Path: utzoo!utgpu!water!watmath!clyde!bellcore!decvax!ucbvax!pasteur!ames!ll-xn!mit-eddie!apollo!nelson_p@apollo.uucp
From: nelson_p@apollo.uucp
Newsgroups: sci.electronics
Subject: electronic time capsule
Message-ID: <3a56593d.44e6@apollo.uucp>
Date: 16 Feb 88 18:45:00 GMT
Sender: user@apollo.uucp
Lines: 56
To: sci.electronics@news


  I was thinking about a rather off-the-wall project the other day 
  and trying to work out some of the technical problems involved.
 
  I had in mind an 'electronic time capsule', i.e., a small box
  perhaps set in a remote location where it wouldn't be accidentally
  disturbed, that would remain in a dormant state for decades or 
  even centuries at the end of which time it would begin transmitting 
  a radio signal to call attention to itself (at least during the
  day when solar power could provide a current source).

  The two main problems I see to overcome would be long-term reli-
  ability and how to count the time before 'waking-up'.
 
  I assume reliability could be achieved by careful selection of 
  components to avoid things like NiCads, electrolytic caps and
  other things that may leak or fail after a relativley short time,
  especially as a result of the inevitable day-night temperature 
  cycling it would be subject to.  Silicon solar cells and CMOS 
  circuitry should be pretty reliable if carefully tested ahead
  of time to get out of the steep part of the 'bathtub curve' 
  and then well sealed against the elements.  All design would
  be extremely conservative.
 
  Timing out decades or centuries is a tougher problem.  I had 
  three ideas but none of them satisfy me: 

     Some modern batteries have very long shelf-lives (decades,
     even).  A CMOS gate input to sense the state of the battery
     has such high input resistance that it would not appreciably
     alter the shelf life.   When the battery finally dies the 
     time-capsule comes to life.   Main objection: too short a 
     time.
 
     Binary day-counter.  A CMOS binary counter could be powered 
     by a few 10's of uF of capacitance using some reliable non-
     electrolytic capacitor type.  The caps would be kept charged
     during the day by the solar cells and would be enough to save 
     the state of the counters at night.  Sensing the output of the
     solar cells with a schmitt trigger would result in one 'count'
     per day.   A 16 bit counter would 'wake' the box up in 179 years.
     Objections:  If the solar cells were covered for an extended 
     period of time by a deep snowstorm, leaves, etc, the caps 
     would discharge, the count would be lost and it would have to
     start over.  
 
     Built-in failure.  Use a component that I *know* will fail
     eventually like a Nicad battery or an electrolytic capacitor.
     When it no longer holds a charge the box 'wakes up'.  Objection:
     the wakeup time is *extremely* unpredictable.  
                                                    
  I know this is a weird idea but sometimes they are the most fun to 
  think about.   Thank you in advance for any ideas this provokes.   
                                                                 
                                  --Peter Nelson (N1CHJ)