Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!mordor!sri-spam!ames!hao!gatech!bloom-beacon!mit-eddie!apollo!nelson_p@apollo.uucp
From: nelson_p@apollo.uucp
Newsgroups: sci.electronics
Subject: time capsule
Message-ID: <3a935422.44e6@apollo.uucp>
Date: 29 Feb 88 15:50:00 GMT
Sender: user@apollo.uucp
Lines: 73
To: sci.electronics@news



 
 Paul Dietz summarizes the options discussed so far for my
 electronic time capsule rather nicely:


>Power sources:
>Batteries	Get serious...
>Solar		How do you make sure that the cells remain exposed?

 I was planning to locate the devices above tree line in
 the mountains, trying to find flat places (minimize effects
 of avalanches)  well off the beaten path to avoid casual
 hikers from stumbling across them prematurely and also to
 take maximum advantage of the pipsqueak RF signal.   During
 the winter they would probably be covered with snow.
 Anywhere else they would probably get covered with vegetation.  
 
 I assume that the output of the solar panel will go down over
 time due to dirt and dust, degradation of the cells and darkening
 of the covering (tentatively Lexan XL, which is UV-stabilized)
 due to UV exposure.  My prototype 6x10" panel yields 6 volts
 at about 150 mA in sunlight.  However, my very low duty-cycle,
 gated-bias transmitter produces a power output of 200 mW (short
 pulses) with a steady state current drain of only about 10 mA.
 A small bank of polyester caps is charged from the cells to
 provide a relatively high current pulse for the transmitter.
 The CMOS oscillator and control circuitry draws less than 1 mA.
 I assume that polyester capacitors are more likely to last out 
 the years than electrolytic ones.

>Nuclear		Not feasible for the the basement project...
>mechanical	(i.e. tides, wind, etc.) subject to failure.
>thermal		Very good reliability, low power output.

 If I can get a reliable 1 or 2 mA for the control circuitry 
 and 10 mA when it 'wakes up' to transmit, I would be happy;
 what do you have in mind?

>
>Timing sources:
>crystal		mechanical reliability problems.
>rc		reliable, accuracy ~1% fairly easy to achieve 
 
 But this requires that the system have a 24 hr source of 
 power to save the count from one day to the next.

>mechanical	won't last if built in a basement...
>chemical	(clock reactions, or how about an Hg filled
>		speaker cable, with a copper plug that gets dissolved
>		away, and allows the mercury to close a circuit?)
>		Reliable, but timing accuracy is poor.
>daylight	Same problems as solar power...
>thermals	Could be a possibility since unit would not have to be
>		exposed, and earth/water (depending where you put it)
>		give a nice slow time constant. (these effects are both
>		daily, and yearly...)
>
>As for putting it underwater, I'm sure it is possible to find materials
>which would be extremely resistant to things like barnical growth.
>(How about teflon, aluminum, or maybe lead?)   
 
 I know the Navy has had a big problem with this.  I know they've
 tried aluminum without success and are currently using an anti-
 fouling coating that is very controversal due to its environmental
 effects but, in any case, it has to be re-applied every so many years.

 Anyway, how would I get the antenna up where it could be heard?

                                     --Peter Nelson