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Author Topic: Key matrix hack / simulate finger press  (Read 2317 times)
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hi !

My project is a matrix keyboard hack that simulate finger press by shorting a row with a column, which are both selected by two multiplexers.

I made this project three or four times with mobile phones or an old PS2 computer keyboard and it worked quite well (sometimes missing a few keypresses).

A few month ago, I made this project one more time to control an old phone (and dial any phone number). It worked pretty well at that time, but now doesn't work anymore.

I checked every connection and my circuitry is the same as http://benjamintomlinson.com/electronics/arduino_schematic_a3.png (except i use CD4097 multiplexers).

The only clue I have for now is that when I connect the rows and columns to the multiplexers a voltage drop seems to occur in the phone (LED is much less brighter).

Many times I suspected something like "maybe the voltage from the matrix scanning is routed in a wrong way". Could it be sucked through the GROUND pin of the multiplexer ?!
I also discovered something strange : COMMON IN/OUT of the multiplexer are shorted with +5V of the arduino. Don't know why...
I think there is some electronic trick that i don't understand here.

If anyone has a clue ?

I can post more details (photos), just tell me.

Thanks in advance
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I have found a clue by myself...
(I changed the hacked phone & made my circuit one more time)

The finger press simulation seems to work only when my laptop is not connected to earth. When the laptop is only powered by its own battery the whole hacking is working.

I found this trick because the phone was making a lot of noise, a huge buzz when the laptop was plugged.

I heard about the common audio problem of "mass loop" which is frequently caused by devices connected to earth. Could this be the problem ? Could this "mass antenna" affect my multiplexer circuitry ?

I would be happy to get some knowledge out of this problem, beyond the momentaneous satisfaction of seeing that my project is working again.
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Hi,

I'm planning on trying a similar setup to control an old radioscanner but have no experience with the Arduino.
Does anybody has some code for this setup?

Thanks in advance,

Martin

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