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Crystal radios don't even need a power supply -- they're powered by the radio waves broadcast by the radio station. You can build one for only a few dollars.
Building them was a craze in the 1920s, and again in the 1950s, and now it's happening all over again.
Crystal sets are endlessly fascinating because of their simplicity and their complexity -- they contain so few parts yet exhibit so many concepts. Crystal sets are a great tool for teaching, and a superb hobby.
The crystal radio was discovered in 1901. It is still the basis of modern-day radio and communications equipment and lives on in a wide variety of radio systems.
You can make a crystal set out of almost anything -- any small box, or just a bit of wood, or a frisbee or a deodorant dispenser (actual examples), or these (recommended for beginners):
A loop crystal radio using computer ribbon cable -- "Perhaps my favourite design," says low-tech guru Doug Edwards. Doug's crystal ardio pages are now hosted at Journey to Forever. See:
Dr Duggee's Crystal Radio
Radio plans page
Ribbon cable radio plans
Quaker Oats box crystal set:
Cigar Box crystal radio set
The Crystal Radio Society (the Xtal Set Society) has books, plans, parts, even kits, and a journal. The society was started in 1991 as amateur interest rekindled in crystal radios -- for the third time. Also on online forum, history and projects.
A miniature crystal radio with ferrite rod tuner, by Doug Edwards
Useful FAQ and other resources from the Crystal Radio Society at its Science Fair Projects and Parents & Teachers Page:
Good set of links to other resources on crystal radio, radio history and radio collections, headphones, amateur science and more.
Broadcast Timeline -- the history of broadcasting, radio, television and electronics (Richard M. Patricia, Warren County Technical School, US):
Scott's Crystal Radios
Crystal Radio Resources
Bizarre: Crystal radio
Bizarre Foxhole radio -- Foxhole and PoW built radios: history and construction
Rap 'n Tap Discussion Group at the Xtal Set Society
radiokits Discussion Group -- for building radios from kits or from scratch (homebrew): shortwave, crystal sets, AM/FM, VHF
One modern drawback to making crystal radios is that it's not as easy to listen in as it was in the old days. The design of headphones and earphones has advanced and cannot pick up the simple crystal signal, which old-style phones had no trouble doing. But the Xtal Set Society now stocks crystal earplugs, capacitors and other parts.
Headphone Repair Page at Scott's Crystal Radios