What makes it tick?

The Brain:

 The picaxe based tracking system comprises of two parts; the DTMF encoder and the DTMF decoder. Both are home made DIY jobs that I made in a couple of weeks. This was one of only a few projects that I actually bothered to etch some PCB's.  The GPS tracker I also use for my car as a theft recovery device.

 The encoder:

 The encoder receives signals from the GPS, and translates them into DTMF tones. These tones can then be transmitted through voice transmitting equipment such as a mobile phone or any type of voice transmitter such as a UHF radio. In my case, it’s the former. The encoder is a picaxe 18X  that grabs the GPGGA serial string. This string has the position fix, and all necessary info to transmit back to base. It deciphers the string and sends each number via the correct selection of frequencies of the DTMF generator IC. The IC is a 5089 DTMF generator.

Sorry, no piccies of the encoder. I should have taken some before I lost it. Its just a little PCB with two chips on it anyway.

The two servos for tilting the camera, and for taking photos were also connected to the same picaxe. 

The picaxe basic code is here. You can tell if it is working by listening to the output. If the GPS has a fix, then you can hear DTMF tones being made in quick succession. If the GPS does not have a fix, you can hear a single tone beep every 10 seconds. If you cant hear anything, then something is wrong. That's what happened to my balloon when I called it. 

The decoder:

Another picaxe 18X at this end as well as a MC145436P DTMF decoder. The DTMF decoder decodes the incoming DTMF signals into binary. The binary is fed into the picaxe which then ensures that the data is not corrupt before showing it on the directly connected LCD screen. A beeper is connected to notify the user of each valid GPS fix update.

The code for the picaxe is here.

Above, when the local GPS is connected, the LCD searches for a position fix and displays the position on the screen. 

Below, once a position fix is obtained, (either remote or local GPS) the Latitude and Longitude coordinates are displayed. 

I thought I'd make a nice face for the tracker as it will be used for my car as well.

If you have looked at the code, there are some excluded references to charging circuits and a mobile phone ring detect circuit. These were not required for this version of the software and were annotated out.

The operation of the tracker is simple. Flick the switch to "local GPS" and the display will read the coordinates of the locally connected GPS, flick the switch to "Remote GPS" and you guessed it, the unit will begin to download and check the DTMF tones connected to the phone via a chopped up hands free.

The original bitmap of the face is here but play around with the scale before printing.

If you want to build the PC boards, then download the artwork for the encoder and the decoder .

Above is the GPS receiver that I used. As with all modern units, it is a 12 channel parallel receiver. The GPGGA NMEA string is the string that is required for the firmware. You will find that most serial GPS receivers (if not all) will be compatible.

Drop me a line if you want to ask a question. philippawlowski at hotmail dotcom