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Here's the new schematic. No major changes, just an extra Transistor to fire the sustainer igniter and the piezo speaker on pin 10.
Here is the altimeter mounted on an electronics bay for the outboard rocket.
Update 2 April, 2006:
I've had an issue with the altimeter I've been trying to resolve for some time. The altimeter seems to intermittently fire the main channel charge. Twice now, I've had a rocket on the pad, armed the altimeter, then after a minute or so the main channel fired. The thing is, I haven't been able to reproduce the problem on the work bench. It seems to work fine for hours on end. The first unit I built has flown many times and never had the problem once. So I thought the problem was somewhere in the second altimeter itself. Last week, I built another new altimeter, and guess what, it did it too, but only once and on the work bench this time.
The problem is twofold: First; the reason I can't reproduce it on the work bench is that it only seems to do it upon first power up. It's like the pressure transducer has to warm up first. Once the unit has been on for a few minutes, it doesn't do it again, regardless of turning it off and back on again. What happens is the pressure transducer is drifting enough upon initial start up that it thinks it is in flight.
The second problem is in my code. Obviously the program isn't running quite as expected. Once lift off is detected, it should loop until apogee is reached, fire the apogee charge and then sample for main deployment altitude. I'll have to look at the code again at some point to figure out why it skips the apogee and goes into the main sub routine.
For the time being, I'm doing a work around. A longer warm up time when the altimeter is first turned on should help, I'm also increasing the altitude for lift off detection from 300' to 600'. Those two changes should prevent the altimeter from prematurely leaving the lift off detection routine. I'm also increasing the apogee detection variance, just in case the unit drifts some at apogee too.
The good news is, this isn't a problem that would threaten one of my flights because I use a main deployment via a PIRM 2, and the worst that would happen if I had early main deployment would be main chute deployment with the drogue at apogee. But if someone uses the altimeter with a more conventional dual deployment, it could cause serious damage if the main chute charge went off early. Just a heads up!
Here's the new altimeter to the right (third build), and the previous version in the Comso 2 electronics bay. This new build has only main and apogee deployment, and is a slightly shorter version.