Here is an APRS
tracker I have built with a single chip Atmel AVR
How does it work (electric diagram of the controller
- GPS NMEA data is read through the serial port and interpreted for position, speed, heading and altitude
- AX.25 frames are formed with a 4-bit resistor DAC and a low pass filter
- Channel busy/free is sensed through the speaker: any audio is interpreted as "busy", thus transmissions are inhibited.
- Two 10K NTC thermistors are read and interpreted as internal and external temperature
- The tracker reads as well its own supply voltage.
There are some LED's for visual check:
- Status LED flashes when correct GPS frames are being received
- PTT LED flashes when the tracker transmits
- Speaker LEDs flash when audio is present
Programming the micro-controller (AVRGCC source code
- On top of the source code you will find tips to compile it with AVRGCC and to download the hex file with SP12
- Before you compile take care to change MyCall, MyQTH and MyMsg so that you transmit your proper callsign and message
- Take care to set the fuses correctly.
- The ATmega88 can function on very low voltage and takes little current. I run the tracker from the +5V supply that comes out of my TH-78 Kenwood transceiver, however the GPS still needs its source of power.
- It is important to adjust properly the squelch and volume on the radio. The speaker LED's should flash when radio is receiving signals on the air and should stay dark when the radio channel is free to transmit. Sporadic squelch openings are OK.