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CRYSTAL
SETS 2
Some
Practical Designs
MAKE YOUR OWN CRYSTAL SET !! |
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Crystal Sets (Part1) | Spider's Web Crystal Set (Part 3
Crystal Set by Kenneth Rankin (Part 4) | Experimental Crystal Sets (Part 5) | Crystal Radio Links |
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CRYSTAL SETS 2: SOME PRACTICAL DESIGNS |
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I
hope that you attempt building one or two of these crystal set designs
and I really do recommend that the components are carefully connected
up
using soldered joints onto a piece of tag-strip for
reliability.
However if you are new to constructing such electronic circuits then
some simple solder-less techniques could be employed and these are
suggested at the bottom of the page. Also see Crystal Sets Part 5
for more ideas on experimenting with crystal sets.
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| An
early and very basic crystal set would have been nothing more than a
coil of wire, perhaps 50 -100 turns, wound around a cardboard tube
about
3 inches (7cm) in diameter, a detector (or cats whisker) and a pair of
special High Impedance headphones (as discussed in part 1). There would be a very large aerial strung up around the garden and the all important connection to earth. The coil would have tapping points (connection points) at intervals of around 5 or 10 turns. See the circuit diagram on the right for details of who the set is wired together. The tapping points on the coil allow the set to be tuned to different frequencies by adjusting the position of tap B. Tap B would be connected to the coil at differently positions by way of a crocodile clip. The fewer turns between the top (aerial end) of the coil and tap B, the shorter the wavelength received (ie the higher the frequency). Tap A would allow the detector to be connected at different positions to vary performance. There is an additional component drawn in the above diagram, the capacitor (value 1000pF), this is included in crystal sets that used the High Impedance magnetic headphones, and bypassed any remaining radio frequencies (RF) to earth. |
 A very basic crystal set circuit. |
 The Standard Crystal Set |
I have not
built the set described
above as it is so basic. Such a crystal set above
would probably have been adequate in 1920 - 1923 when there would have
been only one local transmitter receivable.
When the BBC expanded transmissions and it became possible to hear more than a single station it would have became necessary to include a more convenient means of tuning the set. This was achieved by including a Variable Tuning Capacitor, of about 500pF (0.0005uF) connected in parallel with the tuning coil forming a tuned circuit. The tuning capacitor would have a Bakelite knob on the spindle to aid tuning. |
Because of the simplicity of crystal sets, it is often difficult to separate stations. When tuned into one station it is often possible to hear another close by station in the background, this is due to lack of selectivity. This can be reduced somewhat by adjusting the positions of the Aerial Tap and Detector Tap. Moving them closer to the bottom of the coil, the earthy end, reduces the load on the tuned circuit and this improves selectivity, however it does also reduce sensitivity which can make the station quieter. Headphones will often swamp a tuned circuit and reduce its selectivity (Q factor), so moving the tapping point lower down improves this situation. Every circumstance is bound to be different though so the best balance has to be found by experimentation. My crystal set has both the diode and the aerial connected to the same tapping point on the coil, about a quarter of the way down.
The modern 'standard crystal set' shown above uses a Crystal Earphone, since suitable high impedance magnetic headphones (of 2000 to 4000 ohms) are no longer widely available. When using a crystal earpiece the 1000pF capacitior shown in the first diagram can usually be omitted an in its place a 47k ohm resistor is connected, this ensures that the Crystal Earphone will work at its most efficient i.e. the sounds will be as loud as possible. The resistor allows DC current to flow through the circuit efficiently - this would otherwise be blocked when using a crystal earphone. In a modern crystal set the detector used is a Diode. Suitable diodes include OA80, OA81, OA90 OA91 and IN94 which are usually available from component stockists.
