Crystal Radio Questions

I'm winding a coil for a crystal radio. The original plan says to wind the antenna coil on top of the tuning coil, whereas another version says to wind the antenna coil *adjacent* to the tuning coil, separated by about

1/4 inch.

If I wind both versions of the antenna coil on the form, will one winding interfere with the other? That is, I would only hook up one antenna coil at a time, so should I short the ends of the other (unused) antenna coil together?

Another couple of questions. I have a few germanium transistors, about a dozen each of NPN and PNP. What connection should I make to use one of these as the detector diode - base and emitter or base and collector?

Would a full-wave rectifier setup using four of these as diodes do any good?

Thanks.

--- Joe

Reply to
Joe
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yes, they will be coupled inductively.

in which case the unused coil will basically disappear.

No, don't do that.

I'm not sure, ideally you want something like an OA47 diode.

probably not. the idea with the detector in a crystal set is to turn on at the lowest possible voltage and then to present a high resistance to give maximum sensitivity and to load the tuning coil as little as possible to keep the 'Q' high. going to bridge configuration doubles the impedance.

IME crystal sets need a long antenna and a good earth connection. modern buildings often use platic pipes, which will make finding a suitable earth harder, you may have to make one yourself.

the earpiece will be another issue, that should be high impedance too, like a piezo-electric "crystal" earpiece. if you can't find one perhaps you could use a small piezo speaker - perhaps with 3.3K resistopr in parallel

Reply to
Jasen Betts

Yes. Built one driving a loudspeaker with usable volume (when I was a kid). We had these point contact germanium diodes, back then. They seemed to work the best for sensitivity and power.

The plans for the one I had:

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Two others

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Master list of designs

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There's a lot of folks still building them and there are lots of good sources on the net, if you care enough to search.

Reply to
default

You can use the base to emitter junction of the transistors, with germanium the pnp transistors were normally the best.

Reply to
David Eather

Sorry Jason - I mistakenly answered the wrong post

Reply to
default

Whatever junction you use you want to short the unused pin to the base.

On silicon transistors the C-B junction is much more robust than the B-E junction; I would expect the same for a germanium transistor.

--
www.wescottdesign.com
Reply to
Tim Wescott

Answered already, with the opposite of what my intuition would tell me -- but my intuition was developed with silicon transistors; perhaps there's some magic to germanium that I don't understand. You may want to try both ways and see what works best and what seems to be robust.

In my limited experience a germanium diode (1N33; I'm not even sure if it's epitaxial or point-contact) lasts until the first lightning strike the next county over, then dies. My kid and I were killing them with a flea-power spark-gap transmitter (a relay made into a buzzer) in the same room.

Hot-carrier diodes (a fancy name for low-power Schottkeys) work almost as well as germanium, and are more robust, if you can lay your hands on them. Silicon diodes work at a severe cost to sensitivity. I always wanted to try a silicon diode with a biased detector.

It won't gain you anything. The whole crystal radio idea is that you're trying to power the headphones from energy you harvest out of the aether; you can't think in terms of volts (or amps) alone, but instead in terms of efficiency. At best a bridge detector could be made to work as well as, but no better than, a single diode.

--
www.wescottdesign.com
Reply to
Tim Wescott

Hi Joe, This is fun to build also, we got many to work. If you have some strong signals in your area. There is history behind it also.

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Tom

Reply to
Tom Biasi

"Robust" is not the desired property of a detector diode and you don't want to short the unused pin to anything. Your post is simply wrong.

Reply to
David Eather

Every germanium diode I have used in a crystal set died as a consequence of either fairly distant lightning or a fairly near spark gap.

_You_ may not want robust, but _I_ do.

--
www.wescottdesign.com
Reply to
Tim Wescott

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il

the coils will inevitably interact, either way. Best to wind them with a gap between them as this minimises the coil capacitance, enabling wider tuning range.

Don't short the unused one, this would kill the inductor's Q.

NT

Reply to
NT

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