Status: This is a project in planning state. Currently, these are just thoughts and ideas noted for future usage -- once I have time and/or materials needed for this. If you want to help me with the materials issues, you can check the WishList for an overview what I am missing. Usually, though, the missing resource is just time.
For summer hiking vacations, it would be nice to have a record of movement through the wilderness. As there is a good GPS reception up in the mountains we are climbing, a GPS logger would be the natural solution to this problem - but power is an issue up there: The device would have to survive and operate for about two weeks without being near any power supply. Regular GPS receivers do not fulfil this requirement.
http://www.mikrocontroller.net/articles/GPS_Logger shows a solution that does GPS logging to an SD flash memory card, though this is not designed for the low power profile I need. The hardware design itself should be usable for my purposes, perhaps slightly modified here and there.
Contents
Notepad
- low-power RTC with alarm programmable by MC waking up MC at measurement intervals
- (AVR) MC goes to deep power-save only waking up at the next measurement interval
- GPS receiver being powered on and off by MC
- Must include timeout for power-down if no position decoded after e.g. 5 minutes.
- button for manually waking up device and making a measurement/mark
- LCD powered on and off for position/status messages on user's request?
- how to charge the device? through usb, and also data transfer through usb?
- this would be elegant
- this could utilize existing usb power supplies (e.g. my cellphone's)
- this would eat up progmem in the MC for data transfer code
- this could save progmem in MC because we don't need an "official" filesystem on the flash card
- ready-made USB-to-serial modules are somewhat expensive
- Using a USB-enabled MC requires a larger USB stack in firmware (but could actually play USB mass storage)
- what kind of battery to use?
LiIon, perhaps cellphone-type?
- Pro: High energy/space*weight ratio
- Pro: Available in many small form factors
- Con: Difficult to charge.
- Con: Tends to explode when abused (e.g. wrongly charged)
- Con: Difficult to get to the US in case of travelling there -- then again, I fear they dislike selfmade electronic devices in general, no matter what batteries they contain.
NiMH "AA" or "AAA" batteries (that's Mignon or Micro for me...)
- Pro: Cheap
- Pro: Easy to recharge
- Pro: Could be replaced by non-rechargable batteries available everywhere in "emergency"
- Con: Less energy in more room, at least from my gut feeling - have to verify that.
- Some rechargable battery plus solar cells recharging/supporting the batteries
- This just came to my mind as slightly absurd idea -- perhaps I find the idle time to actually think about this
- make casing, all i/o ports and LEDs and buttons rugged enough to survive that kind of vacation...
- must be waterproof, at least against soaking wet backpack top pockets, and direct rain.
really waterproof would be a bonus - just in case the device is found floating somewhere while I got lost.
- must also be reasonably shockproof. backpacks are handled carelessly by transport operators, and are frequently hit by branches while walking through the lower regions.
- No need to worry about temperature ranges so far - at least -20 decC doesn't seem to be an issue for all devices investigated
- either must record in a "standard" file format, or there must be some data transfer/conversion utility.
Parts considerations
Parts marked 
are in stock.
Board design
GL-P01 GPSLogger firmware for GL-B01
GPS receiver
Ordered the "GPS Micro Mini" by Sparkfun from http://www.tinkersoup.de, but this one was too sensible regarding its surroundings: It acquired no fix within five minutes, under open skies, in a protoboard resting on my notebook computer.
Therefore, ordered the "EM-408 SiRF III" receiver module from same source. This works fine, and I am going to use it in this design.
Microcontroller
Atmel AVR, most probably. These seem to be standard in my designs, and I have some at hand for prototyping.
I have bought an AT90USBkey, which is Atmel's development board for the AT90USB class of microcontrollers, and will do the firmware and interface development with this one. Changing to a USB-enabled controller gives me the option to transfer data from the device not through special protocols but have it be a USB mass storage device...
Real Time Clock
PCF8583 - these are I2C controlled, and have an interrupt output pin for programmable alarm. However, I might also want to use a RTC with built-in crystal oscillator for improved accuracy... Have to check availability and pricing, and judge considering the (possible) frequent time corrections from GPS data. ![/!\](/modern/img/alert.png "/!\"){kind=small}
Battery Charge Controller
LM3622 perhaps - can digest a wide range of input voltage and charge "1 or 2" LiIon/LiPoly batteries. As I want to use a cellphone battery, I need some charger circuitry. However, I might have to investigate this field a bit more before choosing one.
- MAX1811 is a single cell Li+ charge controller, and available from my favourite supplier.
http://www.mikrocontroller.net/topic/163211 discusses some charge controllers.
Power converter
From the LiIon battery, I can get like 7V as far as I remember ( check this!), so it must be converted down to 3.3 for power savings in the MC, and for operation of the GPS module and SD card. Perhaps use MAX667, which has programmable output voltage? On the other hand, a fixed voltage regulator may be cheaper and more exact, and a switching regulator might be more power-efficient... Must check this, too.
Data storage
I changed my mind about the SD card interface - I don't think I want one now. As the amount of storage space needed is not that high, I will go for a flash memory IC on board. This is more comfortable to use (the one I plan to use even seems to have internal RAM buffering for handling the 1k blocksize writes to flash itself), and cheaper, too.
Details are on another page together with the data format specification: /DataStorage.
Display
If I finally decide on including a display, it will be "the usual" 2x16 alphanumeric LCD. I have some "DEM 16217 SYH-LY" in stock.
Case
No idea yet.
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