| A Better Diode For
Increased Efficiency The OA47 will be of particular interest since it has the lowest forward bias voltage of any of these diodes which will make the crystal set somewhat more sensitive and therefore louder. The US equivalent of the British OA47 is the IN34. On the right you will see my real working example of a crystal set. The large plastic knob on the front turns the variable tuning capacitor. This set receives the three UK national stations and also three local radio stations very well at my location. There is a small 3.5mm jack socket mounted on the front of the plastic case (MB5 from Maplin Electronics) that the crystal earphone plugs into. The coil can be seen inside the case, it is 70 turns of 30 gauge enamelled copper wire wound around the centre of a toilet roll and tapped every 10 turns, by scraping off the enamel insulation and making a small twist. The croc' clips can be seen clipped on to these twists to connect to the aerial and detector tap points. |
 A real working crystal set. Radio as if by magic with no battery or mains power. |
| THE
MEDIUM WAVE COIL - MORE DETAILS |
|
 PHOTO
SHOWING THE INSIDE OF
THE COMPLETED CRYSTAL SET |
Medium Wave Coil The number of turns of wire required on the coil will vary depending on the size of the former (in this case the inside toilet roll) and the thickness of the wire. So to obtain the correct coverage of the medium wave band may need a little experimentation. I usually find that between 50 to 90 turns is right and I generally use enamelled copper wire that is between 30 s.w.g. and 26 s.w.g (i.e. 0.315mm and 0.45mm diameter), so it's best to start with too many turns and then work down. The more turns that you use the lower the frequency range will be, i.e. too many and the coverage of the top end of medium wave around 1500 - 1600 kHz will be lost, while too few and the coverage down to 500 kHz will be lost. It is also important that the coil former is non conducting, i.e. not metallic. It could be wood or cardboard or a short piece of PVC piping and with a diameter of between 1½ and 4 inches (4 to 15 cm) are common sizes. You could try using a ferrite rod too, see below. |
| This particular set has a coil wound onto a toilet roll tube which consists of 70 turns of 30 s.w.g. (0.315mm dia) enamelled copper wire tapped at every 10 turns. It also has the additional small trimmer capacitor that helps match the aerial to the tuned circuit thereby improving selectivity, see below. | |
| USING A FERRITE ROD AS THE
COIL FORMER |
The aerial coil could be wound
onto a
ferrite rod.
A piece of 10mm diameter ferrite rod of between 3 and 6 inches long (80 to 150mm) will be most suitable and will require between 50 and 90 turns of enamelled copper wire to provide coverage of the medium wave band: First make a paper tube that is held together with sticky tape that will easily slide up and down the ferrite rod. Then wind the coil over this with the windings neatly side by side. Make tapping points every 10 or 15 turns so that the aerial and diode tapping points can be adjusted. Adjustments to the tuning range can be made by removing some wire from the coil so it is best to start off with too many turns and then work down. Fine adjustments can be made to the completed coil by sliding it up and down the ferrite rod. |
| AN IMPROVEMENT TO THE DESIGN |
|
The crystal set above also has one small, but significant, improvement over the standard crystal set and that is an Aerial Trimmer. A trimmer is a variable capacitor, very similar to the tuning capacitor, except smaller and adjusted with a screwdriver. The value of the trimmer is usually around 10 - 50pF, but if a small tuning capacitor is available that will probably be just as effective. In the absence of such a variable capacitor, individual fixed ceramic capacitors of e.g. 10pF, 50pF and 100pF can be tried in this position to judge which gives the best results with the particular aerial being used. The trimmer
capacitor
adjusts the coupling to the tuned circuit, reducing the load of the
aerial on the tuned circuit will improve the selectivity (Q), and it
will be easier to separate stations. Again tapping points are
used
and I find this to be an excellent arrangement.
