Arduino 3 Wire Matrix Keypad

Keypad connected with only 3 wires to Arduino.

Background:
A matrix keypad can be connected to an Arduino board so that numerical data can be entered by the user.
Instructions exist in various forums on how to do this. The main problem is that this usually uses up a total of 7 of your digital input/output pins.

The problem:
For me this is a big problem as I want these pins free to control external devices such as solenoid valves and actuators. I also need some to send data to a serial LCD display.

Options are either
a) Use an Arduino Mega with lots more pins or
b) Find a way of interfacing a keypad using less input pins.

Solution:
By using ONE single analog input pin and a grid of resistors, we can wire up the buttons on the keypad such that when each is pressed a different voltage is sent to the analog input pin.

Although it is a hassle wiring up all the resistors, the big advantage is that all your digital pins remain free to be used for other things.

This has been described before on AVR forums but I have adapted it for Arduino and written some code for you to get started.

Step 1: Identify the Keypad Pins

On base of keypad are 7 solder terminals.

Keypad can be considered to have 3 "columns" and 4 "rows" - see photo.

First step is to work out which solder terminal represents each row and which represents each column. Write everything down as you go along, very easy to mix everything up.

This has been described and you need to follow the instructions on how to do this given in this Arduino keypad tutorial:

http://www.arduino.cc/playground/Main/KeypadTutorial

Step 2: Wiring Up the Resistors

The next thing is to wire up the matrix of resistors.

As you can see there is one single wire to analog input pin 0 on the Arduino plus a connection to +5V on the Arduino and also one to GND (ground)....3 wires in total.

Having worked out your rows and column wires, you need to follow this diagram.

I use ribbon cable as it makes things (relatively) neat.

The tiny capacitor is there to prevent interference induced in the wires being transmitted as a voltage to the analog input producing errors.

Step 3: Test It!

If you download the first Arduino sketch on the title page of this Instructable, also attached to this page, you get a simple routine that displays the value being "read" by the analog pin 0 (on a scale of 0 - 1023) representing 0 - 5V in the serial viewer window of your PC which you need to set to 9600 Baud.

There will be slight variations depending on tolerances of the resistors but the values you get should be roughly similar to those which I obtained.

Step 4: Write Software to Read Your Kepad

If you download the second Arduino sketch on the title page of this Instructable, also attached to this page, it gives you an idea how to write code to read the keypad and do useful things with the values obtained.

In this example, when you press a key, the value on each key will appear in the serial viewer window (i.e. 0 - 9) on your PC.

You can see that there has to be some code that takes the value, then waits for you to release the key before continuing, to prevent a whole sequence appearing of the same value.

The code is a bit clunky but it works. You will have to adapt this for your own applications but it should get you started.

The variable "keypressed" is the value of 0 - 9 which you can then use in the rest of your program.

Have fun.