So a couple of years ago we had a pretty bad ice storm here on the east coast and there were thousands of homes left without power for a week or more.  As with any disaster one can expect the criminals to come out of the woodwork and make a bad situation worse.  Well as luck would have it my brother in law was one of the people that had their homes robbed while they “seeking shelter” elsewhere.  Among the items stolen was their sons’ Sony PlayStation 3.  Of course the requisite police report was filed and as is usually the case, none of his property was ever recovered.

Now someone can easily have an alarm system installed in their home which can deter some burglars, but let’s be honest, even the best alarm systems have “holes” in them.  So I got to thinking, why spend the money on a home security system that might deter a burglar when we can catch the burglar instead and possibly get him off the streets.

I spoke with several law enforcement personnel and sure enough, video game consoles are one of the most popular items stolen from home break ins.  So a quick search on Ebay showed several empty Sony PlayStation 3 cases in the $20 range that would work for my project.

What’s Inside

The tracking device is an Arduino based system, using the I²C GPS Shield and is interfaced to a SIM900 GSM board, using a T-Mobile Prepaid SIM card, to allow communication via SMS and/or email.  The device is powered by a single cell lithium polymer battery.  The charging circuit consists of an off the shelf AC to USB adapter connected to Sparkfun single cell lipoly charger.  The rest of the circuit consists of a Lipo Fuel Gauge, to let me monitor the battery voltage, a BMA180 accelerometer, to let me monitor movement of the device, a couple of level shifters and a boost converter.

How it Works

Upon initial power up, the Arduino turns the GSM modem on and checks to see if any SMS or email instructions were received while the unit was powered down.  If there are no stored instructions the Arduino powers down the GSM modem to conserve power.  It then monitors the interrupt pin on the accelerometer which is set to detect any motion of the unit.  If it senses motion (indicative of the unit being pulled from it’s storage location) it powers up the GSM modem and sends a Security Alert text message to a preprogrammed number (my cell phone) with a text formatted as an HTML Google Maps link.  The link contains the time, date, latitude, longitude and optionally, if the unit is moving over a certain speed, the direction and speed it’s moving.

Once the unit transmits the initial Security Alert message it keeps the GSM at full power because it assumes the unit has been stolen and it is waiting for additional SMS/email instructions.

Communication works via SMS/email using a preformatted instruction string comprised of a personalized 4 digit security code and a one digit option setting.  Most of the options available are used to tell the unit where to send the tracking information too such as a default email address, default SMS address or simply reply back to the number or email address where the instruction was sent from.  There are other functions such as returning the unit back to it’s power on motion detect mode and requesting the state of charge of the Li-poly battery.

As I mentioned earlier, I’m using a prepaid T-Mobile SIM card for the GSM.  T-Mobile seems to have the cheapest price out there (at least in the US) for SIM cards.  For $10 you get 30 minutes which expires after three months.  Sending and receiving texts is $0.10 each way so you can get 50 tracking requests per $10.  Another benefit to T-Mobile is they have a pager number setup so you can send a numeric page to the unit instead of sending an SMS or email.  The benefit to this is you get charged $0.10 each way on an SMS and only one way when sending a page to the unit.  Basically you can double the amount of tracking requests.  Another benefit to using a prepaid SIM card is that you don’t need to renew your account after the three month expiration to retain the phone number.  Once you refill the account with another $10, the number is reactivated immediately…this is key.  Let’s say you have several tracking devices in your home and cars and you don’t want to have to keep paying monthly subscription fees to keep the accounts active.  Just let the accounts expire and when and if you get robbed/car jacked just log onto your T-Mobile account online and refill the SIM with your credit card.  Once you hit confirm on the payment screen, the SIM card is immediately reactivated and you can track the unit.  It’s a great way around monthly service charges.

Some other modifications I hope to make to the system, in the near future, are adding a microphone and speaker to the GSM modem so I can listen in on the unit if needed.  I also need to look into ways of minimizing power usage to get a longer life out of the battery. I have a piezoelectric speaker that I pulled from my mother-in-law’s broken carbon monoxide detector (thanks Nana) that I want to install so I can set off an audible alarm if needed. Nothin’ says lovin’ like setting off an ear piercing siren at a burglar’s home just as the police show up!

In Action

Since the GPS has a very high sensitivity (-165 dBm), I wanted to put it to the test and make sure I could maintain a good signal lock.  Remember the GPS is already enclosed inside a plastic case so the signal quality will get knocked down some.  Since my SUV has an under floor storage area, I decided to put the device down there and head to the post office.  Once I got to the post office, I dialed up the device, requested the coordinates, waited the 30 seconds or so it takes to respond and sure enough no problems whatsoever.

Arduino sketch available here