Arduino Uno Keypad Simulator: Wiring, Code and a Live Circuit
This is a 4x4 membrane keypad wired to an Arduino Uno and scanned live with the Keypad library: open it in the editor, press a key, and the serial monitor prints it.
Eight wires and nothing else: the rows go to 9, 8, 7, 6 and the columns to 5, 4, 3, 2.
- 5 V logic
- Library: Keypad.h
- 8 pins, no resistors
- Debounced in the library
Wiring the Keypad to the Arduino Uno
Every connection in the circuit above, with the board’s own pin names.
| Keypad pin | Arduino Uno pin | Why |
|---|---|---|
| R1, R2, R3, R4 | pin 9, pin 8, pin 7, pin 6 | The rows: the library drives one low at a time. |
| C1, C2, C3, C4 | pin 5, pin 4, pin 3, pin 2 | The columns: inputs with pull-ups, read for each row. |
A membrane keypad has no power pin: it is sixteen switches at the crossings of four row wires and four column wires. Eight pins of the Arduino Uno scan it, using the chip’s internal pull-ups, so no resistors are needed.
Pins to leave alone on the Arduino Uno
Pins 0 and 1 are the USB serial port on a real Uno, so anything wired there fights the upload and the serial monitor. Pin 13 carries the onboard LED.
On a real bench
On a real bench the Uno stands beside the breadboard: the Keypad goes into the breadboard and male-to-male jumpers run from the Uno’s female headers to its rows. Power the Uno from USB while you build, and unplug it before you move a wire.
The code
The sketch the circuit above runs. Open it in the editor, change a line and press Run: it compiles in your browser.
#include <Keypad.h>
const byte ROWS = 4;
const byte COLS = 4;
char keys[ROWS][COLS] = {
{'1', '2', '3', 'A'},
{'4', '5', '6', 'B'},
{'7', '8', '9', 'C'},
{'*', '0', '#', 'D'},
};
byte rowPins[ROWS] = {9, 8, 7, 6}; // R1 to R4
byte colPins[COLS] = {5, 4, 3, 2}; // C1 to C4
Keypad keypad = Keypad(makeKeymap(keys), rowPins, colPins, ROWS, COLS);
void setup() {
Serial.begin(9600);
Serial.println("Press a key");
}
void loop() {
char key = keypad.getKey(); // 0 when nothing new was pressed
if (key) {
Serial.print("Key: ");
Serial.println(key);
}
}
makeKeymap(keys) tells the library which character sits at each crossing. rowPins and colPins list the board pins in the order of the keypad’s connector, R1 to R4 and C1 to C4.
getKey() scans the matrix: it pulls each row low in turn and reads the four columns, so a column that reads low is a key held in that row. It returns the key once, when it is first pressed, with a debounce built in, and 0 otherwise.
Serial.begin(9600) opens the serial monitor. Serial goes over the USB cable, and the serial monitor in the editor reads it at whatever baud rate the sketch sets.
Common mistakes with the Keypad on the Arduino Uno
Rows and columns swapped
Swap the two arrays and every key prints its mirror across the diagonal, so 2 prints 4. If the wrong keys come out in a pattern, swap rowPins and colPins.
Pins listed in the wrong order
The connector order is R1 to R4 then C1 to C4 on most keypads, left to right with the keys facing you. List the pins in exactly that order or keys move around.
Running out of pins
A 4x4 keypad takes eight pins, which on the Arduino Uno leaves little room. An I2C port expander such as the PCF8574 scans it over two wires instead.
Questions and answers
Can I simulate a keypad with an Arduino Uno?
Yes. The circuit at the top of this page is an Arduino Uno with a 4x4 keypad, running in your browser on a simulated ATmega328P. Click a key to press it; the simulated keypad connects that row to that column the way the membrane does. Press Run it in the editor to change the wiring or the code; it is free and needs no account.
Can I detect two keys at once?
Keypad.h has getKeys() for that, and up to about three keys work. More can ghost: a missing key appears to be pressed because of the paths through the others.
How do I make a password lock?
Collect the keys into a string until # is pressed, compare it with the code, and drive a relay or a servo when it matches. The keypad lock project does exactly that.
Does a 4x3 keypad work the same way?
Yes, with COLS set to 3, three column pins and a 4 by 3 keys array. The simulator has a 4x3 keypad part too.
Build your own Keypad project
Open this circuit in the editor, change the wiring or the code, and keep your version in a free account.