Crystal Sets 4 - Crystal Set by Kenneth Rankin

	CRYSTAL SETS 4 A Crystal Set By Kenneth Rankin
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CRYSTAL SETS Parts: 1 2 3 5

CRYSTAL SETS 4: A CRYSTAL SET BUILT BY KENNETH RANKIN

MDS975.co.uk reader Roddie Rankin kindly sent in the details of a crystal set built by his son Kenneth for a school project which won him great praise and merit. The crystal set is so nicely made and the project so well written that I thought it deserved a special page of its own.

Photograph Of Kenneth Rankin's Rather Super Crystal Set

CONTENTS
1 - What Is A Crystal set?
2 - How A Crystal Set Works
3 - History Of Crystal Sets
4 - How I Built My Crystal Set
5 - The Results I Obtained
6 - Where I Found The Information
7 - Quiz

1 - WHAT IS A CRYSTAL SET?

Perhaps you think, like I did, that a crystal set is just a collection of lovely stones. In fact a crystal set is a very early, simple domestic radio. The photograph above shows the crystal set I built.

The crystal set does not have a battery. It runs completely from the radio waves it picks up in the air. It is absolutely free to run!

Please read on to find out more about this exciting invention!

The birth of domestic radio: A deluxe crystal set produced by Marconi for the reception of programmes from the first of the BBC's radio stations, 2LO in London.
[from Pageant Of The Century - Odhams Press]

2 - HOW A CRYSTAL SET WORKS

The diagram below shows the circuit diagram of the crystal set I have built. It consists of a coil (L1), a tuning capacitor (C2), a diode (X1), an aerial, a crystal earpiece and an earth. (Capacitors C1 and C3 are not essential.)

Circuit Diagram Of The Crystal Set

First the aerial picks up radio waves from radio stations all over the world. Then the required station needs to be selected, this is called tuning, and this is achieved with a Tuned Circuit. The Tuned Circuit in the crystal set consists of two items; a component called a variable capacitor and a coil of wire.

By adjusting the value of the capacitor and coil tuning is achieved. The coil taking unwanted signals of low frequency straight through to the earth, which gets rid of them, while the capacitor takes unwanted signals of high frequency straight through to the earth. The remaining signal, in between the high frequency and low frequency signals, is the signal of the required station which is not passed to earth by either the coil or the capacitor. This is the frequency to which the set is now tuned in.

You normally change the frequency to which a set is tuned in to by turning the knob on the tuning capacitor, thus receiving different stations by varying the capacitance of the variable capacitor. The number of turns on the coil can also be changed [called varying the inductance of the coil] and this will give different tuning ranges. A larger number of turnes provided longer wavelengths [lower frequencies], while a smaller number of turns of wire provides shorter wavelengths [higher frequencies].

[ It is also worth noting that the inductance of a coil can be increased by inserting a component called a Ferrite Rod into the centre of the coil. ]

Once tuned into the required station this remaining signal goes to earth by travelling through the diode and earpiece. The diode takes the sound signal out of the radio signal [called demodulation], and passes it to the earpiece. The sound signal travels through the crystal in the earpiece which vibrates the air in the earpiece giving us the sound of the radio station to which you are tuned.

3 - THE HISTORY OF THE CRYSTAL SET

The history of crystal sets begins with the first broadcast stations being opened. This happened in England in 1922. The simplest form of receiver for these broadcasts was the crystal set.

In those early sets they had "cat’s whiskers" instead of diodes. A cat’s whisker was a fine piece of wire which was adjusted to make a suitable contact with a crystal. The crystal would be made of a substance such as coke or galena. An example from 1923 is shown in the picture below. Notice how well made these instruments were.

There were several limitations to the crystal set: it needed a big aerial, an earth connection, the clumsy cat’s whisker, and the weak signal could only be listened to by one person at a time with headphones. Very quickly the crystal set began to be replaced by valve radios with loudspeakers, powered by battery (see the picture from 1923).

In World War II, crystal sets were used by prisoners of war in prison camps to listen to news from home. As they were often badly treated it was hard to get all the right parts to build the set.

By this time crystal sets were no longer used in the home. Instead radios like the attractive 1937 model in the picture were being used.

Nowadays people only use crystal sets as a hobby. There are various different kits available on the market (see websites in the references below).

Photograph Of A Crystal Set From 1923
[from Pageant Of The Century - Odhams Press]
Crystal sets were primitive instruments used for 'listening in', and were fitted with the now famous cats whisker. Listening was done though a pair of headphones which meant that only one person could listen at a time.

Photograph Of a 1923 Radio
[from Pageant Of The Century - Odhams Press]
Later valve amplification could be added to radios which allowed them to power a loudspeaker so that everyone in a room could listen in.

4 - HOW I BUILT MY CRYSTAL SET

The crystal set is a fun and easy thing to build.

