My twin sons had a birthday recently, and their aunt wanted to give them crystal radio sets. It’s something I’ve been meaning to do for some time but never got around to, so I was glad that she thought of it. The kit is from the Xtal Set Society, and as you can see in the picture it cleverly is built on the side of a round oat box. The directions include a full size schematic that you print out and tape onto the side of the box, and all of the pieces you need to build to build the radio come with it, including a coil of antenna wire if you order it.
The crystal sets I remember had small gauge enameled copper wire wound around a tube, and a strip of brass that could be slid along the coil. The wire was lightly sanded to remove the enamel insulation along the path the the brass strip so it could make electrical contact.
In this design insulated hookup wire is used for the coil, with breaks in the insulation periodically to which you can hook an alligator clip. The radio is then tuned by moving alligator clips from point to point rather than by sliding a brass strip. For a ground, I bought a replacement extension cord end at the hardware store and connected the ground wire from the radio to the ground prong of the plug. This kit can be built without soldering, but I don’t recommend it. Our version did not work reliably until I soldered all the connections.
For those who don’t know how they work, a crystal set is the simplest possible radio., consisting of an antenna to convert the radio wave into current, a resonant circuit that selects one station and rejects others, a detector that converts the high frequency radio wave into a lower frequency audio wave, an earpiece that converts the audio wave into audible sound, and a ground. It is unpowered, a circuit that creates a path between the air and the ground, and which is driven by the current that flows through it.
The detector, which was literally a crystal in the early days of radio, but which is replaced by a solid state diode today, works by rectifying the RF signal that comes out of the resonant circuit. The AM signal cannot be listened to directly because the audio component of the wave is present on both sides of the axis, averaging to zero. With the diode removing all of the signal that lies below the axis, what is left has a DC offset that replicates the audio component of the signal. The RF is sometimes bypassed by a small capacitor in parallel with the earphone. The crystal radio page on Wikipedia has lots more information.
A diode with a small forward voltage drop is preferred due to allowing more of the RF signal to pass through.
Christopher is thrilled with his radio, and got a giant smile on his face when he connected the antenna and heard a station in the earphone. Unfortunately, from my parent perspective, the AM radio is mostly populated by talk radio these days, whereas it used to have top 40 music. Benjamin, his brother, is anxiously awaiting our Sunday trip to the grocery store, when we will buy a second oat box and he can build his own set. I think our next project will be to learn about transmission, first using a spark gap and then an actual audio transmitter.
Go get one for your kids and watch their amazement at building an actual radio that works, and one that doesn’t need batteries to boot.
Design News Gadgeteer.