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Hugh Blemings's Blog, A Diary and Miscellaneous ramblings
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Sat, 05 Mar 2005
Have had the PocketTracker off the Trike for the last month or so. Trial runs I did back in February went well so all that remained was to finish putting bits together properly. The time to do so was elusive however!
I put together a battery pack out of 6 NiMH AA cells, 2300mA capacity should allow for around 20 hours continuous operation. This ought to be enough for a normal weeks worth of commuting between charges. Battery pack has a resettable thermal fues in line to protect against shorts - don't fancy things getting too hot...
A waterproof multi pin connector allows for charging the battery without removing it from the trike as well as configure the Pockettrack unit via a serial port. A switch adjacent to the connector switches the Pockettrack between the serial port and the GPS module.
A 3.3V rail is continually powered from the battery pack to provide a backup supply to the GPS module. This allows it to retain ephemeris data and keep the real time clock running for a faster restart/relock when power comes back up. Since this only needed to be a few uA I was going to just use a zener and a current limiting resistor but after talking to Gavin, a longtime friend and electronics/firmware/software hacker extraordinare, I went for an ultra low standby current 3 terminal regulator instead.
Final bit of circuitry was a switch to allow the status LEDs to be either dimmed or switched off. The need for this became clear after being all but blinded by them on an after dark ride one time :)
Last night I put it all together - bolting the connectors, switches and battery pack in to the box and wiring it all up. Quick test on the bench looked encouraging and continued pottering with it this afternoon. Put some photos up to this point starting here.
Michael dropped over as I was bolting the unit back on, and we wheeled the Trike outside to do some real world tests.
Unfortunately it seems there is some RF feedback or distortion of the transmitted signal so need to do a bit of digging into that. This had occured once before and was able to cure it with internal shielding so fairly sure can be sorted. Following a hunch of Michaels am suspecting RF is being coupled in through the GPS antenna cable as it was in close proximity to the transmit antenna coax.
Aside from sorting out said interference all that remains is putting together a more permanent wiring harness for the status LEDs, hoping to enlist Rachael's help with that again so she gets some soldering practice :)