So I'm going to attempt my first Arduino project (and really my first hardware based project) outside of the site tutorials. I'm going to try and mod an old-school rotary phone to detect incoming GSM signals and trigger the ringer.
For starters, I'm not attempting to re-create the rotary cell phone project from Sparkfun. I'm attempting to incorporate proximity-based GSM signal detection into the phone. The phone itself would serve no function beyond ringing (and possibly serving as a charging station for a cell phone, but that's another mod entirely). What I hope to do here is lay out some ideas and get any feedback that anyone would care to share.
I'm planning on having this project focus on using an Arduino board to detect the signals coming from a MoPod (http://www.thinkgeek.com/gadgets/cellphone/a125/) and ring when a call is incoming. For those who aren't familiar, a MoPod is just a little keychain that will flash some surface mount LEDs and trigger a small motor that spins a toy monkey when it detects a GSM signal. I'm hoping to use this as an input device for the Arduino and, when a signal is detected, trigger a ringer.
A couple things I've figured out so far:
- The MoPod is quite sensitive. It will trigger at the detection of any GSM signal. This includes when the phone intermittently signals back to the tower for a lease refresh (sorry if this is the incorrect terminology, I'm no GSM expert)
- Due to the device's sensitivity, I can't just wire the leads currently going to the motor into the Arduino. I need to find a way to be able to discriminate between an incoming call and the stay alive signals.
- After looking at Sparkfun's rotary cell phone project, I think I'm going to avoid trying to use the phone's bell ringer. Trying to figure out how to get that much voltage can wait for another day. I'll have to whip up a new ringer circuit or pull one out of a newer phone.
Based on how I've heard from putting my cell phone near my computer speakers before, I'm wondering if there is a specific transmission pattern that I could attempt to detect in order to discriminate between incoming calls and other GSM signals. Another idea that I've had is checking to see if there's a difference in signal strength using the analog inputs.
That's where I'm at right now, very early stages. If anyone has ever played around with GSM before and has suggestions feel free to comment. And if anyone feels like this is a stupid project and I'm wasting my time, well piss off. 
-n0/0ne