Hooking up Output Transformer Crystal Radio Plans Help!! Help!!!

[John Woodgate]

I read in sci.electronics.design that Jim Meyer
ing.google.com>) about 'Hooking up Output Transformer Crystal Radio
Plans Help!! Help!!!', on Fri, 2 Jan 2004:

A diaphram isn't really needed. Simply place the homemade mike
so that it touches the earphone/speaker. It then becomes a contact
mike. Probably even more efficient than passing the acoustic energy
through the air.

I just don't see it as being practicable to do that with your pencil
lead balanced on two razor blades. The practical carbon amps I've seen
(at the Science Museum in London) have the two diaphragms joined by a
short metal rod, not relying on acoustic coupling. Actually, I don't see
why one diaphragm would not be enough, and some may have been made like
that. Damn clever, our ancestors. (;-)

 

[N. Thornton]

Two single-edge razor blades mounted in a block of wood with their edges
pointed up and a piece of carbon from the inside of a "lead" pencil laid
perpendicular across the edges, will create a very usable carbon mike. Make
electrical connections to each blade, of course.

I doubt that would give enough gain though. With a real carbon mic
youve got 100 or so grain bondaries in series with each one varying
its R. With a pencil lead youve got all of 2 contact boundaries
varying their R. A proper carbon mic can drive a speaker directly.

IIRC one of the first radio telephony transmitters was a 500w high
frequency alternator with a carbon mic in the ground lead: the mic
handled the lot, no tronics, modulated the alternator output directly.


Regards, NT

 

[Michael A. Terrell]

Carbon amps were popular in the 20s, especially with crystal sets. As
a pair this was your working class radio, no valves, so the huge
expense of most radios was not there. A carbon amp is basically a
moving iron earphone mated with a carbon microphone: the thing
amplifies quite heavily, if not too faithfully. You can make one by
gluing a toothpick from a carbon mic diaphragm to moving iron earpiece
diaphragm, both removed from a 70s or ealier dial phone.

It works with no toothpick just putting them face to face, but the
toothpick will up the gain aplenty. 12v is the rated working voltage
for the mics.

Where to get them from? I dont know a supplier, I've just kept my eyes
open for scrap old phones.

Regards, NT

I scrapped several hundred business phones that use these. I kept all
the elements, networks, and the touch tone pads.

 

[Sven Franklyn Weil]

I scrapped several hundred business phones that use these. I kept all
the elements, networks, and the touch tone pads.

Michael, were these Western Electric 2500 sets?

I'm looking for the face plate for one -- grey or black (and the case
in black) if you happened to save these...and the clip that held it in
place.

There's one on the phone right now, but it was cannibalized
from some other brand of phone sometime in the distant past and it
doesn't fit very well. There's a small piece missing of the case
also.

 

[Robert Baer]

When I was in college the RF prof built a crystal set that drove
a loudspeaker rather well off a long-wire antenna to the next
building over. ;-) Indeed he got enough juice out of the air to
light an LED (more interesting than a crystal or any other diode
they tried) used for the detector and drive the speaker. The
thing was still playing (continuously) in the lab the last time I
was there (over 25 years later).

Many years ago, a friend of mine lit up a flashlight bulb from a
crystal radio that used a 100 foor antenna.
Brightness changed withthe music or speech.

 

[James Meyer]

I'm not too sure that a full wave detector won't produce more
power than a half wave detector will.

I'll simulate both in LTSpice to see what happens.

Jim

While searching for a good SPICE model for a 1N34A, I ran across this
site: http://uweb.superlink.net/bhtongue/index.html.

It has a lot of good scientific information and I'm going to wade
through it before I attempt my own simulations.

Jim