Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cwjcc!neoucom!wtm
From: wtm@neoucom.UUCP (Bill Mayhew)
Newsgroups: sci.electronics
Subject: Making a galena crystal
Summary: Do It Yourself crystal set
Message-ID: <1844@neoucom.UUCP>
Date: 28 Nov 89 04:46:31 GMT
Sender: wfd@neoucom.UUCP
Organization: Northeastern Ohio Universities College of Medicine
Lines: 91

It is possible to make your own ersatz galena crystal in your very
own basement and/or garage.  Regular solder contains enough lead to
suffice.  It is best to start with plain solder wire that does NOT
have a flux core.

Cut up some solder wire and place it in some sort of refractory
vessel such as a small steel or ceramic cup and heat it until it
melts.  A handy propane torch should suffice.  Sprinkle in some
sulfur powder when the solder melts.  You can get sulfur power at
most well endowed drug stores.  When the resulting glop cools, a
mess containing at least a little PbS will form.  If you do this,
please be extremely careful, as molten solder and blow torches can
burn you or burn your house down if you get careless.

The detector can be formed by cutting off the end of an electrical
conduit and drilling 'n tapping a hole for a screw to clamp in the
ersatz galena crystal.  A wire can be attached to the screw that
holds in the crystal.  It is left as an exercise for the reader to
device a nice way to attach the crystal to a piece of pine or oak
"breadboard".  A steel safety pin can serve as a _cat's_whisker_ to
probe for the sweet spot on the crystal.  Screw the cat's wisker
down on the breadboard next to the crystal.  A small piece of pencil
eraser over the business end of the cat's whisker is helpful to
serve as an insulator to keep your finger from damping out the
signal while you probe around for the sweet spot.

Next comes winding the coil for the tank circuit.  A round oatmeal
box works.  I wound enough #32 AWG wire to cover most of an oatmeal
box.  The series resitance is about 78.2 ohms, with an inductance
of about 14.05 mH and Q factor of about 1.1 at 1 KHz according to
my trusty Fluke 710B LCR bridge.  The inductor is the best place to
do the tuning.  Sand off a the varnish along the side of the
make-shift coil, and use a paperclip or a strip of tin can screwed down
to the breadboard.

Give the great performance of the tuning coil, it is probably best
to go with a parallel input tank cirucit.  You can make a condenser
(capacitor to you young whipper-snappers) by stacking sheets of
waxed paper and tin foil.  I'll leave figuring out the capacitance
versus area as an exercise.  Its fun to experiment anyway.  I still
haven't made my condenser, so I don't have any Fluke figures yet.
Starting with too big a size condenser is probably a good idea, and
then trim down with a pair of scissors.

For tuning up, you might want to use a germanium transistor as a
diode bfore adding the complexity of hunting for the sweet spot on
the galena crystal.  Sort of like Bell dialing 555-3341 to call
Watson ("Mr. Watson, come here I need you.") to tell him that he
had just invented the telephone.  Who said we can't be modern,
anyway.

Getting the headphones is tough.  Only the most enterprising
readers would want to make their own.  The best place to look is to
talk to guys with long beards and boxes full of bulbous looking
vacuum tubes at electronic flea markets at ham radio "hamfests".
They'll be able to set you up with a nice pair of Hi-Z phones.  Be
sure to tell them you're a nut building a crystal set from scratch.
They might even be able to fix you up with some Fahnstock Clips for
making your connections and giving your project that read old-timey
look.

This is the basic schema:

   ___ (ant)                      __
    |        ------------>|------O  O-----
    |        |      (crystal)  (phones)  |
    +--------+-\  C                      |
    |           \ C                      |
   === (cap)      C                      |
    |             C                      |
    |             |                      |
    +-------------+-----------------------
    |
   --- (gnd)
  ///


A cold water pipe works well for the ground.  Don't use a hot water
pipe or a gas pipe.  The latter for safety's sake, and the former
beause hot water tanks often are electrically isolated from the
cold water supply and hence earth ground.  A ridiculously long
piece of wire is best for the antenna.  Best to keep the antenna
indoors, as stringing wire outside can serve as an attractat for
lgihtning, which is very uncool.

Standard disclaimers, of course.  Build at your own risk.  Not
responsible for screw-ups, silliness, suitability for intended use,
etc.

Bill
wtm@neoucom.uucp


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