Over a period of time I have been playing around with APRS and my main problem
is how to get the whole system into a manageable size that has not too many
trailing wires! My first prototype was a wirey wonder.
It started off as 3 discrete objects. The GPS, Tiny Track and the Radio. This
setup is still running in my car as ZL1SWW-2. It all comes on with the ignition.
The GPS is a module made by Ashtech, the same people who make Magellan units.
This unit is an 8 channel industrial type unit that is called a G8. It is now
superseded by other units now.
The aim of the game was to get it all in one box. My problem is finding a box to
suit and I came across some old cases without lids at a surplus store here in
Auckland. It had slots that would nicely take a PCB.
I wanted to be able to control both the GPS and the Tiny Track by a common
serial port. (The GPS can be talked to, to output different sentences.) While
away down at Papmoa I knocked up a circuit that would multiplex the serial port
to two devices. All this was done by two CD 4052s and a CD 4069. Using two poles
of a 4 way dipswitch, I can control several modes. Program the GPS, program the
Tiny Track and loop the GPS to the Tiny Track as well as outputting the NMEA
sentence from the GPS to my AVR GPS display or the Laptop.
The schematic is as attached in this PDF . The GPS and Tiny Track circuit is not
drawn, as I was concerned with the multiplexing.
Here is the PCB composite layout.. (Overlay and silk screen)
Here is the unit with the covers removed.
Top left shows the Ashtech GPS module with the small RF lead going to the
back panel with the SMA connector that feeds DC up and RF down from the
antenna which is active and needs about 5v to run.
Top right shows the Backup battery for holding the Almanac and the GPS
user settings.
Bottom left is the Tiny Track section with PIC micro. The other chips are
the MAX 232 (top chip) 2 * CD 4053 Mux chips and the bottom one, a CD4069
inverter.
The dip switch controls the Serial port redirection and Tiny Track
beaconing etc..