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I can't figure out a phone key matrix Answered

Hello everyone,
I'm Anthony and I'm a game designer / game master in an escape game, here in France. I work on Arduino most of the time and I generaly don't reuse electronics of other systems in order to modify it. But now I have to modify a phone to integrate some custom functionnalities.

I followed a tutorial on this website to find out how to get a key matrix of the phone circuit (link to the tutorial)

I'm quite a beginner to electronics and i admit that the key matrix I'm in front of looks really complicated. It has 9 wires soldered on it, but I can determine how much groups of keys it gets. I tried to use a multimeter with the continuity function but nothing happened. Do you have any tips for me in order to find it out ?

Also, the small black lines that cross the circuit's wires are conductive but I have no idea of what the big black dot (on top of the circuit) is... It's hidding the paths so I can't see where the wires go.


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Jack A Lopez
Jack A Lopez

1 year ago

I forgot to mention DTMF signaling.


Another possibility, for your button keypad , is to just leave the push-button telephone intact, but give it some power, like 5 or 6 volts DC, so that it can make its little, "beep-beep-boop-bop" noises; i.e. DTMF signaling.

Then build an Arduino circuit that can decode those beep-boop noises, and translate these tones into numbers (0-9), and other phone button symbols, like star (*) and pound (#).

Some examples:

Using just an audio amplifier, plus one GPIO pin.


Using a tone decoder IC (MT8870) and (looks like 4) GPIO pins


Jack A Lopez
Jack A Lopez

1 year ago

I think your button array is wired the same way as almost every other telephone keypad out there...

...with the important difference that yours has an black box IC (integrated circuit) sitting in between the button array, and the gray ribbon cable, with 9 wires.

This is the thing you call, "big black dot." I call it black box IC, because it is like a black box. 9 wires go in on one side, and maybe 8 wires go out on the other side, but what is it actually doing?

Who knows? And who cares, if what we want is just an ordinary button array that can be scanned by an Arduino using well known, published techniques.

By the way, I have attached a picture of what I expect the usual button array wiring to look like.

I found it by asking DuckDuckGo to show me images of,

"phone keypad circuit"


I noticed that many of these diagrams used the same names for their nodes (wires), e.g. {R1,R2,R3,R4,C1,C2,C3,C4}, and after staring at this for a while, I realized "R" stands for "row", and "C" stands for "column". Note that is 8 wires total, 4 Rs + 4 Cs, for a 16=4*4 button array. A keypad of the same style with only 12=4*3 buttons, will have 7 wires, e.g. 4 Rs + 3 Cs.

Also here is a link to the page I stole that image from:


Coincidentally, that page is a tutorial explaining how to connect this kind of keypad to the Arduino.

In regards to your troubles with measuring continuity, or resistance, of these traces with your multimeter, part of the problem is the bright green is some kind of protective paint or enamel.

If you carefully scrape at that paint with knife or sandpaper, you will discover a layer of shiny copper underneath. Touching the multimeter probes to this copper layer will give good conductivity (i.e. continuity, i.e. low resistance) to any other place on the same node (i.e. wire, i.e. trace)

Also, once you have a layer of exposed copper trace, you can solder to this using a soldering iron, and thus attach wires directly where you want them.

Also the copper traces on the board can be cut deeply, so as to make breaks in these traces.

So, if you have some skill in these techniques, you can essentially cut out the black box IC, by cutting the traces that connect to it, and put your own wires in the places where they need to go.

Yet this is kind of a lot of work, (cutting and soldering work) compared to just buying a keypad that was wired the correct way to start with.

I dunno. Maybe there is something special about the appearance of this keypad, with its numbers, 0-9, arranged all in a circle, and this is the reason why you wanted to use this particular keypad for your, whatever it is you are building.


1 year ago

The black bits are jumprs, they go over the other traces to prevent a slightly more expensive double layer board.
I guess your main problem though it the big, round dot.
In there is a controller.
It is quite likely that the signals from your buttons are processed here, in that case you won't get any output unless the chip is powered.
Is there anything labeled on the contacts where the cables attaches to the original device?
At least something traceable for a supply voltage and ground?