Building a Great Crystal Radio

A Gift of a Crystal Radio

The Annual Christmas Dilemma

Christmas gifts for my two great-nephews are always a challenge, and with one in the first grade, I wanted to up my game a little. The standard Lego-based gifts are always popular, but I sensed that something more involved might be a real hit. By the time I was in Grade School, I was fascinated by all things electrical and radio, so why not give something along those lines?

Lately, I have also been interested in classic old radios, especially those from the first half of the 20th Century, and wanted to build some of the radio projects that were just out of my reach in my youth. Perhaps a kit of some kind . . . ? Then, it occured to me that I could build one of those simple radios and take it apart for my nephew to re-assemble — he would love that. The idea of a Crystal Radio kit was born.

The Popular Electronics Crystal Radio

But which one? A little research on the Internet revealed an entire sub-culture of Crystal Radio enthusiasts from those who collected and restored original models to those who made replicas or even those who re-imagined old designs with modern technology twists.

And there were literally dozens of designs! Some clarity came from a massive online collection of old radio and tv magazines and I eventually settled on a design that appeared in the October 1989 edition of Popular Electronics. It was simple enough for a (nearly) 7-year-old to assemble with enough performance to be interesting. I began collecting the parts.

A Tapped Coil to improve performance

Most crystal radios are very simple, usually with nothing more than a coil, variable capacitor, detector, headphones and antenna wire. This design improved on the run-of-the-mill radio by adding a tapped coil so that the antenna and detector were connected at better impedance points. In theory, the coil would maintain a higher Q, and therefore better selectivity than the simpler designs. The coil consisted of 80 turns of #22 magnet wire wound on a 2-inch or so paper tube. I found that a mailing tube was just the right size. Once cut to the recommended 6-inch length, I painted it with black semi-gloss paint. After all, it had to look cool. Winding the coil with 7 taps, one every 4 turns, and then the rest of the coil was a little challenging to keep tight and even. Once wound, a coat of clear lacquer was applied to protect the coil windings.

Phenolic chassis and brass fittings

The chassis was made from 1/4″ phenolic sheet stock, cut on a table saw. Phenolic has a tendency to chip, and a little painter’s tape along the cut lines helped to reduce the chipping. The coil connections were brought out to 8-32 brass machine screws attached with brass nuts so that the length of the screw faced up from the chassis. This would allow experimentation with moving the detector and antenna taps for best operation. The coil body was attached to the chassis with shorter 8-32 screws and nuts.

Diode and variable capacitor

Connections for antenna, ground, detector (1N34 diode), and headphones were made with brass push terminals. The variable capacitor was mounted with the shaft facing up by tapping two small holes in the capacitor frame for 6-32 screws. All wires were terminated with crimp-on ring lugs to make assembly easier.

Testing the radio

Once assembled, it was time for a test. A couple of hundred feet of #22 stranded hook-up wire was strung through the trees for an antenna and a ground was connected. The radio didn’t work at first however, and some fiddling was necessary. For one thing, the crystal (piezo) earphone turned out to be of such high impedance that it wouldn’t force the 1N34 diode into conduction. A 10K ohm resistor across the headphone terminals solved this problem. Once corrected, a strong radio station was audible in the earpiece.

A Crystal Radio at night is still fun!

Night was a different story however. As I turned the big tuning knob, several stations were audible. Some weak, some stronger, but still surprising for an un-powered collection of wires and small parts. I was fascinated as a kid and it is still amazing today. Reluctantly, I disassembled the radio and packed it into a gift box. Finally, I wrote a little instruction manual to include in the box. This was going to be a memorable gift.