|
 Improved Crystal Set design, with good selectivity |
 Layout Of The Crystal Set - Although this is soldered together an alternative to tagstrip would be a 5amp mains connector block so that components can be trapped in place with screws. See article below. |
The
picture on the right shows the
general layout of the crystal set above. The coil is of
approximately 70 turns is wound on the centre of a toilet roll, and has
tapping points at 10 turn intervals. The trimmer is soldered between the Aerial terminal and the piece of 5-way tag strip, and a wire goes from there to a croc' clip which is clipped onto a tap on the coil. The Diode is also soldered onto the tag strip, one end connected to a piece of wire going to a second croc' clip & connected to a tapping point on the coil, the other end of the diode is connected to the 3.5mm jack socket that the Crystal Earphone plugs into. The 47k resistor is also connected to the earphone end of the diode and goes to earth, the earth terminal wire is soldered to the tag strip at this point too. The tuning capacitor has two terminals, one connected to each end of the coil, and one of them is also connected to earth as shown. [Where the wires cross over in the diagram, they do not touch and are not connected together]. |
| LONG WAVES |
|
In most areas around Europe and certainly around much of the UK you will be able to hear a Long Wave station. To receive Long Wave on a crystal set will require an aerial coil with a greater number of turns to increase its inductance. As a good general guide a coil wound on a piece of 10mm diameter ferrite rod will require about 250 turns of enamelled copper wire: First make a paper tube that is held together with sticky tape that will slide up and down the ferrite rod. Then wind the 250 turn coil over this, the windings will have to be made over the top of each other. Make tapping points at, say, 50, 75 and 100 turns to tap the aerial and diode to. As with the medium wave ferrite rod aerial, adjustments to the tuning range can be made by adding or removing some wire from the coil, and fine adjustments can be made to the completed coil by sliding it up and down the ferrite rod. The longer the ferrite rod the better and anything between 3 and 6 inches long (80 to 150mm) will be very good. |
| SHORT WAVES |
|
If you like experimenting, then reducing the number of turns on the coil to say 10 to 30 will allow reception of the higher frequencies, the Short Waves. I have found that winding the coil around a 'ferrite rod' often works even better with short wave reception. Obtain a ferrite rod about 7 to 15 cm long and about 1cm in diameter. Make a couple of small tubes of card, about 4cm long, that will fit tightly over the rod. On one tube wind two coils using 0.5mm diameter enamelled copper wire - one coil of about 30 turns and a second one of 2 or 3 turns wound over the top of the first. Secure the windings in place with Sellotape. On the other card tube wind a similar coil, but use about 15 turns for the first coil and for the second coil wind about 3 to 4 turns over the top, and secure with Sellotape tape. |
 Example of a layout using a connector block to wire up a crystal set |
These coils will
provide
coverage of short wave in two bands using the first coil for the longer
wavelengths, typically 60 to 31 metre bands and the second coil for the
shorter wavelengths typically 25 to 19 metre band. Wire up
the
circuit as shown in the circuit diagram below.
Even better selectivity performance can be achieved by winding the inductors (coils) on a ferrite toroids (T50-2 yellow, or green will do). The aerial trimmer need not be used if selectivity and sensitivity is found to be adequate. It's all about experimenting, and I find it best to use a trimmer or small coupling capacitor to obtain the best selectivity.
Up to 30 turns of 0.5mm enamelled copper wire can be used for the longer short waves below 10 MHz, while a winding of around 15 turns will provide coverage of the shorter short waves above 10MHz.
| USING A
TOROID INDUCTOR FOR SHORT WAVES |
Even better selectivity performance can be achieved by winding the inductors (coils) on a ferrite toroids (T50-2 yellow, or green will do). The aerial trimmer need not be used if selectivity and sensitivity is found to be adequate. It's all about experimenting, and I find it best to use a trimmer or small coupling capacitor to obtain the best selectivity.
Up to 30 turns of 0.5mm enamelled copper wire can be used for the longer short waves below 10 MHz, while a winding of around 15 turns will provide coverage of the shorter short waves above 10MHz.
