[GreenKeys] off-topic -- crystal radio

[Chris Elmquist]

There's a good discussion of a coil here:

http://www.techlib.com/electronics/crystal.html

When I was a kid, I lived about 3 miles from a max power AM station
on 1500 KHz.  No matter what tuning arrangement I used-- however much
effort I put into coils, caps and all the care in the world-- that's
the only station I could ever receive.  No matter where or how I tuned it.

If you are close enough to a transmitter, my guess is that you can be
pretty loose with the LC front-end and you will still receive something.

Chris NØJCF


On Tuesday (10/07/2008 at 09:03AM -0700), 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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-- 
Chris Elmquist
mailto:chrise at pobox.com