The first thing I had to do was put up an aerial and earth. For the aerial I drilled a hole in the ceiling and fed wire through the hole. Then I went into the attic and took the wire to the other end of the attic, fastening it to rafters as I went. When I got to the other side I fed the wire out of an air vent to the outside of the house. I then went outside tied wire to a nearby tree and joined it up to the wire coming out of the air vent. This gave me an aerial of about 25 metres.

Next the earth. For this I drilled another hole in the ceiling and fed another wire through and out of an air vent at the other side of the house (see previous paragraph). I attached this wire to the metal frame of my swing. This is a good earth because its poles stick deep into the ground.

The coil used enamelled copper wire wound on to a piece of plastic pipe with a diameter of 5 cm. There were 120 turns of wire. Every 10 turns I made a loop on which a crocodile clip could be fixed. See the picture below:

Photograph Of The Coil Showing The Tapping Points & Crocodile Clips

The Tuning Capacitor - Being Carefully Adjusted

For the crystal set itself, I first had to plan what I would be using and where the pieces would be placed (see picture and the plan drawing). I was using the circuit shown above. I built the crystal set on a wooden board about 20cm by 15cm. I drilled 4 holes 4cm apart. I put screws with cup washers in each hole and fastened the wires of the parts of the crystal set and the aerial and earth wires under the cup washers. I fastened the capacitor to the board using Meccano brackets and screws. The coil was fixed to the board using glue. The earpiece plugged into a socket which was fastened to the board using a thunderbolt shaped piece of Meccano.

Drawing Showing Layout Of Components

5 - THE RESULTS I OBTAINED

At first all I could hear was a foreign station. But then, I tried late at night and could sometimes hear a station from Florida in English! By tuning the capacitor and moving the crocodile clips up and down the coil I managed to receive a few foreign stations on Short Wave. What I really wanted was to tune in to Medium Wave. I wanted to receive English stations like Radio Scotland and music stations. I tried everything including a longer aerial and using different parts but to no avail. I came to the conclusion that I could not get a strong enough Medium Wave signal in Northerly area of Scotland where we live.

Here is a picture of me operating my crystal set:

Photograph Showing The Crystal Set In Operation

6 - WHERE I FOUND THE INFORMATION

The History Of Crystal Sets

Book: The Pageant of the Century, Odhams Press, 1933.

www.oldradioworld.de/gollum/dhistor.htm

How I Built My Crystal Set:

Web page describing how to build a crystal set:

http://dspace.dial.pipex.com/town/pipexdsl/r/arar93/mds975/Content/crystalsets2.html (THIS SITE)

Book:

"Making A Transistor Radio" written by Rev. G.C. Dobbs, Ladybird, 1972

Kits Are Available From:

www.midnightscience.com/kits.html

www.oldradioworld.de/gollum/xkits.htm

Other Hobby Websites:

www.schmarder.com/radios/crystal

MDS975.co.uk (The Site You Are Reading This On!)

7 - QUIZ QUESTIONS

CRYSTAL SET QUIZ QUESTIONS:

1. What is a crystal set?
2. When were the first crystal sets sold?
3. Name 3 essential parts for a crystal set.
4. What does the earth do?
5. Does the coil pass signals of low frequency or high frequency?
6. How far apart were the screws on my crystal set?
7. What happened when I made the aerial longer?
8. What is this an anagram of: i a c c p r o t a?

(ANSWERS ARE AT THE BOTTOM OF THE PAGE)

PARTS THAT ARE REQUIRED


1	Wooden Base Panel
1	Coil Former 5cm dia. UPVC Pipe
1	Tuning Capacitor approx 500pF Air-Spaced or Solid Dielectric
1	Reel Of Enamelled Copper Wire for coil
1	Germanium Diode OA91, OA81, 1N94 etc
1	Crystal Earphone
1	47 k Ohm Resistor across earphone
6	½ inch No. 6 Brass Screws and Screw Cups
1	3.5mm mono Jack Socket
	Mounting Hardware (Mecano) for coil, capacitor and jack socket
	PVC covered wire for aerial and earth

A Superb RGD Valve Radio from 1937 - carefully restored in 1994 by Roddie Rankin

With thanks to Kenneth and Roddie Rankin for sending in this excellent article and photgraphs!

CRYSTAL SETS Parts: 1 2 3 5

	Crystal Sets - Introduction	Radio Stations & Memorabilia
	Resistor Codes & Capacitor Conversion Tables	Home Page

LINKS:

BOWOOD ELECTRONICS - A friendly, helpful and very speedy source for many of your electronic components at prices that won't frighten your wallet!

THE FOXHOLE and P.O.W RADIOS - Simple crystal set receivers used by soldiers during the war and by prisoners of war (P.O.W.'s).

QUIZ ANSWERS

1. The first kind of radio
2. 1922.
3. Coil, diode, capacitor, earpiece, aerial, earth.
4. Gets rid of signals you don’t want.
5. Low.
6. 4 cm.
7. It did not make much difference.
8. Capacitor.

CRYSTAL SETS Parts: 1 2 3 5

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