Bottle Radio | Make:

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By Steve Hobley
Time Required: 3-4 hours

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Crystal radio technology has been around for many years. This “bottle radio” take on a crystal radio requires no power source, operates on the power from radio waves, and receives signal from a long wire antenna. As radio stations slowly move away from the AM band, the “window of opportunity” to experience this remarkable technology is dwindling. On that note, the closer you are to an AM station or transmitter, the more likely you are to hear distinct radio waves with this project.

The “crystal” in question is contained inside a germanium diode, and is used to rectify the radio signal so that our ears can hear it. Since this project has no power source, the signal can be quite weak – crystal earphones are a fun way to hear low-amp signal (due to their sensitivity), or, alternatively, you could hook up a 9V mini-amplifier.

For more information on this battery-less technology check out this Wikipedia article.

Steps

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Step #1: Winding the coil.

Unsurprisingly, the first thing we'll need is a bottle. I'm using a brown glass bottle. Remove the label and rinse it clean.
Next, we need one of our three spools of magnet wire. Here we'll be using the red 30-gauge wire.
When winding a coil, it's intuitive to wind "hand over bottle," but this introduces a twist in the wire, something we don't want. A better way to wind a twist-less coil is to put the spool of wire on some kind of spindle and rotate the bottle slowly. Start by sticking down the free end with tape (start at the bottom of the bottle, leaving around 6" of lead wire for connecting later).
Be careful of the wire. You don't want it to "cross over" itself while winding. Try to keep a constant tension and apply tape as needed to save your position as you wind. You want to make the coils as smooth and closely packed as possible.
Once complete (using the whole spool) you should have something that looks like image 2 (leave about 8-10" of lead wire at the top, for connecting later). Next we will sandpaper the wire, but before that, apply fast drying sprayable lacquer or shellac (if available) to hold the wire in position. If you don't, you risk loosening the wire when you sandpaper the wire. The lacquer will hold it all together. Also, don't use too much, otherwise you risk waiting over an hour for it to dry.
The last step is to take a small piece of 150 grit sandpaper, and carefully sand off the red coating in a vertical line (image 3), then gently sand the ends of the wire to expose the copper.

Conclusion

There are actually many ways of making a crystal radio set; this is just one of them. If you can find a 365pF variable capacitor, you can try making a crystal radio like this one, reputed to be an excellent fixed-coil radio.

Steve Hobley

This week, I have been mostly working on...

I've been tinkering around with bits of technology since I was five years old. I used to take the telephone apart at home, just to see how it worked.

After a couple of years I could even put it back together again - and sometimes it would continue to work.

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