Kids' "crystal radio", making an amplifier thereof

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.

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.

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.

Should be OK. Have you tried the local library?

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.

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

-- A science is any discipline in which a fool of this generation can go beyond the point reached by a genius of the last generation. -- Max Gluckman

--

I couldn't get it to stay put.

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

Look at

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

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

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.

ISTR that POWs in Germany built radios like that.

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.

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

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:

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

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Nice!

Ed

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

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"

That sounds like fun. Any links to share?

Regards bob

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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

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

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

Cheers! Rich