step 2: Importing Necessary Extensions in MicroCode

Open the MicroCode editor in your web browser. Click on the 'Extensions' tab, usually located in the left sidebar of the editor. Search for the extension that corresponds to your digital load cell or the HX711 amplifier module (if there is one). Add the extension to your project by clicking on it. This will add custom blocks necessary for interacting with the load cell.

Step 3: Configuring Scale Pins

Use the 'set scale pins' block to define which pins on the micro:bit are connected to the load cell. Specify the pins for the clock (SCK) and data (DT) lines based on your wiring configuration (e.g., SCK: P0, DT: P1). Place this block inside the 'on start' block to ensure that the pin configuration is set when the micro:bit is powered on.

Step 4: Zeroing the Scale

Read the scale with no load to establish a baseline and eliminate any initial offset. Use the 'read scale with zero load' block inside the 'on start' block. This will set the current reading as the zero point for future measurements.

Step 5: Reading and Displaying Mass Continuously

Place the 'read scale in grams' block inside the 'forever' loop. This ensures that the mass is continuously read from the load cell. Assign the mass reading to a variable, such as 'mass,' using the 'set mass to read scale in grams' block. Use the 'show number mass' block inside the 'on button A pressed' block to display the current mass reading on the micro:bit screen when button A is pressed.

Step 6: Calculating and Displaying Force

To calculate the force acting on the load cell, multiply the mass by the acceleration due to gravity (approximately 0.0098 N/g). Use the 'set force to mass * 0.0098' block inside the 'on button B pressed' block. Display the calculated force on the micro:bit screen using the 'show number force' block. This allows you to switch between displaying mass and force by pressing buttons A and B, respectively.

Step 7: Calibrating the Load Cell

Use a known weight to calibrate the load cell and improve accuracy. Place the 'calibrate with 50g load' block inside the 'on button A+B pressed' block. Follow the instructions to place a 50-gram weight on the load cell and press buttons A and B simultaneously to initiate the calibration process. This will adjust the calibration factor to ensure accurate mass readings. The 50g is just an example, any load cell could be selected.