 The circuit diagram of the Short Wave Crystal Set |
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| AUSTRALIAN
DESIGN |
|
 The front panel of the Australian Crystal Set |
Moving back to the Medium Waves, here is a circuit for a very interesting Australian design that promises extremely good station separation (selectivity), and having built it I can vouch for that claim, it's really excellent. I receive three national stations and three local stations at my location with excellent clarity using a modest antenna and standard crystal earphone. The coil is different to the other crystal sets described above, it is much bigger at 3¼ inches (8cm) diameter and 5inches (12cm) long. I made my coil former out of the cardboard from a breakfast cereal box - just like Blue Peter! The design is often referred to as The Mystery Crystal Set, by Proton. |
| Two
distinct coils are wound on it, the first one consists of about 50 turns of
24
s.w.g (approx) enamelled copper wire. The second coil is 25 turns, very
close wound right over the top of the first coil using 30 s.w.g. (approx)
wire, try to get this second coil wound in between the windings of the
first, for better inductive coupling. Then carefully wire up the set according to the diagram. Notice that the tuned circuit is not connected to earth and has no direct connection to the detector circuit. The detector circuit is connected to earth however. The two aerial terminals offer alternative selectivity performance, terminal A gives very good selectivity while B is very wide. I never bother with B.  |
 Make the coil carefully and wire up this crystal set according to the circuit diagram opposite and you will be rewarded with a really high performance crystal set of a type that was used in the very early days of broadcasting in 1930's in Australia. This is probably my favourite crystal set! |
| THE DENCO PCC1 COIL |
||
The
PPC1 coil was a commercially manufactured by Denco Clacton Ltd and was
popular among hobbyists not keen on going to the bother of winding
their
own fiddly little coils. As a child I wanted try one of these
coils and sent away for one by mail order. It arrived a few
days
later in a little cloth bag, like a miniature pump bag, with protective
wrapping inside.
The coil windings are entirely enclosed in what I can only describe as a cylindrical ferrite 'shell', the four very thin connecting wires exiting, two either side, from small apertures in the 'shell'. The performance of the circuit shown below I seem to remember was quite pleasing. Unfortunately I cannot find the set or the PPC1 coil at the moment, but here is a reproduction of the circuit diagram and data:
|
| THE REPANCO DRR2 COIL |
|
I recently rediscovered an old Repanco DRR2 Longwave / Mediumwave coil that must have been kicking around in my junk box since the 1970's. The DRR2 coil was made by Repanco in Coventry. It came with a page of suggested circuit diagrams which I thought had been lost to the mists of time, but it recently came to light again, so I have now copied it below. Once again I included an aerial trimmer which can be adjusted to improve selectivity. |
 The circuit diagram of the
crystal set
using the Repanco DRR2 coil
|
 A 'lash-up' of the Repanco crystal set |
Repanco Ltd was formed by
two ex-army signals engineers and from the earliest days of radio
supplied crystal set kits and coils to radio construction enthusiasts.
The Repanco DRR2 coil was for medium wave and long wave intended for use in when building simple crystal set and valve radio circuits. It consists of three coils; a Medium Wave coil at the top that includes a tapping point (for the aerial); a coupling coil or tickler in the middle; a lower coil which can be connected in series with to top coil to provide Long Wave reception. I have built a quick crystal set with the coil and it provides good reception with excellent selectivity, so it must have a very good Q factor. |
The
Repanco DDR2 coil was provided with a simple Foolscap size information
sheet that showed four different radio circuits. Sadly the sheet does
not give a huge amount of information and my copy is
now rather tatty and faded - it is copied below:
Simple crystal radio type circuits using the Repanco DRR2 coil
DIODES - For Crystal Set Use - some notes by Felix Scerri
Germanium diodes for crystal set use.
Although I'm a fan of these new silicon schottky BAT 46 diodes, good germanium diodes still have a lot to offer, especially in terms of 'weak signal' sensitivity. Last night I did an experiment.
I sorted through quite a few of my hundreds of acquired random germanium diodes looking for particularly 'sensitive' ones. I tested this by tuning in a weak AM station and comparing the detected DC output level and also the apparent 'loudness' of the audio signal.
Even amongst germanium diodes of the same type, there was enormous variation all the way from excellent to poor! For very weak signals, germanium diodes 'detect' in the 'square law' region below the diode conduction 'knee', in a rather different part of the curve than with much stronger signals (way beyond the diode knee).
When testing germanium diodes for weak signal sensitivity, the inherent capacitance of the diodes is also a factor, and the 'tuning' may change somewhat and will need to be readjusted with every diode tested!
