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4.3 Electrode Keyboard with MPR121

In this lesson, we’ll learn how to use the MPR121 capacitive touch sensor to create a touch-sensitive keyboard with the Raspberry Pi Pico 2. The MPR121 allows you to detect touch inputs on up to 12 electrodes, which can be connected to conductive materials like wires, foil, or even fruits like bananas!

What You’ll Need

In this project, we need the following components.

It’s definitely convenient to buy a whole kit, here’s the link:

Name	ITEMS IN THIS KIT	LINK
Newton Lab Kit	450+	Newton Lab Kit

You can also buy them separately from the links below.

SN	COMPONENT	QUANTITY	LINK
1	Raspberry Pi Pico 2	1	BUY
2	Micro USB Cable	1
3	Breadboard	1	BUY
4	Jumper Wires	Several	BUY
5	MPR121 Module	1

Understanding the MPR121 Sensor

The MPR121 is a capacitive touch sensor controller that communicates via the I2C interface. It can handle up to 12 touch inputs, making it ideal for creating interactive projects with multiple touch points.

The MPR121 sensor detects changes in capacitance on its electrodes. When you touch an electrode, the capacitance changes, and the sensor registers a touch.The sensor communicates this information over I2C to the Raspberry Pi Pico 2.

Circuit Diagram

Wiring Diagram

• Connect wires or conductive materials to the electrode pins (labeled E0 to E11) on the MPR121.

• You can attach the other ends of the wires to conductive objects like fruits, aluminum foil shapes, or touchpads.

Writing the Code

Note

• You can open the file 4.3_electrode_keyboard.ino from newton-lab-kit/arduino/4.3_electrode_keyboard.

• Or copy this code into Arduino IDE.

• Select the Raspberry Pi Pico 2 board and the correct port, then click “Upload”.

• The Adafruit MPR121 library is used here, you can install it from the Library Manager.

  

#include <Wire.h>
#include <Adafruit_MPR121.h>

// Create an instance of the MPR121 sensor
Adafruit_MPR121 cap = Adafruit_MPR121();

// Array to hold the touch states of each electrode
bool touchStates[12] = { false };

// Variables to store current and last touch states
uint16_t currtouched = 0;
uint16_t lasttouched = 0;

void setup() {
  Serial.begin(115200); // Initialize serial communication at 115200 baud
  while (!Serial);    // Wait for Serial Monitor to open

  // Initialize the MPR121 sensor with I2C address 0x5A
  if (!cap.begin(0x5A)) {
    Serial.println("MPR121 not found, check wiring?");
    while (1);
  }
  Serial.println("MPR121 found!");
}

void loop() {
  // Get the currently touched pads
  currtouched = cap.touched();

  // Check if there is a change in touch state
  if (currtouched != lasttouched) {
    // Update the last touched state
    lasttouched = currtouched;

    // Iterate through each electrode
    for (int i = 0; i < 12; i++) {
      // Check if the electrode is touched
      if (currtouched & (1 << i)) {
        touchStates[i] = true;
      } else {
        touchStates[i] = false;
      }
    }

    // Print the touch states as a binary string
    for (int i = 0; i < 12; i++) {
      Serial.print(touchStates[i] ? "1" : "0");
    }
    Serial.println();
  }

  delay(100); // Small delay to stabilize readings
}

After uploading the code, touch the electrodes connected to the MPR121 sensor.

• Observe the binary output in the Serial Monitor indicating which electrodes are being touched.

• For example, touching the first and eleventh electrodes will display 100000000010.

Understanding the Code

1. Including Libraries:

   • Wire.h: Handles I2C communication.

   • Adafruit_MPR121.h: Provides functions to interact with the MPR121 sensor.

2. Initializing the MPR121 Sensor:

   Creates an instance of the MPR121 sensor.

   Adafruit_MPR121 cap = Adafruit_MPR121();
   

3. Setup Function:

   • Starts serial communication for debugging.

   • Initializes the MPR121 sensor with the I2C address 0x5A.

   • If the sensor is not found, it prints an error message and halts the program.

   void setup() {
     Serial.begin(115200); // Initialize serial communication
     while (!Serial);    // Wait for Serial Monitor to open
   
     // Initialize the MPR121 sensor with I2C address 0x5A
     if (!cap.begin(0x5A)) {
       Serial.println("MPR121 not found, check wiring?");
       while (1);
     }
     Serial.println("MPR121 found!");
   }
   

4. loop() Function:

   • Retrieves the current touch state from the MPR121 sensor. Each bit in the currtouched variable represents the touch state of an electrode (1 for touched, 0 for not touched).

     currtouched = cap.touched();
     

   • Checks if there has been a change in the touch state since the last loop iteration.

     if (currtouched != lasttouched) {
       // Update touch states
     }
     

   • Iterates through each electrode and updates the touchStates array based on whether each electrode is touched.

     for (int i = 0; i < 12; i++) {
       if (currtouched & (1 << i)) {
         touchStates[i] = true;
       } else {
         touchStates[i] = false;
       }
     }
     

   • Prints the touch states as a 12-bit binary string to the Serial Monitor. For example, if the first and eleventh electrodes are touched, it will print 100000000010.

     for (int i = 0; i < 12; i++) {
       Serial.print(touchStates[i] ? "1" : "0");
     }
     Serial.println();
     

   • Adds a short delay to stabilize the readings and prevent flooding the Serial Monitor.

     delay(100);
     

Extending the Electrodes

You can enhance your project by connecting the electrodes to various conductive materials:

• Fruits: Attach wires to bananas, apples, or other fruits to turn them into touch-sensitive inputs.

• Foil Shapes: Cut shapes out of aluminum foil and attach them to the electrodes.

• Conductive Paint: Draw patterns with conductive ink or paint.

Note

If you change the electrodes (e.g., connect different materials), you may need to reset the sensor to recalibrate the baseline values.

Further Exploration

• Creating Interactive Projects:

  Build a touch-controlled LED matrix where each electrode controls an individual LED.

• Implementing Key Debouncing:

  Enhance the reliability of touch detection by implementing debouncing techniques to filter out false touches.

• Combining with Other Sensors:

  Integrate the MPR121 with other sensors like temperature or light sensors to create more complex interactive systems.

• Developing a Touch-Based Game Controller:

  Use the touch inputs to control game elements, such as moving characters or selecting options.

Conclusion

In this lesson, you’ve learned how to use the MPR121 capacitive touch sensor with the Raspberry Pi Pico to create a touch-sensitive keyboard. By detecting touch inputs on multiple electrodes, you can build interactive interfaces for your projects, such as custom keypads, control panels, or creative input devices. Understanding how to read and process touch inputs is a valuable skill for developing responsive and user-friendly electronics projects.