THE CRYSTAL SET



The humble crystal set So, this is it. This is where my interest in radio began. When I was just a little boy, I know it was before I left primary school, my Dad showed me how to make a crystal set. The circuit diagram to the right is just as he drew it for me and explained what the various components do. He made it out of bits from an old radio, screwed onto a wooden base. The antenna was a long length of old wire that ran out of the open window and was trailed around our yard and down the entry. The earth wire was wrapped around a lead water pipe in the kitchen. To me it was magical and if I'm honest, it still is!

I sit in front of my HF radio and, depending on time of day and propagation, I can hear stations calling from all over the world. There are no cables joining us together, no telephone lines, just .....well, nothing! That's got to be magic hasn't it ? I look through the window and I see my antenna, just thin copper wire attached to the radio with co-axial cable and realise that those voices or morse dits and dahs are coming to me down that wire from thousands of kilometres away, with just atmosphere between us. That's why radio has been, is and always will be the most fascinating hobbby that I can think of. I often wonder if other Radio Amateurs think the same as me or whether they are more interested in the science behind radio propagation. Are they visualising the electric and magnetic fields pulsing from their antenna at the speed of light ? Then bouncing off the ionosphere and down again before inducing a tiny current in the receiving antenna . A tiny signal that has to be teased and coaxed from out of the mish-mash of all the other signals trying to be heard.

Just imagine, wherever you are on this earth, radio signals from all directions will be charging around you, close to the speed of light, 300,000 km a second. Your length of wire has many, many signals induced in it, but that coil of wire with the old variable capacitor wired across it, make a tuned circuit that can pick out just one signal for you to listen to. The diode extracts the audio from the radio frequency and the earpiece or headphones provide just enough volume for you to hear it. The other capacitor C2 just lets the radio frequency part of the signal go to ground. And don't imagine that this circuit was only for the bad old days, it will still work and it doesn't need batteries! I once made a crystal set when I was in my late teens that probably still holds the record for audio output. Mind you, I was a guitarist in a group and could feed the output of the crystal set into my Marshall 100 watt amplifier!

So why don't you give it a go ? If you can't find or scrounge the parts to make a set you can always put your hand in your pocket and buy them. Wind as much wire as you can get onto a piece of plastic pipe or whatever, providing it's not metal, throw as much thin wire as you can outside, over trees or fencing and connect the ground terminal to a good earthed point. DON'T use the household earth or ground, it's not safe, but a copper water pipe that runs underground or a length of metal driven into the ground will do the job just fine. Connect your crystal earpiece or high impedance headphones, (ordinary stereo headphones are no good for this), and see what you can hear. If you wind enamelled wire onto a former ,you can scrape the enamel off at intervals, give the wire a twist and produce tapping points. If the wire to the capacitor is fastened to these tapping points with a small crocodile clip, you'll be able to tune across different lengths of coil when you move the clip to other taps. This will enable you to listen to different parts of the band of frequencies that you can cover with that coil / capacitor combination.

You just never know. You may fall under the same spell that grabbed me when I was little. You may go on to study radio or electronics, either for a hobby or even as a job, you just never know...........

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