Step 49: Repurposing the System: GPS Car Tracking
This is something that I’ve always wanted to build/own but the costs of such systems are staggering. At the time of this writing a similar system designed to be end user programmable, affordable, and easily configured was recently released for use in the US. The base cost for this subsystem was over $1500 and all it did was feed GPS latitude and longitude data over a cell phone line. These costs did not include the costs for the cell phone service.
A basic cell phone and a decent GPS reciever can be purchased for less then $50. If purchased in quanties and at the OEM level a competitve GPS car tracker can be built to integrate with the remote home access system for very little cost.
Figure 34: GPS Car Tracker Block Diagram
The block diagram for the GPS car tracker can be seen in in Figure 34. The existing system can be seen on the right hand side, and the new module on the left. The idea is to call the second module from the user’s home. A cell phone will silently auto answer the call and wake up microprocessor #1 seen above. This microprocessor will pole a GPS unit which will return a NMEA string containing latitude and longitude data.
The microprocessor will convert the NMEA string to an X and a Y value to feed to a DTMF encoder circuit. The DTMF encoder will send the X and Y value back through the cell phone to the calling home phone line where it will be decoded by the existing DTMF decoding system. The decoded X and Y value will be displayed on the LCD screen mentioned above. New X and Y values will be sent automatically back to the calling line until the phone call is disconnected. The software flow diagram seen in Figure 35 illustrates this process.
Figure 35: GPS Car Tracker Software Flow Chart
While it seems like a big leap from home automation to GPS car tracking it isn’t much more then what has been done so far. The GPS decoding module has been built for a Robo-Magellan mobile robot in another project and will easily interface to the existing system because both were designed to run on the Atmel AVRs. A picture of the robot using GPS and a variety of other sensors can be seen in Figure 36.
Figure 36: Robo-Magellan Robot
The only component that hasn’t been built is the DTMF encoder circuit but there are many chips available for just such a thing (Like the HT9200 ). A DTMF encoding sister chip to the CM8870 has been obtained and testing will begin on it this summer.
Another sensor that has been interfaced with the Robo-Magellan robot that would be of use in this subsystem is the HMC6352 digital compass. Using DTMF to relay GPS and heading data to a remote location can be used to tell not only where in the world the vehicle is, but also in which direction it is heading. Some of the potential uses for this system include finding a stolen or misplaced vehicle, keeping tabs on company property, and tracking taxis and buses along their routes. The information that this system can provide would be useful for validating claims and determining the outcomes of issues taken to court.
The GPS receiver is a Rand McNally product with RS-232 output. Many comparable receivers can be obtained for around $20 on the internet. The compass was $55 (the sure dont cost that now) and obtained from Sparkfun Electronics . Figure 37 is a photograph of the GPS receiver and the digital compass being used on the Robo-Magellan robot.
Figure 37: GPS Receiver and HMC6352 Digital Compass on a Robo-Magellan Robot
This subsystem can be exceptionally useful as an automatic trigger for the home automation system. If traveling home from a long trip the system can be polled to detect the distance left until a person arrives. Depending on which vehicle was detected a variety of other subsystems can be controlled. Temperature and other predefined settings can be set based on a predicted arrival time. This way the user’s home will be just the way they like it as soon as they arrive.