Alt-BEAM Archive

Message #03049



To: beam@corp.sgi.com
From: Jean auBois aubois@trail.com
Date: Thu, 06 May 1999 13:28:06 -0600
Subject: [alt-beam] Re: HYPER WALKER 1.0


At 07:41 AM 5/6/99 , NSX - wrote:
> i dont have a clue how to draw the schematic out based on his text
> like below and most of all i dont have the getting started in electronics
> book by radio shack...

I believe that your best plan is to do precisely what Mark suggested.
That is, get a copy of the Radio Shack book: either by buying a copy
or seeing if your library has one. There may also be very similar books
at your library.

Along with looking for those books, searching the Web might be useful.
See the end of this note for an example.

In any case, what Mark describes probably works as follows:

A crystal radio is one that is powered entirely by the electromagnetic field
in which it is placed. This particular project depends on the field created
in a building as a result of "live" electrical wiring. As a result, the radio
would be "tuned" using a "tank circuit" (a capacitor parallel to a coil -- was
someone looking for a BEAM-like LC circuit? ) to a frequency of
50..60 Hz (depending on where you live). The tank circuit is connected
on one end to some kind of an antenna and possibly the other end to
some sort of "earth ground")

The following refers to the usual use of such a receiver:
The "crystal" in the description refers to the use of galena crystals (galena
is a lead compound) that acts like a rectifier -- like a diode, it allows
electricity to flow much more easily in one direction than the other. As
a result, only half of the signal (say from ground to some positive voltage)
is permitted to flow to the rest of the circuit. Since the signal that a
crystal radio usually detects is usually very high frequency (above the
range of human hearing), what is heard is only the signal that
rides "on top of" the "carrier". In other words, the carrier had its
amplitude (strength) modulated (varied) by something like music or
voice, etc. but what you hear is that rectified (remember, a diode is
also sometimes called a rectifier) or demodulated signal (the music
or whatever.)

One would probably use a germanium diode rather than a silicon
diode because they have a lower threshold (turn-on) voltage... the
voltage that is developed across the tank circuit isn't likely to be
very large and you want to get as much of it as you can. I suppose
you still can get galena crystals but I hardly think it would be at
all worthwhile.

The following refers to this use of such a receiver:
Generally speaking, the electromagnetic field in any part of such a building
is pretty much constant -- that is, its amplitude is not modulated very
much. Three things that can change this situation are:

(a) changes in power being carried by the wire as devices (lights, washing
machines and so forth) are turned off and on as well as power outages
and greyouts and other changes to the system as a whole,

(b) such electrical devices getting nearer or further from the receiver,

(c) changes in the permeability and/or dielectric constant of the space
between the receiver and the wiring, cause by objects entering or leaving
that space.

This device only relies on the third sort of disturbance. When something
like a cat wanders by whatever is being used as an antenna, the amplitude
of the signal changes. The biggest times of change are just as the object
arrives or leaves and the description given indicates that the change is
translated into spikes.

I'm not sure why Mark uses the term "sample and hold" (hold, yes -- but
sample???) because what he is describing is really not something that
fits the usual definition of that phrase. What I believe he means to
describe is something like a solar engine that isn't triggered by the
capacitor's voltage but instead by the spikes that you get when you
demodulate the electromagnetic field. It would then power something
like a motor like any other good solar engine thus scaring the cat or
whatever it is you want to do. You still have to use a capacitor & solar
cell or some other source of power to run that motor or other load --
the receiver is only being used as a sensor.

Anyhow, like I wrote above you can find pretty much what you want on
the Web if you can't find the exact book in question. For example, I
used AltaVista to search on (+"crystal radio" +plan) and found:

http://www.ametro.net/~ignatz/crystal.htm

which is a step-by-step set of instructions including a schematic. Of
course, you'd have to re-calculate the values for the coil and the tuning
capacitor to get its resonating frequency down to whatever your
house runs on and you'd have to figure out what to use instead of the
"phones". Not at all surprisingly, he refers to that good old Radio
Shack book which is more properly referred to as:

GETTING STARTED IN ELECTRONICS by Forrest Mims

Apparently, Mark's advice is still the best.



---------------------------------------------------------------
jab

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