In the end, out of a large number of germanium diodes tested, I found three or four germanium diodes with excellent weak signal sensitivity and the rest were poor. One other interesting thing, good germanium diodes 'sound' different, rather more 'rounded and smoother' than the schottky's which tend to sound mercilessly clean, almost clinical. I also found almost no variation in weak signal sensitivity with my BAT 46 schottky diodes. Take your pick!
Regards, Felix Scerri VK4FUQ 14/03/2012.
LINKS:
^Top Of Page
Simple crystal radio type circuits using the Repanco DRR2 coil
Valve circuits using the Repanco DRR2 coil
Repanco Ltd no longer produces radio
coils and crystal set kits for the radio construction enthusiast, as it
did in the early days of radio. In 1986 it was renamed Repanco Bartlett
Ltd when it merged with Bartlett Electronics. The company moved from the
Foleshill Road to Unit 24, Albion Industrial Estate, Endemere Road,
Coventry CV6 5NT and now specialises in transformers and wound
components and can design and manufacture to commercial customer
requirements, their website is: http://www.repancobartlett.co.uk/
| CRYSTAL
EARPHONES |
|
Here is a good idea and well worth trying, to maximise the use of sound output from your crystal set why not use dual crystal earphones? Having an earphone in each ear helps to block out extraneous noises helping the listener to better concentrate on any weaker stations received. Using the circuit below, one earphone makes use of one half cycle of the radio wave while the second earphone uses the other half cycle of the wave that would have previously gone to waste when using just one diode. Ensure that the diodes are connected up according to the diagram i.e. one diode is connected the opposite way round to the other. Also try to make sure that the diodes and crystal earphones are similar to obtain the best results. (You could simply connect two crystal earphones to the same terminals of the single diode, but this would not be as efficient and the sounds would be much quieter.) |
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A note about Crystal
Earphones: It will be worthwhile buying several
different
ones from different sources as performance varies between manufacturers
quite markedly. I have found the ones marked 'Japan' on the
back are the most sensitive and therefore loudest,
whereas ones marked 'Receiver' 'Taiwan' are often a little less
sensitive and therefore quieter and sometimes more 'tinny' sounding.
As mentioned previously it has been noted that the OA47 diode will be of particular interest since it has the lowest forward bias voltage of any of the common diodes available. This will make the crystal set somewhat more sensitive and therefore louder. The US equivalent of the British OA47 is the IN34.
As mentioned previously it has been noted that the OA47 diode will be of particular interest since it has the lowest forward bias voltage of any of the common diodes available. This will make the crystal set somewhat more sensitive and therefore louder. The US equivalent of the British OA47 is the IN34.
| 'SPIDERS
WEB' COIL |
Here is an interesting concept sent in by Chris Dorna of the Vught North Scouts in the Netherlands. It is a crystal set made out of a coil wound in the form of a spiders web: See more HERE. |
 Chris Dorna's Crystal Set with Spider Web Coil |
 Detail of the germanium diode |
 Close up of Chris Dorna's Spider Coil |
| LOOP THE LOOP! |
|
|
A crystal set can also be made that does not need a large long wire aerial. If you have ever made a loop aerial for medium wave or long wave DX-ing, then it is a simple matter to add a diode, resistor and a socket to connect a crystal earphone that will allow reception of nearby stations. See my section on Loop Aerials and ATU's for more constructional details. |
|
 |
 The
circuit diagram of the Loop Crystal Set. The loop is 10 turns
of
7/0.2mm 'hook-up' wire wound on a 40cm (17") former made of
attractive plastic edging strip available from many DIY
stores.
The loop is very directional in its pick up pattern, which can help
eliminate interference from some stations by rotating the
loop.
The
switch and additional capacitor allow tuning of the lower medium wave
band from about 650 to 520 kHz. Having a loop with 50 to 60
turns
of wire will tune into the Long Wave band.
|
DIODES - For Crystal Set Use - some notes by Felix Scerri
Germanium diodes for crystal set use.
Although I'm a fan of these new silicon schottky BAT 46 diodes, good germanium diodes still have a lot to offer, especially in terms of 'weak signal' sensitivity. Last night I did an experiment.
