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Electronic TechnicianPosted - 22 July 2013 23:20  Show Profile  Email Poster  Edit Message
When I was in the Eight grade in school, I built my first crystal radio. It used a toilet paper roll as the coil form and had 3 windings on them. There were 20, 30 and 80 turns on it. It used a crystal earphone and a plastic insulated capacitor. The diode was likely a 1N34a. I suspect the smaller windings were for antenna and detector, but I don't remember the circuit. Does anyone here know this circuit?
gzimmerPosted - 23 July 2013 1:37  Show Profile  Email Poster  Edit Message
> I suspect the smaller windings were for antenna and detector

You are almost certainly correct. There's not many other interpretations.

A search on Google for "toilet paper roll crystal set" turns up dozens of hits.

Toilet paper rolls are not ideal however. You end up with a long skinny coil with a fairly poor Q due to the shape factor.

Better to go to the Post Office and buy a mailing tube.

............ Zim


Edited by - gzimmer on 7/23/2013 1:38:44 AM

golfguruPosted - 23 July 2013 2:36  Show Profile  Email Poster  Edit Message  
Or go modern and use a HDPE screw-top capsule bottle or similar. (Ours are 3.5" diam. x 3" winding length)

Similar to this:
http://www.ebay.com.au/itm/Pharmalife-Odourless-Fish-Oil-Tablets-1000mg-200-Capsules-X2-Bottles-Brand-New-/330956636058?pt=LH_DefaultDomain_15&hash=item4d0e8d7f9a

............

Edited by - golfguru on 7/23/2013 2:51:52 AM

homebrewPosted - 23 July 2013 8:22  Show Profile  Email Poster  Edit Message  
I learned early on that parafin soaked tubes made better coils. And were easier to wind. (free parafin from mom's canning supplies) Reception was about the same as oatbox and salt box sets. Tried other materials, canning jars, shrink wrap and wood laminate rolled into tubes, and the barrel of a old plastic cannon. Four inch bakelite coils from a old battery radio probably worked best. From rural central Illinois WLS in Chicago and WLW in Cincinnati were good high power targets.
Smaller wire, 26 to 30 ga scrounged from old motors mainly and wound the same length as the diameter worked best. Q of the ones I've managed to keep over the years ran in the low 200's.

Edited by - homebrew on 7/24/2013 2:39:38 PM

_J_Posted - 29 July 2013 21:38  Show Profile  Email Poster  Edit Message
I made one with toilet paper coil as a kid with 3 "windings" (described as windings, don't remember the numbers) but it was a coil-only tuner, the 3 windings were connected together as (as one with taps) and the taps were used to tune stations. No capacitor, only coil and detector (galena).

John Davidson

Edited by - _J_ on 7/29/2013 9:41:57 PM

Electronic TechnicianPosted - 30 July 2013 22:47  Show Profile  Email Poster  Edit Message
I know there are much better circuits. I thought it would be fun to build the same radio I built as a child. Seems kind of close to the "standard crystal set".
Electronic TechnicianPosted - 30 July 2013 22:48  Show Profile  Email Poster  Edit Message
I know there are much better circuits. I thought it would be fun to build the same radio I built as a child. Seems kind of close to the "standard crystal set".
Electronic TechnicianPosted - 30 July 2013 22:48  Show Profile  Email Poster  Edit Message
I know there are much better circuits. I thought it would be fun to build the same radio I built as a child. Seems kind of close to the "standard crystal set".
homebrewPosted - 31 July 2013 7:15  Show Profile  Email Poster  Edit Message  
You can have a lot of fun with a basic circuit. Most of them make good antenna tuners for more advanced sets.

_J_Posted - 31 July 2013 18:33  Show Profile  Email Poster  Edit Message
I do like simple circuits.

John Davidson

W0XIPosted - 7 August 2013 10:39  Show Profile  Email Poster  Edit Message
Nice to see some posts here. I've been playing with an infinite impedance detector, using a JFET as a source follower, thus preserving the Q of the tuning circuit. Of course it's not traditional in that one needs a battery in the drain to provide the "self" bias! Phil, W0XI

Phil, WXI

What Would Maxwell Say [1831-1879]


Bill SteinhourPosted - 28 August 2013 22:14  Show Profile  Email Poster  Edit Message
I recently tried a zero bias MOSFET in an old xtal set I pulled out of storage and using the circuit from the QST archives. It had a loud. hissy, tearing sound in the audio, varying with the modulation amplitude. Then the hiss went away and all I got was the same volume as with an 1N34A.

I finally figured out the circuit in QST used only a ferrite loop rod with no direct antenna connection. The article also included a SW circuit with an antenna connection and included a protection diode that would limit the gate voltage from strong signals. I think the hissy tearing sound was the breakdown of the device's gate insulation and subsequent complete device failure. I'm awaiting arrival of another MOSFET (and a spare)for another try.

bs

gzimmerPosted - 29 August 2013 1:7  Show Profile  Email Poster  Edit Message
Hi Bill,

I'm always puzzled why the ZB FET gets so much attention, when a normal RF FET works as least as well (although it does require adjustable bias).

And yes, the Z.B. FET has a protection diode which will conduct on strong signals, and is easily damaged.

For what it's worth, that ZB FET article is one of a long string of similar designs using both FET and Bipolar transistors.
Here's one (in Russian via Google translate) using push-pull bipolar transistors.
http://translate.google.com.au/translate?sl=auto&tl=en&prev=_t&hl=en&ie=UTF-8&u=http://the-mostly.ru/misc/radio_with_no_power.html

I have a collection of similar articles if anyone is interested.

......... Zim


Edited by - gzimmer on 8/29/2013 10:08:12 PM

_J_Posted - 9 September 2013 23:42  Show Profile  Email Poster  Edit Message
Zim,
You are puzzled because you know better than to believe their underlying thesis -that low forward offset voltage has anything to do with it.

How a simple little diode behaves with small signals must be the most universally misunderstood topic of electrical engineering. And the impact of misunderstanding that profoundly interferes with rational crystal radio design.

John Davidson

RichardPosted - 10 September 2013 8:6  Show Profile  Email Poster  Edit Message  
John, in twenty five hundred words or less, would you care to elaborate? If you do please use small words. ;)
homebrewPosted - 10 September 2013 9:51  Show Profile  Email Poster  Edit Message  
Graphs are easier to understand most of the time. I cannot completely agree with the article but it is a place to start.
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