Kids' "crystal radio", making an amplifier thereof

[Michael A. Terrell]

Well you already have enough responses, but I'll dive into this fray
anyway. The crystal set was my first introduction to electronics. I
was five and could barely read enough to follow the directions to
build that first "Philmore" crystal set kit

Add an amplifier? That's cheating.

Good antenna and ground and you should get enough signal to drive a
loudspeaker in many cities. The problem today is it is hard to find
those "Cannon" 2,000 ohm dual coil magnetic headsets - crystal
earpieces are no where near as good.. Equally hard to find a 2K ohm
to 8 ohm matching transformer to drive a speaker.

Use the common and 2.5 watt taps a 70 volt line transformer, and you
have a 2000 ohm primary. The secondaries are usually 4, 8, or 16 ohms.
Cheap, and easy to find.

Here's the mother load of crystal set designs
http://www.crystalradio.net/crystalplans/index.shtml

The one I built in the 50's that worked a loudspeaker

#74 on the crystal radio site (same site as above)
http://www.crystalradio.net/crystalplans/index.shtml

someone else with the same radio on their web site

http://hibp.ecse.rpi.edu/~john/xtal.html
http://hibp.ecse.rpi.edu/~john/xtal_pict_b+w.gif
http://hibp.ecse.rpi.edu/~john/hi_power_xtal_set.gif

There was one in the 60's or late 50's that used a single transistor
that drove a speaker - that needed a volume control. It was a reflex
type - one transistor amplified RF and audio. I think it was in Radio
Electronics but I haven't found it on line.

Then Philmore came out with a two transistor kit that could drive a
speaker with no ground or external antenna.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Kryten]

OK... Are there any modern parts under $5 that could make this work
satisfactorily?

A reel of copper wire for aerial
A germanium or schottky diode

If those don't fix it then there isn't enough RF to drive a crystal
earpiece.


But don't despair! Your kid will only think you're a big failure if you give
up and show no idea of why it didn't work. <grin!>

If you encounter problems, find out the causes, explain them and try new
ideas, then that can be a better educational experience than if the thing
worked first time.

We can all cope with easy success.
How we deal with difficulty is the key to progress.

 

[Kryten]

I didn't have any problems making one when I was about 12.

Good for you.

Just my bad luck that the guy at the component shop was less than useless,
and we didn't know anyone with rudimentary electronic skills to ask for
help.

My dad later bought a mail-order transistor radio kit.
I opened it up expecting a nice shiny PCB, and found a pair of tag strips
inside a cheap plastic box. Trying to solder huge tags without an equally
huge soldering iron resulted in yukky looking joints and no doubt
heat-damaged components. No surprise that did not work either.

It also expected a huge PP3 lantern battery - gross even in those days. In
fact, the only radios I know that want that size battery are from Roberts -
a British company for whom design fossilised in the 1950s so much that they
now have the cheek to call it 'retro' style.

 

[Homer J Simpson]

OK, I am about 25 miles west of CHicago, I will definitely try just a
good ground and a long antenna!!! My son is very interested, I want to
keep this going.

Should be OK. Have you tried the local library?

 

[Frnak McKenney]

The knowledgeable would either have some germanium diodes lying
around, or know where to get them. Indeed, the knowledgeable would
know why a germanium diode is better than silicon.

It's been a while, but I recall one design that used a razor blade, a
safety pin, and a (graphite) pencil lead as its detector. And I once
built one using a variable capacitor made from two squares of masonite
covered with aluminum foil that you tuned by rotating the squares
around a screw joining them at one corner. It used waxed paper as the
dielectric... I never _did_ get that one tuned properly. <grin!>


Frank McKenney, McKenney Associates
Richmond, Virginia / (804) 320-4887
Munged E-mail: frank uscore mckenney ayut minds pring dawt cahm (y'all)

 

[Kryten]

You're supposed to use a Cat's Whisker.

I couldn't get it to stay put.

And it wasn't any good even after I immobilised the cat.

 

[Sjouke Burry]

OK... Are there any modern parts under $5 that could make this work
satisfactorily?

i

Look at http://www.scitoys.com/
where you find all kinds of thing incuding radios
to make for almost zero cost.
It also gives you something nice to read!

 

[Michael A. Terrell]

It's been a while, but I recall one design that used a razor blade, a
safety pin, and a (graphite) pencil lead as its detector. And I once
built one using a variable capacitor made from two squares of masonite
covered with aluminum foil that you tuned by rotating the squares
around a screw joining them at one corner. It used waxed paper as the
dielectric... I never _did_ get that one tuned properly. <grin!>

They used a "Blued" blade, which is no longer made.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Michael Black]

Good for you.

Just my bad luck that the guy at the component shop was less than useless,
and we didn't know anyone with rudimentary electronic skills to ask for
help.

My dad later bought a mail-order transistor radio kit.
I opened it up expecting a nice shiny PCB, and found a pair of tag strips
inside a cheap plastic box. Trying to solder huge tags without an equally
huge soldering iron resulted in yukky looking joints and no doubt
heat-damaged components. No surprise that did not work either.

