Path: utzoo!mnetor!uunet!lll-winken!lll-lcc!ames!ll-xn!mit-eddie!uw-beaver!cornell!rochester!PT.CS.CMU.EDU!SPEECH1.CS.CMU.EDU!phd
From: phd@SPEECH1.CS.CMU.EDU (Paul Dietz)
Newsgroups: sci.electronics
Subject: Re: time capsule
Message-ID: <1004@PT.CS.CMU.EDU>
Date: 1 Mar 88 07:22:21 GMT
References: <3a935422.44e6@apollo.uucp>
Sender: netnews@PT.CS.CMU.EDU
Distribution: na
Organization: Carnegie-Mellon University, CS/RI
Lines: 86
Keywords: Underwater popups

In article <3a935422.44e6@apollo.uucp> nelson_p@apollo.uucp writes:
>>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.  

The problem I see with this scheme is that it gives you lots of reliable
power now, but not one hundred years from now, when it really needs it.
>
>>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?

I'm no expert on thermo-couples, but I would think it would be possible
to reliably generate a milliamp or two by digging a deep hole (say 10
feet or so) and using the temperature difference between the surface
and the ground underneath. I tried to find some real numbers for the
current you could expect from a junction, but I couldn't find any.
Maybe somebody on the net could help with that one. As for the voltage,
Nickel-10% Chromium versus Constantan will give you about 50uV/degree C
per junction. I'm not sure what sort of temperature difference you
could expect, but I suspect that careful placement could give something
that averages near 10 degrees C. (In a dessert, for instance.)

>>Timing sources:
>>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.

Yup. But by careful design, this can be made exceedingly small. One of those
multi-farad caps made by Sohio could probably drive it for several days
before it self-discharged.

>>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.

Actually, I've had the extreme displeasure of applying bottom paint to a
boat. The poisons that they put in don't seem to work particularly well.
However, my own observations were that those beasties tended to prefer
porous surfaces. I don't recall ever seeing any on glass surfaces, for
instance. (By that I mean real glass, not fiberglass.)

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

That's the best part of putting it underwater! You keep it submerged
until the very last moment. Then, you can trigger a small chemical
reaction that either: 1. releases some weight, allowing it to be
floated to the surface; or 2.blows the tanks, which causes it to float
to the surface. At the same time, you can use all this energy to
expose your solar panel, so that when it gets to the surface, you have
enough power to put out a serious signal.

(Here's a real simple version: attach the weight with something that you
know will corrode away. Encase the electronics box in another box that
will explode (well, maybe just break) off when its internal pressure becomes
very large compared to its external pressure. Fill it with something
viscous, like Vasaline, under high pressure. At this point, use
straps to hold it together that will decay quickly under water. Thus,
when the weight breaks off, the whole thing quickly rises to the surface
and the sudden pressure change bursts off the external casing, leaving
your long protected electronics free to float to the surface and do their
dirty work. You don't have to worry about a perfect seal on the
outer box because everything still works fine so long as the leak
time constant is long compared to the time to float to the surface.)

Hope this is helpful. It's certainly an interesting diversion!

Paul H. Dietz                                        ____          ____
Dept. of Electrical and Computer Engineering        / oo \        <_<\\\
Carnegie Mellon University                        /|  \/  |\        \\ \\
--------------------------------------------     | | (  ) | |       | ||\\
"If God had meant for penguins to fly,             -->--<--        / / |\\\  /
he would have given them wings."            _________^__^_________/ / / \\\\-