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

Comments

[roachburn]

How do i get it to display the * or the # signs?

[Dominion-Network]

Has anyone actually done this? I'd love to see pictures of the end result.

[thiagottjv]

Can i use another capacitor size? like a 100nf?
And the resistors? how did you calculate the sizes?

[XenonJohn]

Capacitor, probably other values work. It helps debounce the keys I think.

The resistor values were borrowed from a post on similar topic on the AVR forums.

Best wishes

John

[Data643]

The keypad in the picture is not the keypad you describe. You say it has 7 pins. The one in the picture has 14.

[XenonJohn]

OK I will go and shoot myself then for wasting your time.

If you turn it over you see that several of the apparent solder terminals don't do anything and there are 7 active terminals.

[budabob07]

Very clever. Great way to save pins without using shift registers or other IC's

[fennecfanatic]

Wouldn't it be a lot simpler to do this with a parallel-in shift register? That would only use three pins, and you could do the same thing with parallel-out regs for outputs. Granted, it would cost a little more.

[yes8s]

There is an error with your diagram above. The "1500 Ohm" resistor should actually be "15000 Ohm".

[XenonJohn]

Thanks for spotting this.

I have changed the diagram to 15000 Ohm.

Best wishes

John

[doby162]

I thought about doing this before. I guess since the keypad was only going to be for one project and it only needed a couple other pins I just took the lasy way out =). Well implemented though, I definantly think this shouldev been featured.

[nadav]

WOW! how this didnt get featured is beyond me... Great 'ible thanks. -Nadav

[XenonJohn]

It's the weekend maybe! Also, until you have personally come up against the "running out of pins" problem you wouldn't realise it is a problem at all. John

[nadav]

yeah, i always like to think ahead just incase there are upgrades to the project im doing. especially if its going to be permanent..