One year ago, you were reading this on GeekDad, by Kevin Kelly:
Crystal Radios are an old standby of GeekDads. For the early mists of time GeekDads have been showing kids how to pull radio stations from thin air with the barest snips from the basement. Here are some very simple radios that you can have your kid make in a few hours. They seem too simple to work. But unbelievably they can hear music or talk programs coming out of this scraps of metal. How does it work?
Ken Reitz’s cool crystal radio
Aha! Glad you asked. They use the ingredients of the first radios, solid state bits of matter wired up in various circuits, including long antennas and coil tuners.
There are an amazing variety of crystal radio types, some of them very sophisticated, and worthy of midnight engineers. The best source for instructions and books and a newsletter and kits for crystal radios is the Xtal Set Society. They sell the classic book Radios That Work For Free, and many other plans books. They also sell a bunch of very cheap but effective starter kits, like this one, the XS500 Basic Crystal Radio Kit, selling for $15.
Another great starter project for kids is the Quaker Oats box set broadcast AM crystal radio. You use the cardboard cylinder of a Quaker Oats box for the wire coil. Download the free PDF free plans. They even give some useful science fair hints for this project.
And for those who get into it big time they issue a paper newsletter with new circuits every six months or so. There’s hours and hours, if not years, of things to play around with here. I’ve found that kids can at least get a weekend of curiosity from one of these.