From: jgd@dixie.com (John De Armond)
Subject: Re: Help with ultra-long timing
Article-I.D.: dixie.8#_v!0a
Organization: Dixie Communications Public Access.  The Mouth of the South.
Lines: 44

wellison@kuhub.cc.ukans.edu writes:

>I have a project that was drooped in my lap that is somewhat a pain to design.
>What I am looking for is a stable ultra-long solid state timer. What they want
>to do is to place this thing on the Antartic ice shelf and measure the amount
>of snow fall over a period of six weeks. Every two weeks, they want to trip a
>selonoid to discharge different colored pellets by gas pressure out across the
>snow. Then by digging down into the snow, the snow fall amount can be measured
>as they come to the different color pellets.

>The problem is trying to build a timer that would stand the cold (-40 degrees)
>and a power source that wouldn't drain. I have looked at the XR-2204 timers and
>the standard NE556 dual timers, but thier temp specs won't go that low. Also,
>two weeks equates to 1,209,600 seconds per firing, which means one big timing
>cap ! I have found 2.2 farad (yes, Farad !) caps that have a working voltage of
>5 volts and are small in size. 

Why are you fooling around with analog for this job?  A single chip
micro and a crystal will do the job reliably and easily.  An 8748 only
costs about $5.  That and a $1 crystal and you're in business.  Embed
the whole thing in a foam insulated blanket, power it from a solar cell,
use the excess power to heat the assembly during the day and rely
on the insulation to hold the heat during darkness.  If you don't want
to try thermal management, contact someone like ICL and have them cut
you a special low temperature crystal.  It'll cost at most $20.

If you use a single chip micro, you're looking at a parts count of 
maybe 7.  A processor, a crystal, two caps on the crystal, a power FET
to fire the solenoid a flyback diode and a battery.  This is fewer parts than 
you can build an analog timer for and is infinitely more reliable.  Add
a power zener diode (for heat) and a solar cell and the parts count
screams up to 9.

PD assemblers are available for all the common single chip micros.  This
application is so trivial you could even look up the op codes in the 
programmer's guide and create the binary with a hex editor.

John
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