MadSci Network: Engineering


Date: Mon May 21 20:48:21 2001
Posted By: Karl Kolbus, Staff, Data processing, Mequon Consulting Corp.
Area of science: Engineering
ID: 988993881.Eg

Hi Josh, 

Your question brings back some fond memories of my youth, and some not-so-
fond memories of the Viet Nam war.

As a kid, I made many crystal radios; most using a Galena crystal and cats 
whisker, and later with 1n34 diodes or transistors. Transistors work well 
because they are basically back-to-back diodes - the base being common to 
the emitter and the collector, so you could use either base/collector or 
base/emitter. The first transistor readily available to the experimenter 
was the Raytheon CK-722, and could be had for 99 cents back in the early 
50's. That would be the equivalant of about $15 in todays money!

In the jungles of Viet Nam, you carried in only what you needed to 
survive. That meant no portable radios, no electric razors, etc. Our 
crystal radios were much the same as the foxhole radios of the WWII, in 
that we used razor blades for the detector, AND for the headset. Uncle Sam 
was kind enough to provide us with C-rations and K-rations left over from 
the WWII and Korean wars - er, excuse me - police action. In both you 
would have cans of crackers and jelly. The jelly was in a smaller can, 
about 2" dia., and 3/4" tall, and was packed inside a somewhat larger can 
with the crackers on top of it. The jelly can is what we used because your 
ear would fit inside it and would be very close to the bottom. 
Construction was as follows:

1. Open the can with your ever-present can opener. (Uncle Sam designed a 
little folding can opener that was about 1" long and was hinged and had a 
hole in it so you could keep it on your dogtag chain. With a little 
practice, you could open a can faster than any electric can opener ever 
2. Eat the contents, or throw it away - your choice. After untold numbers 
of cans of grape jelly, you really got tired of it.
3. Punch a rectangular hole in the bottom, slightly smaller than the razor 
4. Smooth off the rough edges of the hole, leaving only one or two SMALL 
sharp points sticking up into the can.
5. Lay the razor blade (NOT stainless steel - it's non-magnetic) over the 
hole, inside the can, with one end of it resting on one of the remaining 
sharp points. Secure the other end to the can by dripping candle wax on 
it. Two or three drops is sufficient.
6. Drive an iron or steel nail (not stainless) through a 1 1/2" long, 1/2" 
dia. piece of bamboo, leaving the head flush against the surface.
7. Wrap your magnet wire around the end of the nail that's sticking thru 
the bamboo, and connect it to your radio. Use very fine wire if possible, 
and put on as many turns as you can. We always carried a quantity of #36 
wrapped around a regular thread spool from our sewing kit. If caught, the 
V.C. would think it was just thread!

To use it, put the can around your ear and place the nail/wire/bamboo 
voice coil against the bottom of the can so the nail head is in close 
proximity to the razor blade. Viola! We either held the contraption in 
place by tying a piece of string around our head, or jammed it between our 
helmet liner and head - not very comfortable.

You could make it more efficient by magnetizing the razor blade, prior to 
sticking it inside the can, by stroking it with a magnet or, if you didn't 
have a magnet, strike one end of a heated blade vertically on a rock a few 
times. The earth's magnetic field would give it a slight magnetism. 

We could usually pick up 3 stations - Voice of America, AFRS, and the BBC.

Here's a link you might find interesting.

Good luck!

Your Not-So-Mad Scientist


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