I wish to control a classic cellphone using an Arduino Nano by connecting the cellphone keys to digital pins, and switch them using a program that I will put on the Arduino. The idea is to make a pseudo-GSM shield, and be able for example to send SMS messages, make a call, or just print a text on the LCD of the cellphone.
The 2 issues that I found :
- The cellphone that I use contains 25 keys, but the nano contains just 13 digital pins, so is there a smart way to use few pins to control the 25 keys?
- To switch on/off a key I thought to use a transistor, where E(mitter) and C(ollector) connected respectively to GND and V+ of the key and B(ase) connected to arduino. by setting HIGH or LOW on that pin it will open or close EC circuit of the transistor.
Question: Is there a smarter way to switch ON/OFF the keys using just few components (because in my case I will use 25 transistors and it's a lot !!)? And should I to use the same power source for the Arduino Nano and the cellphone?