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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

sch_mpr121_ar

Wiring Diagram

wiring_mpr121_ar

  • 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

Let’s write a MicroPython program to detect touch inputs on the electrodes and print out which ones are touched.

Note

  • Open the 4.3_electrode_keyboard.py from newton-lab-kit/micropython or copy the code into Thonny, then click “Run” or press F5.

  • Ensure the correct interpreter is selected: MicroPython (Raspberry Pi Pico).COMxx.

  • Here you need to use the library called mpr121.py, please check if it has been uploaded to Pico, for a detailed tutorial refer to Upload the Libraries to Pico.

from mpr121 import MPR121
from machine import Pin, I2C
import utime

i2c = I2C(0, sda=Pin(4), scl=Pin(5))
mpr = MPR121(i2c)

# check all keys
while True:
    value = mpr.get_all_states()
    if len(value) != 0:
        print(value)
    utime.sleep_ms(100)

After running the program, touch the connected electrodes or conductive objects. Observe the printed output in the Thonny Shell. You should see messages indicating which electrodes are being touched.

Understanding the Code

  1. Import Modules:

    • machine: Provides access to hardware-related functions.

    • mpr121: The library to interface with the MPR121 sensor.

    • utime: Contains time-related functions for delays.

  2. Initialize I2C Communication:

    • i2c = I2C(0, sda=Pin(4), scl=Pin(5)): Sets up I2C communication on I2C0 bus using GP4 (SDA) and GP5 (SCL).

  3. Create an MPR121 Object:

    • mpr = MPR121(i2c): Initializes the MPR121 sensor using the I2C communication established.

  4. Main Loop to Detect Touch Inputs:

    • get_all_states(): Returns a list of electrode numbers that are currently being touched.

    • If any electrodes are touched, it prints out their numbers.

    • The loop runs continuously with a short delay of 100 milliseconds.

    while True:
    value = mpr.get_all_states()
    if len(value) != 0:
        print(value)
    time.sleep_ms(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.

Experimenting Further

  • Detecting a Specific Electrode:

    If you want to monitor a specific electrode, you can use the is_touched(pin) method, it will return True if the specified electrode (pin) is being touched; otherwise, it returns False.

    from mpr121 import MPR121
from machine import Pin, I2C
import utime

i2c = I2C(0, sda=Pin(4), scl=Pin(5))
mpr = MPR121(i2c)

# check all keys
while True:
  if mpr.is_touched(0):
      print("Electrode 0 is touched!")
  utime.sleep(0.1)

  • Create a Musical Instrument: Map each electrode to a musical note and play sounds when touched.

  • Interactive Art: Use conductive paint to create touch-sensitive artworks.

  • Game Controller: Design custom touch controls for a game.

Conclusion

In this lesson, you’ve learned how to use the MPR121 capacitive touch sensor with the Raspberry Pi Pico 2 to create a touch-sensitive electrode keyboard. This opens up possibilities for interactive projects that respond to touch inputs in creative ways.