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