The device designer: Tim Moffat

8/26/2010 3:52 PM EDT

Over the next few weeks, I will be talking about how and why we built an Altitude Logger (with GPS assist) using PIC® microcontrollers with eXtreme Low Power (XLP) technology.

By choosing technology that has low active current draw and sleep modes, we can run this logger for almost 60 days on only 2 AA batteries. Chris and Darren will then take the project with them up to the top of Mt. Kilimanjaro to log their path along the way.

Below is an Altitude Logger board photo, and you can see several more on my Flickr page.

Since many aspects of this project were new to me, there was a large learning curve over a short period of time. I needed to learn a new CAD software program called EAGLE. This software allows me to draw the schematics and lay out the PCB. This is also the first time I have designed RF into any of my boards. Holding a GPS antenna so close to a human body is a real challenge! .

The board might work fine sitting on a table, but once you take that board and put it inside Chris or Darren’s jackets, or even inside their packs, you have a very different environment. For this project I also learned how to use a live X-ray machine, to make sure the GPS chip is installed correctly. And, I learned how to use a network analyzer to make sure that we can actually receive 1.575 GHz correctly..

On the board, we have a few different types of sensors to log their trip. We have a temperature sensor, which will let us know just how cold that night got in basecamp. There is a barometer (pressure sensor), which will tell us what altitude the hikers are at. The GPS that we added to the board will help us calibrate the barometer as the weather changes, but will also give us so much more. GPS will allow us to keep an UTC time stamp on all of the data we collect; the chip will also output our latitude and longitude, which we will also log. We will log temperate and Pressure once a minute, then once an hour we will log GPS data. If we have power to spare, we can log GPS once every 10 minutes. All of this data will be collected over the trip then stored on one of four 1 MBIT EEPROMs. When Darrin and Chris get off the mountain, we can pull the data log off the board, convert the data to a KLM file and then Import the data to Google Earth so that you can see their entire trip..

I decided to use a PIC18LF14K50 8-bit microcontroller (MCU), due to its low power consumption, and it just happened to have the exact amount of pins we needed. The PIC18LF14K50 also has the peripherals we need, such as I2C™, USB and UART, in a 20-pin package. The MCU only draws 24 nA when in sleep mode, which we want it to be in as much as possible. Saving power is key to this project, so that Chris and Darren do not need to worry about the logger when they are on the mountain. When the sensors are not in use, they are either switched off, or put into low-power mode..