I sorted through quite a few of my hundreds of acquired random germanium diodes looking for particularly 'sensitive' ones. I tested this by tuning in a weak AM station and comparing the detected DC output level and also the apparent 'loudness' of the audio signal.
Even amongst germanium diodes of the same type, there was enormous variation all the way from excellent to poor! For very weak signals, germanium diodes 'detect' in the 'square law' region below the diode conduction 'knee', in a rather different part of the curve than with much stronger signals (way beyond the diode knee).
When testing germanium diodes for weak signal sensitivity, the inherent capacitance of the diodes is also a factor, and the 'tuning' may change somewhat and will need to be readjusted with every diode tested!
In the end, out of a large number of germanium diodes tested, I found three or four germanium diodes with excellent weak signal sensitivity and the rest were poor. One other interesting thing, good germanium diodes 'sound' different, rather more 'rounded and smoother' than the schottky's which tend to sound mercilessly clean, almost clinical. I also found almost no variation in weak signal sensitivity with my BAT 46 schottky diodes. Take your pick!
Regards, Felix Scerri VK4FUQ 14/03/2012.
| SOLDERLESS
CONSTRUCTION IDEAS |
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For a novice the use of a soldering iron may seem a bit daunting at first and while the most reliable results will be obtained with a good soldered joint using a tag strip as shown below, the circuits can still be made without the use of a soldering iron.
The very simplest circuits could be wired together ,with a little ingenuity, with the component wires being held together in the grip of solderless crocodile clips, whereby the connecting hook-up wire is fixed to the croc' clip by a screw rather than solder. For more the slightly more complex circuits a plastic Terminal Block (sometimes referred to as a choc' or chocolate block) can be utilised very effectively indeed. These are used in mains wiring and are available in various sizes; 2 Amp, 5 Amp, 15 Amp and 30 Amp. The 5 and 15 Amp Terminal Blocks I have found to be the most suitable. The various component wires can be trapped securely with the screw at each junction point. This method also makes it easy to change the components around when experiment with different circuits. See The EXPERMENTAL CRYSTAL SET for more details in Part 5.
The Ladybird book called 'Making A Transistor Radio' (also shown on the TRF Radio pages) detailed a very novel approach using brass screws with screw-cups to trap the component wires at each junction point:
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No
AM radio stations or transmitters in your locality or country?
 Has your
local medium wave broadcast station closed or been moved to VHF/FM or
Digital? Don't worry. You can
still build and experiment with crystal sets and TRF radios by also buying
or even building a simple low power AM transmitter. So, not only can you use your
crystal sets but you can also run your own radio
station that can be heard in and around your home - playing the music
or programmes that you want to hear!
SSTRAN
AMT3000
Superb high fidelity medium wave AM transmitter kits from SSTRAN.
Versions available for 10kHz spacing in the Americas (AMT3000 or
AMT3000-SM) and 9kHz spacing in Europe and other areas (AMT3000-9 and
AMT3000-9SM). Superb audio quality and a great and well
designed little
kit to build: http://www.sstran.com/pages/products.html
 http://www.sstran.com/ Other AM transmitters available:
Spitfire & Metzo Complete, high quality ready built medium wave AM
Transmitters from Vintage Components:
http://www.vcomp.co.uk/index.htm Vintage Components offer a choice of the high quality Spitfire and Metzo transmitters: SPITFIRE AM Medium Wave Transmitter with 100 milliwatt RF output power:
 METZO AM Medium Wave Transmitter with built in compressor:  AM88 LP A basic AM transmitter kit from North County Radio. http://www.northcountryradio.com/Kitpages/am88.htm |
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).
VINTAGE COMPONENTS - A great resource for crystal sets, components, valve radio kits and medium wave AM transmitters!
6V6 - Electronic Nostalgia and Vintage Components
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).
VINTAGE COMPONENTS - A great resource for crystal sets, components, valve radio kits and medium wave AM transmitters!
6V6 - Electronic Nostalgia and Vintage Components
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