To complete this project, you need:
- A suitable GPS receiver
- 26 gauge wire
- 100pF or 47pf ceramic capacitor
- Active external GPS antenna
- BNC panel mount connector (may vary, get one to match connector for the antenna)
- 1/8 inch mini jacks
or similar for power and serial output (optional)
- Tools: Soldering iron, pliers, wire strippers, drill
We wanted a low cost dedicated GPS
receiver that could be mounted somewhere in the
guts of our vehicle. We picked up a cheap GPS device made for the Palm III and used the instructions from Walter at radiohound.com
to remove the 12 channel GPS board from the unit. It's
just a matter of disassembling it and moving a few components around. The result is a stand alone GPS card. Power it up
and it starts spitting out NMEA sentences
via the serial port.
The GPS board isn't too
shabby, but it does lack a backup battery (for faster acquisition) and support for an external antenna. By adding a power injector circuit
, we can use a standard external
active antenna. To make life easier, we used an antenna that accepts 5-12 volts DC. The GPS board we used accepts 4-16
volts DC. If your hardware needs a lower voltage like 3-5 volts DC, you'll need a power regulator to get power from
your car. We scored a pair of 100pF capacitors, and a male BNC panel mount connector from RadioShack. The 3/32 inch
drill bits (you only need one) came from Sears and the antenna from eBay.
How it works
Satellites orbiting the earth
transmit radio signals down to the earth. The GPS antenna receives the signal (active antennas amplify the signal a
bit) and sends it down the line to the receiver. The power injector does what it says, sends power up the antenna cable
to the antenna. The signal is passed to the GPS receiver. Based on the time and the satellites the receiver can 'see' it
calculates its position on the earth.
The Power Injector
The power injector is a simple circuit resembling the sound crossover you'd find in any decent set of
speakers. The coil is a small inductor that acts as a low pass filter. It keeps the GPS frequencies from the power
supply, and keeps interfering signals from the power supply out of the GPS while allowing the DC voltage to pass
through. The capacitor acts as a high pass filter. It allows the GPS signals to pass while blocking the DC voltage from
entering the GPS receiver
We'll make the coil from scratch, and the capacitor is either a 47pF or 100pF
Building the coil
To make the coil,
you'll need a piece of 26 gauge wire that's about .4mm thick. We pulled apart some 18 gauge stranded wire to get the
right size. The digital caliper was oscillating between .39 and .40 mm, so we're good.
To make the coil, we wound our
26 gauge wire around the 3/32 inch drill bit six complete turns. It's easiest to wind it, squash it together and then
evenly separate it evenly using your fingernails.
We decided to mount the hardware inside a penguin mint tin. To drill the large hole for the BNC
connector, start with a small drill bit and work your way up in size. We used four or five bits to drill the hole.
Finishing up the job is pretty
easy. A couple lines of hot glue provide a simple insulator to keep the board from shorting on the bottom of the tin.
The coil is connected to the power pin on the GPS board. The capacitor runs to the signal line of the coax to the
board. The shielding is soldered to the outer tab of the BNC connector. We couldn't get the mini-jacks in time for the
photo shoot, so just imagine them being wired up. There's plenty of room for them next to the BNC connector.
The mod is so simple, it's almost a let-down. Fortunately, the results aren't. The lock speed
of the GPS card decreased to a consistent 60 seconds, down from over two minutes. The extra shielding helped along with
the now externally mounted antenna. If you don't want to crack open your shiny GPS, you an build a re-radiating antenna
using the same circuit with a passive
antenna in place of the GPS unit.