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History This project has taken me, off and on, over four years to get to this stage. It would have been quicker but there were more interesting things to do at times such as aviation and getting married! During this development I maintained a diary of changes made to the software/hardware. I did this both to educate others from my mistakes and also to allow me to backtrack to an appropriate old archived version of the software when things didn't work. A kind of rudimentary change control system! To make digestion easier, I've split the diary up into years accessable using the links below. The more observant reader will notice that the altimeter went through around three or four board spins and design changes. It started life with an analogue vario but I gave up on this as too difficult to maintain the zero point. I also dabbled in deriving the AD7705 clock from the PIC and feeding the PIC off an RC oscillator instead of crystal. Despite compensating the timer for temperature drift (there is a temperature sensor on board!) I was never really happy with this scheme so returned to an XTAL. Throughout the project the constant battle was with keeping noise down to a minimum. Even today I'm not entirely happy but seems satisfactory in practise, though the slow vario update may need some thought. Back to top ...