A crystal radio is often a kid’s first introduction to building something electronic. [Billy Cheung] is a crystal radio builder who wants to “make crystal radios as easy to use as regular radios.” He’s built many sets, but his latest is one that not only fits in a matchbox, but uses the matchbox as a variable tuning inductor.
There’s no oatmeal box in this design and just a few components. The matchbox contains some ferrite rods and two different windings. By moving the inner part of the matchbox, you can tune different stations. Although the design calls for two fixed capacitors [Billy] found he had enough self resonance (presumably from stray capacitance) that omitting them didn’t hurt his reception of strong signals.
[Billy’s] web site has a variety of crystal radio builds. You can find radios built into pens and even some capable of receiving FM stations. Because of their simplicity, many people have built them from junk they find lying around.