How to Use MPR121 Capacitive Touch Keypad - Backside: Examples, Pinouts, and Specs

The MPR121 Capacitive Touch Keypad is an innovative electronic component that utilizes capacitive sensing technology to detect touch or proximity. This keypad is designed to interface with a wide range of microcontrollers, including the popular Arduino platform, making it an ideal choice for adding touch input to your projects. Common applications include touch-based user interfaces, interactive installations, and custom keyboards.

• Operating Voltage: 2.5V to 3.6V
• Supply Current: 29uA (run mode), 3uA (sleep mode)
• Number of Inputs: 12 capacitive touch inputs
• Communication: I2C interface
• Response Time: < 30ms
• Sensitivity: Adjustable with internal registers

1. Connecting the Keypad:

   • Connect VDD to the 3.3V output on the Arduino.
   • Connect VSS to the ground (GND).
   • Connect SDA and SCL to the corresponding I2C pins on the Arduino.
   • Optionally, connect IRQ to an interrupt-capable pin on the Arduino.
   • If address selection is needed, connect ADDR to GND or VCC.

2. Library Installation:

   • Install the MPR121 library through the Arduino Library Manager or download it from a reputable source.

3. Programming the Arduino:

   • Include the MPR121 library in your sketch.
   • Initialize the keypad and check for successful communication.
   • Read touch states and respond to touch events in your code.

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

// You can have up to 4 on one i2c bus (connecting ADDR to different pins),
// but this example will just use one
Adafruit_MPR121 cap = Adafruit_MPR121();

void setup() {
  Serial.begin(9600);
  while (!Serial) { } // Wait for serial console to open, if necessary

  if (!cap.begin(0x5A)) {
    Serial.println("MPR121 not found, check wiring?");
    while (1);
  }
  Serial.println("MPR121 found!");
}

void loop() {
  // Get the currently touched pads
  uint16_t touched = cap.touched();
  
  for (uint8_t i=0; i<12; i++) {
    // Check if each pin is touched
    if (touched & (1 << i)) {
      Serial.print("C"); Serial.print(i); Serial.println(" touched!");
    }
  }
  
  // Optional: add a small delay to avoid overwhelming the serial port
  delay(100);
}

• Ensure that the power supply is stable and within the specified voltage range.
• Avoid running long wires to the touch pads, as this can introduce noise and reduce sensitivity.
• Use proper pull-up resistors for the I2C lines if they are not included on the breakout board.
• Calibrate the sensitivity according to your application's requirements using the MPR121's internal registers.

• Keypad not responding: Check the wiring and ensure that the I2C address is correct. Also, verify that the power supply is within the specified range.
• Inconsistent touch detection: Ensure that the touch pads are not too close to each other or to other conductive materials that may affect capacitance.
• Noisy readings: Keep the wires to the touch pads short and consider adding filtering capacitors if necessary.

Q: Can I use the MPR121 with a 5V microcontroller? A: Yes, but ensure that the MPR121's VDD is connected to a 3.3V supply, and use level shifters for the I2C lines if necessary.

Q: How many touch sensors can I connect to a single microcontroller? A: You can connect up to 4 MPR121 sensors to a single I2C bus by using different ADDR pin connections for each sensor.

Q: Can the MPR121 detect touch through materials? A: Yes, the MPR121 can detect touch through non-conductive materials like plastic or glass, depending on the thickness and the calibration.

For further assistance, consult the MPR121 datasheet and the library documentation for in-depth information on configuring and using the device.