PeterH:
It sounds as if you've got an unexpected voltage difference between the Arduino and your DTMF decoder. Can you measure the difference between the two grounds when the system is wired up in the way that works? Can you measure the signal voltage on those four pins relative to Arduino ground? If the grounds are close and you are getting a positive voltage in the right sort of range, it should work. So presumably you aren't. So you need to see what's actually happening on those pins.
I'm powering my arduino via USB and my circuit from external 5V (230V) supply. Ground difference is 0,36V. If I join grounds only, half of the keys stop working. If I join grounds and external 5V + arduino 5V it starts working again. Does this make sense? Does is maybe have to do something with laptop power supplies and/or some 230V installation having GND an N joined together?
@iggykoopa & @Graynomad:
Yeah, that's what I had in mind. I understand the code, but for some reason it's not working. It's doesn't enter any case statement. I've played with both codes for a bit, but didn't get it to work. :~
I mean it's not a huge problem if I don't use a tweaked code, but as you said, it would help me if I start adding more devices. Thanks anyway, I'll try some more minor changes and see if it starts working.
@RandallR:
Yeah sorry, could've picked a better suited title. Although I'm also interested in how to decode DTMF tones with Arduino... 