However, I have discovered a problem - The i.trek unit will *not* work
unless the cigarette lighter power plug (Which is part of the PDA to
GPS interface cable) is plugged in. I didn't know this ahead of
time.

However, I have discovered a problem - The i.trek unit will *not* work
unless the cigarette lighter power plug (Which is part of the PDA to
GPS interface cable) is plugged in. I didn't know this ahead of
time.

For anyone who is interested, I remedied this problem myself, after
not finding any immediate solutions anywhere else.

5 Years of electronics courses are paying off.. :-)

Basically, I pulled the GPS to PDA cable apart at the PDA connector,
and at a junction connector, and start probing with my multimeter...

The circuit that was providing power to the PDA while the cigarette
lighter was plugged in did *not* backfeed power when the cigarette
lighter power source was removed, but the GPS was left on. If it
did, it would have backfed the required +5v to the junction box, and
hence, to the GPS...

However, one of the other pins on the GPS connector *did* have a ~+5v
output when the PDA was on..or off, for that matter..but that wasn't
terribly important. It seems that this was a direct connection to
the PDA battery from this pin.

So, I got busy with the soldering iron, and did the following...

After opening the PDA end of the connector, I drilled a small hole in
the housing to allow two small guage wires to pass through. I
soldered one end to the constant +5v pin (Which was unused), routed it
through the cover to a small switch mounted externally, and then ran
the other wire back into the housing, and soldered to the +5v feed
wire that supplied voltage to the PDA when the cigarette lighter is
plugged in.

With the switch closed, the PDA now backfeeds power to the small
junction box, which in turn feeds the GPS successfully.

When I want to plug in the cigarette lighter plug, I simply open the
switch, and things revert to normal.

The reasoning behind the switch is that I'm unsure what will happen if
voltage is fed back to the alternate circuit I used to draw power
from. My indications are that this circuit connects *directly* to
the PDA battery - if voltage is backfed to this circuilt via the
cigarette lighter charger, it would be bypassing any charging circuits
within the PDA. Also, voltage would now be provided at 2 points on
the PDA connector.

Maybe no problem...possibly a big one. The switch isolates the
changes I made, so there is no concern.

Works like a charm - the PDA now successfully powers the GPS for about
2 hours and 20 minutes on a full charge. A flip of the swich, and the
PDA and GPS is powered by the cigarette lighter again, and the PDA
hapilly charges itself.

Problem solved!