iFOB

The Pro Mini with its tiny form factor is meant for concealment in final projects, and the iFOB project takes advantage of this face nicely. The iFOB is a homebrewed hack to combine the RFID capabilities of a Nike+iPod device with the versatility of the Pro Mini to control a car key fob:
So here in lies the goal of iFOB: design a system that listens for a unique ID (kind of like active RFID) and does something when the correct ID is heard. In this case, unlock the doors to my car.
So here is iFOB! In the middle is the Arduino Pro Mini running at 3.3V. The Mini is powering the iPod serial board (at 3.3V) and has serial lines connected (TX and RX) so that the Mini can configure and then listen to serial responses from the iPod receiver.

The receiver of the Nike+iPod is connected to the Pro Mini. Whenever the Nike+iPod transmitter is within range of the receiver, a sequence is sent to the Pro Mini via the serial interface. The Pro Mini then proceeds to triggering the unlock function on the key fob.
So what is the point of the project? Well, the entire apparatus stays in the car, so that’s one less key fob to carry around in your pants. Instead, you would be carrying around the Nike+iPod transmitter, but since it’s designed to live in a shoe, you could optionally place it there (just remember to wear the correct pair of shoes when going out!). Although there is a probably security risk involved, it is easily overshadowed by the coolness factor.

More information and source code for the project is available at the source link below.

Source: http://www.sparkfun.com/tutorials/135
Intermediate Project
Difficulty: 3.5/5.0