Help with crystal radio description.

[hamster_nz]

I'm going to help a couple of 9-year-old kids to build their first radios. Being a slacker and wanting a pre-made PCB I went to Jaycar to get a crystal radio kit (yeah, I know, bad move, but...)

The instructions are terrible. It is like they have been translated by Google through every language - let's hope the kits is better...

These aren't my kids, so so I'm trying to make an explanation that is actually almost intelligible to a 9 year old that they can keep with the radio. It doesn't have to be 100% accurate, but enough that everything in the design has a reason to be there, and it gives them the general idea,

Sorry I can't include the schematics - copyright and all :-(

Is anybody keen to have a read of my draft and maybe edit bits of it into 9 Y.O. speak if I've got carried away?

Mike


Crystal radio kit
=========

About 200 years age some very bright people discovered how electricity worked, and the explanation told them something unexpected. When electricity moves, it creates waves that spread out like ripples on a pond, through air and even through empty space. At the time this was completely unexpected, and it took another 50 years or so until somebody invented a machine that could create these waves (a radio transmitter), and a second machine that could allow us to listen to these waves (a radio receiver).

This kit will allow you to create the simplest of radio receivers, one which doesn't need any batteries because you are listening directly to the radio waves themselves!

Before we start we need a few technical names for the parts, (as names are very important!)

Capacitor - An electronic component that stores small amounts of electricity.

Coil - A coil of wire, sometimes with a metal bar in it. It too can store small amounts of electricity.

Resistor -  An electronic component that only allows a limited amount of electricity to flow through it.

Wire - A wire is thin bit of metal, and metals allow a lot of electricity to flow through it.

Diode - A very special electronic component that has a very special crystal which only allows electricity to flow in one direction.

The difference between the capacitor and a coil is quite tricky to explain. A capacitor is much like a bucket for electricity - when you poor water into a bucket it stays there, however a coil is like a water balloon - as soon as you stop adding electricity it tries to squirt it back out!

Here's how the circuit works:

1) The antenna

When the radio wave pass by the antenna, it causes a small amount of electricity to flow from one end of the wire to the other, and as the wave passes it then flows back to where it started. Capacitor C1, allows this signal to get from the antenna into the rest of the receiver, and the resistor R1 allows just a tiny bit of power to return back to the antenna.

2) The resonant circuit

The variable capacitor  (VC1) and the coil (L1) form a "resonant circuit". For electricity a resonant circuit acts much like a swing at a playground.

Power held in VC1 goes through the L1, storing nearly all the energy in it (like the downswing of a swing which gives you speed), and the energy stored up in L1 generates electricity that goes back into VC1 (much like how a swing reaching the high point at the other side), and then cycle repeats over and over again.

The small signal coming in from the antenna acts like tiny pushes when you are on a swing. If you get the timing just right, then many tiny small pushes can have you swinging quite big fast swings!

The key to good radio reception is for the receiver to be able to adjusted to select which signal it receives - the frequency that the station transmits on. So how is this done? VC1 is called a variable capacitor because you can alter how it stores energy. It is a bit like changing the length of the swing to have either short ropes or long ropes - the longer the ropes the slower the pushes need to be. By changing VC1 it allows us to select which radio waves will increase the electricity held in the VC1 and L1.

3) The detector

The speed at which the energy moves between VC1 and L1 is very, very quick, and can be over a million times a second. This is so fast we can't directly connect it to a speaker and hear it.  The diode 'D1' has an essential role in allowing us to hear the radio broadcast. A special crystal in the diode only allows electricity to flow in one direction, and if the amount of energy sloshing around in VC1 and L1 is increasing it takes a tiny bit of it will pass through the diode and be stored in capacitor C2. If the radio is tuned to a station and the station is transmitting a loud sound, over time this will store more and more electricity in C2. If the same station then transmits a quite sound, the resistor R2 allows some of the electricity in C2 to escape, causing the amount held in C2 to drop.

4) The earphones

The earphone connected across allows us to hear how much electricy is in C2, therfore allowing you to hear the radio station

[Tac Eht Xilef]

No, that's pretty good for a 9-year-old level!

