Well, let's look at the problem you are trying to solve from a simple perspective. As I see it, you need two things to happen.
- You detect the motion
- You wirelessly notify yourself that motion has been detected
Simple enough. Now, it depends of course on what your requirements are. Things like:
- How big you want the device to be
- How much you want to spend on it
- What kind of experience you have with electronics
You didn't provide any of this information, so I'll tell you what I would do if I were in your situation.
Detecting the Thief
Use an Ultrasonic Sensor
Grab the HC-SR04 ultrasonic sensor. It is super easy to interface with most microcontrollers, and very accurate. Very cheap too (~$4)
Use a PIR Sensor
They are also very cheap, and as feasible a solution as is the one above. This one would be a little more compact, and have a wider field of view though. Probably a little more expensive than the HC-SR04.
Transmitting the Information
Use an xBee WiFi Shield
You'll need just one of these for your car. Hook it up to your WiFi connection, and it can send data directly to your computer or even your mobile phone if you are willing to write a quick app which can alert you.
Use a pair of 2.4GHz Transceiver Modules
You can try the cheap-o NRF24L01 modules. You'll need two, one for your car, one for your room. The main advantage is that they are unbelievably cheap (~$2.50 a pop), and there is good sample code out there too.
A Microcontroller to Process the Data and Interface With Your Components
A typical recommendation would be to use the Arduino. All the components I listed above have lots of sample code available which should work flawlessly with the Arduino. You would likely need two in any of the scenarios, unless you are up to using the xBee shield to send the data directly over your WiFi connection. I have never tried it, but it is well documented and shouldn't be very hard to do.
Or you could go the cheaper route and use a standalone AVR or PIC microcontroller. Also not a bad solution, as you can keep things much more compact and portable. If you really want to use the Arduino code, you can install an Arduino bootloader on most AVRs, or you can just code them in C. The Arduino route is easier from a coding standpoint I suppose - but that's arguable.
Lastly, you'll need power. You have two options:
- Use a battery pack. Go for rechargeable NiMh packs if you choose this route.
- Use the cigarette adapter in your car.
Hope this helped!