ESP32 Keypad Simulator: Wiring, Code and a Live Circuit
This is a 4x4 membrane keypad wired to an ESP32 DevKit 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 13, 12, 14, 27 and the columns to 26, 25, 33, 32.
- 3.3 V logic
- Library: Keypad.h
- 8 pins, no resistors
- Debounced in the library
Wiring the Keypad to the ESP32
Every connection in the circuit above, with the board’s own pin names.
| Keypad pin | ESP32 pin | Why |
|---|---|---|
| R1, R2, R3, R4 | GPIO 13 (D13), GPIO 12 (D12), GPIO 14 (D14), GPIO 27 (D27) | The rows: the library drives one low at a time. |
| C1, C2, C3, C4 | GPIO 26 (D26), GPIO 25 (D25), GPIO 33 (D33), GPIO 32 (D32) | 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 ESP32 scan it, using the chip’s internal pull-ups, so no resistors are needed.
Pins to leave alone on the ESP32
GPIO 34, 35, 36 (VP) and 39 (VN) are input only: they cannot drive a pin or take an internal pull-up. GPIO 0, 2, 5, 12 and 15 are strapping pins, read at power-on, and GPIO 6 to 11 are the flash chip.
On a real bench
The thirty-pin DevKit straddles a breadboard with one row of holes free on each side, which is where the Keypad’s jumpers go. Many ESP32 boards are wider and cover both rows; then use two breadboards side by side. Hold BOOT while uploading if the IDE cannot connect.
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] = {13, 12, 14, 27}; // R1 to R4
byte colPins[COLS] = {26, 25, 33, 32}; // C1 to C4
Keypad keypad = Keypad(makeKeymap(keys), rowPins, colPins, ROWS, COLS);
void setup() {
Serial.begin(115200);
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(115200) opens the serial monitor. Serial is UART0 on the USB bridge. ESP32 sketches usually run it at 115200 baud, which this one does.
Common mistakes with the Keypad on the ESP32
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 ESP32 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 ESP32 DevKit?
Yes. The circuit at the top of this page is an ESP32 DevKit with a 4x4 keypad, running in your browser on a simulated ESP32 (Xtensa LX6). 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.