Crystal sets hold a fascination for some people. I am one of those people. I suppose it is due to having played about with them a great deal in childhood.
I think one of the fascinations is that, even as a child, I always felt that I would be able to put together a set from odds and ends should the need ever arise. If ever I found myself in a Stalag, a Colditz or its equivalent, would I be able to assemble a simple radio from the inside of a toilet roll, a discarded auto-erecting umbrella and a shoelace tag; (properly called an aglet, I believe)? I always felt that a crystal set would be the way to go and that I would manage somehow, except perhaps for the mysterious crystal itself. That might just be the item which stopped me.
In practice, it is more likely to be the headphones which would be difficult to manage, but more of that later. These thoughts, forgotten since childhood, were suddenly regenerated when I came across a site talking about, what was called a foxhole radio. The original site disappeared but has now reappeared at,
Apparently in World War II, soldiers in their foxholes had put together simple crystal type receivers to listen in for news and entertainment. With incredible ingenuity they had found that a razor blade could be used as a detector. Upon reading this, I had to try it. I cobbled together a simple crystal set and then tried to fit it with a razor blade detector. The major problem was in finding a razor blade. It seemed that what was necessary for good detection was one of the old blue blades. These are no longer in production having long since been replaced by stainless steel. Eventually I managed to find one inside an ancient gadget which had been designed to hold a razor blade and so allow it to be used as a simple cutter. The gadget had not proved useful and had been thrown into a drawer where it had stayed for many years, but in my desperate searching, I took it apart and there inside was one of the precious blue blades. So it went into my set and - it worked. I felt the anxiety of years lift from my shoulders as I realised that any future spell in a Colditz, which I might experience, was less likely to be without news and entertainment. Before progressing further let me say a few words about crystal sets in general. Interestingly, the fascination with them seems to hold for quite a few people and so there are plenty of internet sites on the subject. Any search engine will yield a goodly haul but for the convenience of anyone who wishes to have a quick look I have put below a few links to sites.
These sites were all active in March 2004.
http://www.radio.mcmail.com/xtal.html
http://www.crystalradio.org.uk
http://www.oldradioworld.de/gollum
Many of the given circuits are very precise in the component details. The gauge of wire, the number of turns, the coil diameter, the capacitor value. Be aware that exactness is far from vital in these constructions, have a go with what you have available but be aware that a good aerial IS vital and a good earth is next to vital.
Most of the modern constructions replace the crystal with a low power semiconductor; a germanium diode. The diode is more readily available than the crystal and is more reliable and efficient but, to my mind, removes the heart and soul of the crystal set. Somehow the crystal and catswhisker seemed to be the little element of magic that got the thing working. All the rest; wires, coils, earphones, etc., was science. This unusual, bright, silver, crystal of Galena was the mysterious heart. The magic that brought everything else to life when the hot spot was suddenly found after minutes of prodding and probing with the catswhisker.
Of course science has long since explained all the whys and hows. The naturally formed irregularities in the homogeneity of the crystal which have semiconducting properties. The ability to pass a low power current in one direction through the junction but not allow it to flow back. That is all it does and that is all it has to do for the unit to act as a radio detector. Since in my incarceration the crystal is unlikely to be available I needed to investigate the substitutes.
According to the web sites there are many alternatives to the razor blade detector and my small success prompted me to experiment further. It seems that coke has been used. I was less able to find coke, (coal less the volatiles, in case there is any misunderstanding), than blue razor blades. I mention it here however, because you never know what is likely to be available in your particular Colditz. I tried one of these thick single edged steel blades which used to be known as a safety razor blade. Mine was going a bit rusty, which appears to be a good thing, and I was able to make this work. Not as well as the blue blade but working nevertheless. Another detector mentioned as a possibility is a hacksaw blade. I tried a junior hacksaw blade which is heat treated to give a blue finish. This worked as well as the blue razor blade and is a lot easier to find. Could be useful to work on cell bars when not listening in.
One last word on detectors. On one of the sites an Australian chap describes a diode which he used to make as a child. It is essentially a small tube filled with copper oxide with a connector at each end. I made one but was unable to get it to work. It seemed that the essential feature in all of these detectors is the oxidisation of the metal which, as a thin film, has semiconducting properties in places. He described how to make copper oxide for his device, by putting copper pieces in a fire. I hammered a piece of thick copper wire flat at one end heated it and waved it about in the air to form an oxidised surface and tried that as a detector. That did work. Not particularly well, but well enough and much simpler to produce than the diode. So it seems there are plenty of possibilities.
A word about the 'catswhisker'. With the blue blade I used a piece of hard pencil lead sharpened to a point. A piece of stiff wire wrapped around the lead allows connection to the circuit. With the safety blade I found the point of a safety pin was more effective. Both these devices are described on the web sites.
As I mentioned above, the real problem is likely to be earphones. These need to be high impedance in a crystal set. They were never the sort of item you were likely to come across casually. One site describes how soldiers made a sort of earphone from wire and nails. I would not fancy that myself. These days I think it unnecessary. The crystal earphone, not available to WWII soldiers, is high impedance and is perfect for the job. "But", I fancy I hear you murmur, "I am no more likely to find a crystal earpiece in Colditz than I am to find a high impedance headphone". True. But I would point out that the functional part of a crystal earphone is the piezoelectric crystal. (It's those magic crystals again). These days the piezoelectric crystal is pretty ubiquitous. They are in toys and novelties, automatic gas igniters, cigarette lighters and many of the beepers and burpers found in everyday life. It should not be beyond the ingenuity of the Colditz internee to put together an earphone from one of these items. Get a relative to send you a musical birthday card; it has a piezo beeper inside. The crystal has a wire connector on either side. Feeding the electric signal down these wires causes the crystal to vibrate. If the crystal is fixed to a diaphragm then the diaphragm vibrates and an audio output is generated. Or to put it another way - you can hear it! By way of encouragement I can tell you that I have taken the flat disc beeper out of a computer, put that in place of an earphone on a crystal set and been able to hear music just by holding the disc to my ear.
Good luck chaps!