CRYSTAL SETS PART 1 - INTRODUCTION
What They Are, Why They Came Into Being and How YOU Can Build Your Own CRYSTAL SET!
What are Crystal Sets? I am glad you asked! It is a fascinating subject dating back to 1920 when Marconi first started experimenting with radio broadcasting from his station 2MT in Chelmsford.
The earliest listeners to Marconi's experimental radio programmes would have mainly used the most basic of radio sets called a Crystal Set. Companies such as the W.G Pye began producing early radio sets in 1921 which used electronic components called 'valves', the forerunner to today's transistors and silicon chips). These glass valves could be arranged to amplify the weak radio signal to make them louder - enough to operate a loudspeaker. However these radios were expensive and needed electricity or bulky lead acid batteries to operate.
THE MAGIC OF A CRYSTAL SET
The crystal set was popular during the 1920's and 30's because it was cheap to buy, much cheaper if you built one yourself, perhaps only a few shillings. Additionally a Crystal Set did not require electricity or expensive batteries, however it could only provide enough volume for headphone listening, and if it was situated in an area of low signal strength the listener would also require a very quiet room as the volume would be low. However In the 1920's hearing voices and music from a station miles away with no wires (hence the term Wireless) really was a magical experience! Listeners in the early twenties were also tinkerers and experimenters, trying different designs of crystal set in an attempt to get the loudest and clearest reception.
Since crystal sets have no batteries and no mains power, they rely entirely from the electrical energy developed between the aerial an earth connections - producing sounds as if by magic.
The crystal set had to be carefully tuned into the station by making adjustments to a tuning coil and condenser, these early crystal sets had the added complication of using a 'Cats Whisker' as the detector. The detector converted the radio waves received from the radio station into an audio wave that could be heard in the headphones. The Cats Whisker was a fine wire that rested onto a piece of galena, the crystal, and had to be very finely adjusted to obtain the loudest and clearest sound. Once the 'sweet spot' was found it was important not to move it, it would be very frustrating if someone bumped into the table and dislodged it!
To work at all a crystal set requires a very large aerial, but this is nothing more than a long piece of wire, perhaps 20 to 50 yards of insulated copper wire, hung outside around a garden between poles or trees, or even around the loft space. For the aerial to be effective it needs to be balanced with an earth. An earth is simply a copper rod or pipe about one yard long driven into some soft damp earth with a mallet. At the top of the pipe is fitted a jubilee or hose-clip to which is attached a length of insulated copper wire which is then fed into the house, along with the aerial wire, to the room where the crystal set will be operated and connected to the set's terminals.
It is quite possible to build your own crystal set. In its very simplest early form a crystal set consisted of a coil of wire, something called a 'detector' and a pair of very special headphones.
THE CONSTITUENT PARTS
The coil of wire is called a Tuning Coil, and the number of turns of wire on the coil determines the wavelength (frequency) that the set is tuned into. Tuning to different stations can be accomplished by varying the number of turms on the coil, or more easily by having many different tapping points on the coil so that adjustments can be made.
To make tuning easier a component called a tuning capacitor can be included in the circuit. In very early sets a tuning capacitor was not always included to keep costs down, or due to their being difficult to obtain.
The Detector converts the radio wave received into an electrical wave that is suitable for the headphones to, in turn, convert into sound waves that can be heard by the human ear. In the very early days of the crystal set the detector consisted of a holder containing a piece of galena crystal that had a very thin and springy wire placed on its surface that had to be very delicately adjusted to find the sweet spot where the radio station could be heard. This was commonly referred to as a "Cat's Whisker". Modern detectors are called diodes and are more efficient than early detectors and cats whiskers. Diodes are still quite readily available and inexpensive. Part numbers for modern diodes include OA90, OA91 and IN34. Diode part numbers that are perhaps now more difficult to obtain include OA47 and OA81.
The headphones have to be of a very special type called high impedance headphones. Because there is no additional power source in a crystal set the current generated in the circuit is tiny - minuscule in fact. Ordinary low impedance headphones, such as Walkman headphones, would present a virtual short circuit to the crystal set allowing the tiny signals to drain away to earth & consequently producing no sound - not very useful!
High impedance headphones, on the other hand, reduce or impede the flow of current down to earth, in effect saving the tiny signals to produce sounds from the headphones that we can then hear.