And that's not insignificant.

I suspect many are quite lucky if their first project actually works.

Mine certainly didn't, I had no real knowledge to know if the parts
that were substituted at the store were proper (or a means of actually
testing them), there were all kinds of things I could wire up wrong
because I didn't know how the transistor pinout was (or that coil),
I was barely familiar with resistor color codes and likely could have
misread capacitor values, and my soldering iron was a wood burning
tool. And on top of that, I could have simply made a wiring error.

What seems obvious once you get deeper into the field is not so
obvious when starting out.

It took two or three more projects before I got something to work.

On the other hand, if one doesn't take those first steps, nothing
can follow.

Michael

 

[John Popelish]

They used a "Blued" blade, which is no longer made.

I think you can buff a small piece of steel to a nice shine, and heat
it, gently, till it turns blue and quench it and get the same result.

 

[Michael A. Terrell]

I think you can buff a small piece of steel to a nice shine, and heat
it, gently, till it turns blue and quench it and get the same result.

It has to be high carbon steel, and a very thin edge.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Homer J Simpson]

It has to be high carbon steel, and a very thin edge.

ISTR that POWs in Germany built radios like that.

 

[John Popelish]

It has to be high carbon steel, and a very thin edge.

I didn't use the edge. I placed the tip of a nickel plated pin
against the flat blued surface. Worked pretty well, if I am
remembering correctly, something I did 40 years ago.

 

[default]

OK, I am about 25 miles west of CHicago, I will definitely try just a
good ground and a long antenna!!! My son is very interested, I want to
keep this going.

Read up on antennas. The long wire should be in line with the signal.

There's something called a "beverage" antenna that is highly
directional and efficient - downside is it has to be very long for
broadcast frequencies.

Another great antenna is the "Ewe" it is more moderate size wise
25-50 feet and every bit as good as a beverage according to the people
with both. I had a Ewe and was able to listen to NYC with excellent
reception on a communications receiver from over 600 miles away.

It is very directional, and takes a small matching transformer and
coaxial lead in to the receiver. The front to back ratio is better
than a beverage almost no signal from the 180 degree position - and
it is concentrated at about a 30 degree up angle from the antenna
(less noise from space)

Overkill for the first crystal set (beverage and ewe) but if you get
bitten by the bug there's stuff to keep you happy and occupied for a
long time.

With crystal sets the selectivity is generally poor - too wide a
bandwidth - hard to single out one station from the rest. Multiple
tuned circuits help there and directional antennas.

There's a series of small integrated circuit "tuned radio frequency"
devices that are the next step up from a crystal set. The Ferranti
ZN414 is the most famous - it is out of production but there's still
stock to be had.

You'll love this site:
http://www.arar93.dsl.pipex.com/mds975/Content/trfradios.html

A good TRF receiver with a good amplifier and speakers will sound much
better than a regular stereo AM receiver.

 

[ehsjr]

Look at http://www.scitoys.com/
where you find all kinds of thing incuding radios
to make for almost zero cost.
It also gives you something nice to read!

Nice!

Ed

 

[Roger Hamlett]

There's a series of small integrated circuit "tuned radio frequency"
devices that are the next step up from a crystal set. The Ferranti
ZN414 is the most famous - it is out of production but there's still
stock to be had.

You'll love this site:
http://www.arar93.dsl.pipex.com/mds975/Content/trfradios.html

A good TRF receiver with a good amplifier and speakers will sound much
better than a regular stereo AM receiver.

Look for the MK484 instead. It is still in production, and is compatible
with the ZN414. It is actually quite a 'useful' IC for more complex
radios, and some other things. You can make a remarkably good small radio,
using a single transistor superhet mixer down to a normal IF frequency,
one amplification stage, and then a ceramic 455KHz filter feeding one of
these. They make great decoders for ultrasonic transducers as well.

Best Wishes

 

[Homer J Simpson]

MK484

Is there an original spec sheet for this anywhere?

"It is our understanding that the MK484 device is a later _improved_ device
which is functionally almost identical to the ZN414Z. However we have not
been able to source any authentic technical data which specifically
describes the MK484; the manufacturer's own data appears to be no longer
available"

 

[default]

Look for the MK484 instead. It is still in production, and is compatible
with the ZN414. It is actually quite a 'useful' IC for more complex
radios, and some other things. You can make a remarkably good small radio,
using a single transistor superhet mixer down to a normal IF frequency,
one amplification stage, and then a ceramic 455KHz filter feeding one of
these. They make great decoders for ultrasonic transducers as well.

Best Wishes

That sounds like fun. Any links to share?

Regards
bob

 

[Rich Grise]

Read up on antennas. The long wire should be in line with the signal.

Well, as long as you understand that this means that the axis - the line
formed by the wire - should be perpendiculat to the direction to the
station. Pointing a wire at a station aims the null response that way.

Cheers!
Rich

 

[Rich Grise]

Especially by night when ionospheric bounce increases the MW range.

I used to pick up WLS in the Minneapolis suburbs, after dark.

Admittedly, that was with a "real" radio. ;-)

Cheers!
Rich