The only think I might change is the bit about capacitors, coils, & resonance, to help visualise how resonance works. I'd go  along the lines of:

"Capacitor - An electronic component that stores small amounts of energy as an electric charge." (Yeah, not 100% right, but...)
"Coil - A coil of wire, sometimes with a metal bar in it. It too can store small amounts of energy as a magnetic field."

And slide that difference into your explanation of resonance - how the energy from one 'swings' with the energy from the other, and if both are adjusted to match the 'speed' of the electric energy swinging from a particular radio station it tends to collect that one station's energy & mostly ignore others.

[hamster_nz]

Oh, I found the instructions online at http://akizukidenshi.com/download/ds/aviosys/2pk2300_k23_crystal_radio.pdf

Polite words escape me when describing it, esp for an introductory kit - it has to be read to believe just how bad it truly is.

[vk3yedotcom]

Is anybody keen to have a read of my draft and maybe edit bits of it into 9 Y.O. speak if I've got carried away?

Mike - I'd simplify it further. 

Though good to relate it to something kids would be familiar with, the swing analogy didn't do it for me.  I see the swing as an oscillator whereas a tuned circuit is more a gate that only passes certain signals.   

It could be worth simplifying some of the component descriptions.  While the one-way diode description is correct the main thing that's important here is that it demodulates the RF signal, leaving only AF, but I haven't found an easy way to describe this. Plus I've skipped over frequency though have described tuning.

Try this (omitting the Ancient Greeks and mangling Franklin, Faraday, Hertz & Marconi):

People have always been curious about lightning and thunder.  A smart American guy flew a kite during a thunderstorm and got an electric shock.  He was lucky not to be killed.  But it showed that lightning was just electricity jumping between two charged parts of the sky.

Years later some other smart people found they could generate electricity themselves.  That let them do experiments without having to wait for the next storm.  Some made a special type of generator called a transmitter.  If connected to a piece of wire, called an antenna, it could send invisible signals across the air.   The signals could be picked up with another antenna, connected to a device called a receiver.   Later on people worked out how to send sounds and pictures via these signals and broadcasting was born.

Radio broadcasting stations are transmitters, sending out waves of signals through their antenna. 

This kit has the parts for the simplest radio receiver you can make.   

There are no batteries.  All the power that drives its earphone comes from the broadcast station.

You don't have to pay them but must listen hard for a signal as it is not very loud. 

This type of radio is called a crystal set (after one of the parts, called a crystal diode), and works like this:

Signals arrive from the long wire antenna you have outside. 

There are thousands of signals jumbled together, including radio, TV, mobile phone, wifi, etc, but you want to hear only one.

The antenna wire is connected to special parts called a coil and capacitor which together select only the signal you want.

If you make either the coil or capacitor bigger or smaller you can select another station.  That's what the tuning or channel change button is for on your radio or TV.

The selected signal is at radio frequencies, which is so many vibrations per second our ears can't hear it. 

This is why we need a crystal diode, which strips off the radio frequency and leaves only a sound frequency.

The sound frequency has fewer vibrations per second but it's still an electrical signal so we can't hear it.

However sending it to the earphones allows you to hear the voices and music as transmitted by the radio station.

[vk3yedotcom]

Oh, I found the instructions online at http://akizukidenshi.com/download/ds/aviosys/2pk2300_k23_crystal_radio.pdf

Polite words escape me when describing it, esp for an introductory kit - it has to be read to believe just how bad it truly is.

And, if the circuit is the same as above, its selectivity will be low because the antenna is attached to the top of the tuned circuit via a large capacitor.

Ideally the coil will have taps allowing both the antenna and diode to be tapped down it. 

Taps nearer earth mean quieter audio but better selectivity and vice versa.

Particularly with a long (ie good) antenna local stations will be all jumbled together with the circuit above.

A ground connection is also important.

[hamster_nz]

Yeah some improvements are needed. Selectivity isn't a big issue (or I don't think it is) as I think we have only half a handful of AM stations.

But agree regarding the earth. What proto-electonics geek didn't have a wire jammed in their bedroom window, running to a pipe in mum's flowerbeds.... Or was that just me?

Oh, and I like the uber-simplified version too. I'll use both, as one kid is a 'know the details' sort, the other is a big picture sort.