There is a problem however, these high impedance headphones that were so readily available in the 1920's and 1930's are to so easy to obtain today, but some specialist vintage radio outlets still stock them though the price can be quite high. They can still be seen in museums of course. All is not lost though, today we can obtain a special earphone called, appropriately, a crystal earpiece very easily and far more cheaply than. An electronic component, called a resistor, must be connected accross the crystal earphone to allow a path for DC current to get to earth. The value of the resistor is usually 47,000 Ohms and without it a crystal earphone tends to block DC current and as a consequence the sound will be very quiet and distorted.
Aerial and Earth: For a crystal set work it needs an efficient aerial and earth. Since a crystal radio has no power - no batteries or mains electricity - it relies entirely on the radio wave energy sent out from the radio station's transmitter and collected by the aerial to work. The aerial simply consists of a length of wire, but it necessarily has to be quite long, usually in the order of 10 to 20 meters. For the aerial to be effective the crystal radio set also has to be connected to a good earth point. A good earth often consists of a 3 or 4 foot copper stake driven into the ground, but sometimes a water pipe can be used to reasonable effect. (Safety: Never use the earth pin of a household mains plug)
The circuit diagram (schematic) for a crystal set is shown below. Practical designs are shown on the following pages.
Above: The circuit diagram for the standard Crystal Radio
Above: A commercially made crystal set from around 1940
There is no variable tuning capacitor (condenser) in the above design. The capacitor is a fixed component and tuning is achieved by varying the coil inductance with the rotating switch. The detector is a galena crystal, seen at the top of the panel. A pair of high impedance magnetic headphones can just be seen to the right of this shot. (photo sent in by our reader Krysatec in Czechoslovakia - thanks!)
Almost certainly when building a crystal set today a crystal earphone will need to be used. A crystal earphone is not quite as good as the old high impedance headphones as it is not quite as sensitive and therefore not quite so loud, however today's transmitters are much more powerful than those of the 1920's and so really excellent results are very often obtained - just don't expect to hear everything that you might on a top quality portable radio!
Crystal sets are fascinating because they are so simple to build, and because there is no power required they remain magical to this day. I have built several over the years, with differing designs, and if you'd like to build one yourself, it's easy, and I have described some circuits in part 2 with some more experimental ideas in part 5.
I have a crystal set that receives BBC Radio 4 on long wave and 3 local stations and 3 national stations on medium wave and another that also receives shortwave stations. Not bad for radios with no battery or other power - and such as set could be very useful in an emergency.
More About Diodes - unconventional and new ideas (by Felix Scerri)
While messing around with diodes for crystal sets recently, I tried something as a theory that actually works very well in practice, and with one or two actual advantages.
Ordinarily the ubiquitous 1N914 or IN4148 silicon switching diodes are pretty hopeless as RF detectors unless RF signals are very strong! However there is a simple way of making these diodes work very well as RF detectors, by the addition of adjustable voltage bias. I have used adjustable voltage bias with diodes before with very good results, but using bias with IN914 or 1N4148’s is a relatively new idea!
Interestingly enough with voltage bias correctly adjusted, ‘weak signal’ performance and sound quality is actually very good, with that typically ‘crisp’ silicon diode ‘sound’. I use a simple bias network with a 1.5 v AA battery and a 1 Megohm potentiometer applied to the diode directly with the incoming RF fed into the diode through a 1 uf plastic film capacitor (as shown). The use of adjustable bias seems to help most with ‘repositioning’ the position of the knee in the diodes curve and weak signal performance is excellent, easily as good as any ‘proper’ germanium diode.
Another ‘tweak’ that is actually quite beneficial is ‘paralleling’ several diodes together. This doesn’t work with ordinary germanium diodes but definitely works with BAT46 ‘germanium equivalent’ diodes and the effect of three of these diodes together is to make a better ‘single’ diode. The turn on voltage is noticeably reduced and the ‘square law; region of detection is definitely improved and so is the rectified audio quality. These diodes are relatively inexpensive (at least in this country), so it is worth trying. 73 Felix vk4fuq 080214.
Having difficulty in finding components? I have added some ideas for component sources here.
Sources For Older Components
Build Your Own Crystal Set (Part 2) | Spider's Web Crystal Set (Part 3)
Crystal Set By Kenneth Rankin (Part 4) | Experimental Crystal Set (Part 5)
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