How a 4x3 Membrane Keypad Works | Continuity Test & Internal Circuit Explained

In this post, we’ll take a closer look at how a 4x3 membrane keypad works — the same type used in calculators, security panels, and Arduino projects. You’ll learn how the keypad detects key presses, how the membrane layer creates continuity, and how each button connects to the circuit to send signals to a microcontroller.

A 4x3 membrane keypad used in Arduino and electronic input projects.

🧩 What is a 4x3 Membrane Keypad?

A 4x3 keypad has 12 buttons arranged in 4 rows and 3 columns, similar to a phone keypad (1–9, *, 0, #). 

The 4x3 matrix keypad layout — each button connects one row and one column.

It’s called a membrane keypad because the button layer is made from thin, flexible plastic sheets with conductive surfaces that close the circuit when pressed. Each button connects one row line and one column line — that’s how the system identifies which key is pressed.

⚙️ Internal Layers of the Keypad

Once opened, you can clearly see it’s made up of three layers:

1. Button layer – where you press the keys.

2. Membrane sheet – contains conductive black carbon pads.

3. Circuit board (PCB) – has the printed traces connected to the pins.

Inside the keypad: buttons, membrane layer with carbon pads, and the PCB.

These layers work together to make or break the circuit when a button is pressed.

🔍 Continuity Test with a Multimeter

To test the membrane layer:

1. Set your multimeter to continuity mode.

2. Place the probes on both sides of a black circular pad on the membrane.

3. When pressed, the meter shows continuity or low resistance, which means current can flow through.

Testing the membrane pad with a multimeter in continuity mode.

✅ This confirms that each pad acts as a switch — it closes the circuit only whcen pressed.

🧠 How the Circuit Works

On the PCB, you can see multiple circuit traces running across the board. Each key position is an open circuit by default, meaning no current flows. When a key is pressed, the membrane pad bridges the two traces, closing the circuit.

Each trace leads to a pin — the circuit closes only when the key is pressed.

On the left side of the PCB, the pins are labeled from pin 0 to pin 9 — each connected to a row or column. When the circuit is closed, the microcontroller detects which row and column were connected, identifying the exact key pressed.

🧪 Testing the Keypad

After assembling the keypad back together:

1. Connect it according to the 4x3 wiring layout.

2. Test by pressing key “2” — you should see continuity between pin 2 and pin 3.

3. Try other keys and compare results — each button connects a unique pair of pins.

Pressing key “2” connects pin 2 and pin 3, confirming a closed circuit.

This is how the microcontroller can detect multiple buttons using only 7 I/O pins.

📹 Video Summary

In the video version of this tutorial:

• We disassembled the keypad

• Tested the membrane using a multimeter

• Explained open and closed circuits

• Checked pin layout and microcontroller detection

• Reassembled and tested real button presses

It’s a simple yet fascinating way to understand how everyday input devices work!

📦 Where to Buy

Shopee:

1. Oscillating Head Fan

Lazada:

1. Oscillating Head Fan

TikTok:

1. Oscillating Head Fan

Aliexpress:

1. Oscillating Head Fan

🎬 Conclusion

A membrane keypad works by completing a circuit through thin flexible layers. Each keypress connects specific row and column traces, allowing the microcontroller to recognize which button was pressed. With just a multimeter and a bit of curiosity, you can easily test and understand the working principle of this common input device.

💡 Don’t forget to Like, Subscribe, and Share if you enjoyed learning how a 4x3 keypad works!