How to Use 4X4 Membrane Matrix Keypad: Examples, Pinouts, and Specs

The 4X4 Membrane Matrix Keypad is a user input device manufactured by Parallax Inc., part number 27899. This keypad consists of 16 buttons arranged in a 4x4 grid, allowing for a compact and versatile method of capturing user input. It is commonly used in electronic projects such as security systems, telephone dial pads, and various control panels.

• Numeric input for security keypads
• Menu navigation for user interfaces
• Control panels for electronics projects
• Prototyping for embedded systems

• Operating Voltage: 3.3V to 5V
• Interface: 8-pin connector
• Contact Bounce: <5ms
• Actuation Force: 160-180g
• Life Expectancy: 1 million operations per key

1. Connect the row pins (1-4) to the digital input pins on your microcontroller.
2. Connect the column pins (5-8) to the digital output pins on your microcontroller.
3. Apply a pull-up resistor or enable the internal pull-up resistors on the microcontroller for the row pins.
4. To detect a keypress, set one column to low and read the rows. If a key is pressed, the corresponding row will read low.
5. Repeat the process for each column to determine the specific key pressed.

• Ensure that the voltage levels are compatible with your microcontroller to prevent damage.
• Debounce the keys in software to avoid false readings due to contact bounce.
• Use a keypad library if available for your platform to simplify programming.

#include <Keypad.h>

const byte ROWS = 4; // Four rows
const byte COLS = 4; // Four columns

// Define the Keymap
char keys[ROWS][COLS] = {
  {'1','2','3','A'},
  {'4','5','6','B'},
  {'7','8','9','C'},
  {'*','0','#','D'}
};

// Connect keypad ROW0, ROW1, ROW2 and ROW3 to these Arduino pins.
byte rowPins[ROWS] = {9, 8, 7, 6};

// Connect keypad COL0, COL1, COL2 and COL3 to these Arduino pins.
byte colPins[COLS] = {5, 4, 3, 2};

// Create the Keypad
Keypad keypad = Keypad(makeKeymap(keys), rowPins, colPins, ROWS, COLS);

void setup() {
  Serial.begin(9600);
}

void loop() {
  char key = keypad.getKey();
  
  if (key) {
    Serial.println(key);
  }
}

• Keys not responding: Ensure all connections are secure and the pins are correctly configured in your code.
• Multiple keys registering at once: This could be due to not using diodes for preventing ghosting or not properly debouncing the keys in software.
• Inconsistent readings: Check for a stable power supply and ensure that the pull-up resistors are correctly configured.

• Double-check wiring against the pin configuration table.
• Implement software debouncing to handle contact bounce.
• Use a multimeter to verify the continuity of the keypad's traces if a button is not working.

Q: Can I use this keypad with a 3.3V system? A: Yes, the keypad can operate at 3.3V, but ensure that the logic levels are compatible with your microcontroller.

Q: How can I prevent ghosting on the keypad? A: To prevent ghosting, you can use diodes on each button or ensure that no more than one key is pressed at a time.

Q: What is the best way to detect a long press on a key? A: In your code, you can measure the time between the key press and release events to determine a long press.

Q: How do I clean the keypad if a button becomes unresponsive? A: Gently wipe the surface with a soft, damp cloth. Avoid using harsh chemicals that may damage the membrane surface.