[GreenKeys] off-topic -- crystal radio

[John J Mccarty]

Gil;

   I don't have it here at the office, but in 73 magazine Sept 1998, there was
an construction article for a "Green Radio" (no relation to Uncle Wayne) crystal
set. The set used tissue paper tubes for coil forms and pop cans for caps. The
pop cans are covered with view foils from an over head projector and strips of
aluminum tape to form the caps. The user slides the view foil with tape up and
down to "tune" the set. The radio in the acticle has two coils, two caps. It
tunes a lot sharper that the usual one coil rig. You can pre-built some of the
radio depending on the age of the scouts. Over the years we built a bunch of
these radios as "kits" for girl and boy scout groups and the kids had a lot of
fun with them. One of the other guys built a version with only one coil for
younger scouts and it also worked pretty well.


73


John n9hrt

On 10/7/2008 11:03 AM, gil smith wrote:
> Hi folks:
> 
> This is obviously off-topic, but since there are lots of radio guys on 
> greenkeys, I thought someone would have some input.
> 
> I want to have my cub scouts build crystal radios next week, and I need a 
> bit of expertise to point me in the right direction.  Being an electrical 
> engineer, you'd think that I would have played with a crystal set in my 
> youth, but I never did.
> 
> So I picked up a cheesy little $10 kit yesterday and put it together -- 
> actually works!  It is the simplest design, using a single coil wound on a 
> cardboard tube, with a brass ball slider to tap it along a sanded section, 
> and uses a 1N60 diode and a standard ceramic hi-z earphone.
> 
> I have eleven scouts, so I can't use up half my year's budget on $10 kits 
> (plus they are a pain to put together).  So I want to use PVC or ABS pipe 
> for the form, and pre-wind the coils for them myself.  I can make wood 
> bases no problem, and just use screws for connecting hardware.  The most 
> expense will be the earphones and diode, which look to be about $3 per kid 
> plus shipping.
> 
> I think the basic one-coil, tapped design will be the best for us, since I 
> likely can't afford to get eleven variable caps used in other crystal 
> designs.  I have googled for hours, and have still not seen any good rules 
> of thumb for designing these.
> 
> Does anyone have any guidelines on coil diameter, wire gauge, number of 
> turns, etc?  I have seen coil winding calculators on some crystal web 
> sites, but they are for getting a desired inductance to resonate with the 
> (typical 365 pF) variable caps, targeting a general frequency of 
> interest.  For the basic design without the cap, there must be some general 
> coil parameters that change the tuning freq range, selectivity, or 
> something, but I have not seen discussion of it.
> 
> Any thoughts on the diode?  1N34 and 1N60 Ge units are typical, but some 
> folks say schottkys with work well.  I'd also like to try the 
> razor-blade/pencil-lead thing, but just as a demo and not for the kids' units.
> 
> Antenna?  Just get a decent length up as high as practical?  I presume that 
> the broad side should face the transmitters (which are all on one mountain 
> here in Phoenix).
> 
> Any other ideas or thoughts?
> 
> thanks in advance,
> 
> gil
> 
> 
> 
> 
> 
> Vaux Electronics, Inc.
> 480-354-5556
> (fax: 480-354-5558)
> www.vauxelectronics.com
> 
> 
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