Powering i.trek from PDA

[Oshawapilot]

Hello everyone..I'm new here, after being referred by someone in the newsgroups. It was suggested I may be able to get some more assistance on a specefic i.trek GPS question with my PDA.

I did try a search of the forums first, but didn't find anything.

Here we go..

I have a Sony Clie TJ25 Palm-OS PDA that I recently purchased an
i.trek mouse GPS receiver for from Semsons & Co. in the USA. I'm located in Canada, myself...if that matters.

Anyhow, I plan on using the GPS mainly for general-aviation navigation (I'm a pilot), as well as the odd Geocaching adventure.

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.

Now, this presents several problems:

First, when Geocaching, I'm obviously not near a 12v cigarette lighter
plug.

Secondly, not all of the rental aircraft that I fly are equipped with
cigarette lighter power sources. Additionally, most Cessna's are a 24v power system, and I'm unsure yet if the power adapter that is part of the i.trek PDA interface cable will accept this high of a voltage. I'd hate to fry the GPS itself, and the PDA, finding out the hard way.

In a pinch, I can band-aid some sort of "portable" 12v power supply
using non-spillable batteries and some sort of backpack for
portability sake, but it starts to get cumbersome at that point. Additionally, with the small planes I fly, every pound counts - carrying around extra batteries to power a GPS isn't ideal.

I know that other "direct" cables for mouse-style GPS units actually
power the GPS from the PDA (Which is what I desire), but I'm stuck
without that option apparently at this point.

I'm wondering if anyone else has run into this problem, and what the
solution might have been? Can I buy a PDA connector for my Sony from
eBay, and hack it into a direct cable successfully? Is there some
sort of modification possible to the cable I have now to enable
powering the GPS from the PDA even when it's not plugged into the
cigarette lighter?

I know that many mouse GPS units *can* be powered directly from the PDA..but unfortunately mine doesn't appear to be one of them..yet.

I'm quite happy with the actual i.trek unit at this point, regardless - it's been performing excellent to date, although I have not yet taken it airborne.

Regarding batteries, the TJ25 has an *excellent* battery onboard, and even with the GPS draw I would still except several hours of runtime - fine for 90% of my flights.

Any assistance appreciated!

Thanks everyone.
Mark

[Oshawapilot]

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!

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