GPS Logger VIII – What happened?


So over two years ago I set out to build a GPS Logger to take hiking and traveling. Everything was assembled and it was good to go (almost) and hasn’t moved from that state. Driving around Australia for 2 years and then taking off around the world has a way of putting a cramp in finishing projects.

Design Flaws

During the assembly phase I discovered a few problems with the circuit as it currently stands:

  • No programming header. Whoops. This was solved by directly attaching to the various pins on the PIC
  • Switching FETs wired up wrong. The board required modification to fix this one and allow the GPS module to turn on.
  • USB Charging Port detection circuit probably wouldn’t work, due to the FET issue above.

Lots of learning experiences with this project. Additionally there were a few nice-to-haves that could have been included:

  • Isolation of the processor & GPS from the battery when charging. Excessive current draw while charging will confuse the charge controller and could result in battery over charging, fire and explosions.
  • Hardware SPI for the SD Card. Seeing as the circuit needs to be redesigned, basing it around a processor with separate SPI and RS232 ports will allow data to be written to the card much faster, resulting in less dropped data and a lower power draw as the processor will spend more time sleeping.

Taking the above items into consideration I redesigned the circuit, shown below, based around a PIC18F2xJ50 microcontroller.

USB Charge Port detection has been removed at this stage. To include it would require simulating the proposed circuit against the specification to ensure correct operation, which I should have done the first time around. Additionally the value for R8 should be revised to ensure that the logger doesn’t consume more than 500mA when operating from a USB (non-charging) Port and charging the battery.

Where to from here…

At this stage there is no intention to finish the GPS Logger, all information will remain here for reference purposes. I may desire to finish it at some stage but currently all the hardware is in another country.

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