[Greenkeys] crystal radio

[gil smith]

Hi Brooke:

I am building a dozen simple (that means cheap) variable-coil crystal 
radio sets -- a coil is wound (and I still have no idea how to 
optimize it), the windings are sanded off in a linear section, and 
some sort of slider is used to tune it as a 
variable-inductor.    There is no variable cap in this design, 
although John White has prodded me to add a small fixed cap to 
perhaps kick up the sensitivity (hopefully not at the expense of selectivity).

My prototype is this cheesy-ass kit I picked up for ten bucks at the 
scout shop.  It was a pain to build, but hey it does work -- I have 
picked out three distinct AM stations:
http://store.sundancesolar.com/crrakitfrsls.html

The schematic is pretty simple:  the antenna goes to the 
coil's-tap/detector and the ground goes to the other side of the 
coil.  The tap also goes through the detector (polarity is not 
important) to the earphone which returns to ground.  Some variations 
include a resistor of about 47K across the earphone, and a cap of 
about .001 uF across the earphone -- dunno why, as all of the 
explanations are very un-scientific.

This was likely among the very earliest of xtal radio designs.

This modern one has a cat's whisker detector, and is very nicely 
built.  I love the vertical coil design, but the slider is more 
complicated than a simple horizontal design (the guy has a machine shop):
http://en.wikipedia.org/wiki/Image:DSC01718s.jpg


The "foxhole" design, in tribute to WWII guys, usually uses a 
razor-blade/safety-pin-graphite detector.  It is the same basic 
design, though I just want to use a germanium diode as opposed to a 
razor blade or cat's whisker, but my final design will likely be 
similar to this:
http://www.radiodaze.com/KIT-CORNELL.htm

Here is a foxhole video (which crashes firefox 3.03 , but runs under IE):
http://www.metacafe.com/watch/1075464/foxhole_radio_fox_hole_radio/

And I need to keep it cheap, so variable caps are out.  My only 
tweaks are how to wind the coil (wire gauge?  diameter?  number of 
turns?), possible parallel cap for a low-Q tank, and the resistor/cap 
thingy across the earphone.  But since I have yet to see any 
technical explanation of any of these, I guess the whole design will 
just evolve on the bench.  Perhaps that is a fitting tribute to the 
early designers anyway.

gil



At 03:57 PM 10/9/2008, you wrote:
>Hi Gil:
>
>Crystal sets are one of my interests, see:
>http://www.prc68.com/I/Xtal1800.shtml
>
>But I'm not clear on the circuit you are using.  Can you send a schematic?
>
>Have Fun,
>
>Brooke Clarke
>http://www.prc68.com/P/Prod.html  Products I make and sell
>http://www.prc68.com/Alpha.shtml  All my web pages listed based on html name
>http://www.PRC68.com
>http://www.precisionclock.com
>http://www.prc68.com/I/WebCam2.shtml 24/7 Sky-Weather-Astronomy Web Cam
>
>gil smith wrote:
>>Hi John and gang:
>>Thanks for the info.  I didn't think of a fixed cap across the coil 
>>-- interesting idea;  was trying to avoid expense of a variable, 
>>but I have lots of 100pF caps lying around.  I need to experiment 
>>tomorrow.  I presume Q of the coil will be low enough that the cap 
>>could help sensitivity without losing too much selectivity.
>>I set up a little gear motor bolted to a plastic pipe cap so I can 
>>wind coils, and made a test coil pretty quickly on 1.5" ABS 
>>pipe.  Ordered a dozen hi-z ceramic earphones and 1N34A diodes from 
>>mouser.  Now I need to figure out the coil turns and possible cap, 
>>to get something workable.  Gotta make a dozen of these things, 
>>without spending a bunch.  Getting harder to find magnet wire these days too.
>>I have still never found, in all my googling, info on optimizing 
>>the coil for a basic (tapped/slide-tuned-coil with no-variable-cap) 
>>crystal design.  All I can say at this point is they all look like 
>>a hundred or so turns on a 1.5 to 2 inch form of some sort.  24 GA 
>>magnet wire seems to be a decent size, just to pull a number out of 
>>my ass.  Not a very scientific way to approach it, but it is all I 
>>have at the moment.  I have no data on wire gauge, or cardboard 
>>tubes versus PVC versus ABS plastic, but I presume they are all 
>>fine.  I plan to use ABS since it is cheap and black.  The 1.5" 
>>(ID) size seems to be about right for typical coils.
>>Local transmitters are about 25-30 miles away from here, but all in 
>>the same spot.  Am I correct in assuming that the (random-length 
>>longish-wire) antenna should be perpendicular to the path?  Seems 
>>that I recall that.  Found out that our home water faucets are 
>>indeed not grounds, since the copper pipe transitions to CPVC 
>>inside the wall.  I dare not tell the scouts to use the ground wire 
>>in an outlet, since I can't have them poking wires in a 
>>socket.  And if the traditional faucet is out, I guess they need to 
>>go sit out by the grounded electrical service box to clip on a 
>>ground lead.  Ah well.
>>Another odd question:  Since analog TV bites the big one this year, 
>>how long do you think it will be until digital radio replaces analog radio?
>>Then none of these crystal sets will work anymore.  Ah progress.
>>Anyway, thanks to everybody who offered insight on crystal radios 
>>-- I'll let you know what I finally come up with.
>>gil
>>
>>
>>Vaux Electronics, Inc.
>>480-354-5556
>>(fax: 480-354-5558)
>>www.vauxelectronics.com
>>
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Vaux Electronics, Inc.
480-354-5556
(fax: 480-354-5558)
www.vauxelectronics.com