Phone Keypad Hacking: Part 3

In parts 1 and 2, I walked through my journey of repurposing the keypad out of a phone from 1980. I learned that a more modern keypad matrix doesn’t exactly function (behind the scenes) in a way I’d expect. I wanted to understand it better so I set out to recreate a 2×2 keypad (kept it simple to make wiring easier) that would function the same way as something you can buy today. It would be a success if it worked with the Arduino Keypad Library.

adafruit-3x4-keypad

From my earlier looks through the code I knew it pulsed power out to a column pin and then read in each row’s key from that column before switching to the next column and repeating the process. I figured that should be enough for me to wire this up and try example programs without going back to look at the library’s code again.

I don’t know why I was thinking this would be more complicated and at least a little more exciting, but it was unbelievably easy. I guess I should be celebrating I understood how it worked. Literally all you do is connect one side of every button in a column to a pin and one side of every button in a row to a pin. No need for connections to power, or ground. No pull up/down resistors.

2x2-keypad-matrix-wiring.png

It immediately worked with the Arduino Keypad library examples, even the MultiKey one. I guess being able to detect multiple key presses at once is where the advantage to this implementation comes in. It worked flawlessly when pressing 2 of the 4 buttons, but when you get to 3/4 there are too many connections to distinguish the keys.

Just to be sure I had it figured out, I added a 3rd column to make it a 2×3 grid and it was just as easy.

2x3-keypad-matrix-wiring.png

I love the beauty of how simple this is. I’ve added Fritzing for both of these to my phone-keypad GitHub repo (2×2 & 2×3). If you check this PDF, in the How it Works section it has a really good explanation and shows the row and column connections exactly like I came up with.

Naturally now I need to do a part 4 and attempt to recreate the keypad implementation I ended up with from the old phone. Due to how it mechanically makes the electrical connections I think it’s going to be a bit more complicated than this was. We shall see…

Update: Read part 4.

2 Replies to “Phone Keypad Hacking: Part 3”

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