Portable CD players are so cheap nowadays that most folks have them, but CD players for cars are still at a premium. If you want to listen to your portable CD player in your car, you need an adapter, like the one that can be bought at your local Radio Shack for about US$20 (Cat. #12-1951). This adapter looks like a cassette tape with a wire hanging out of it and a mini phone plug attached to the other end. To use it, you simply insert the adapter into your car's cassette slot and attach the mini-plug to the output on your portable CD player.

But why spend $20 when you can make your own adapter, out of junk, for free!

Here's what you need:

• An old cassette tape (with a screwed on shell)
• A couple of feet of enamel-covered, thin copper wire (around 30 gauge).
• A pair of old stereo headphones with a stereo plug on it.

Here's how to do it:

1) Find a cassette tape that you don't listen to anymore, like that Bee Gees Greatest Hits tape your mom gave you for Christmas. Make sure that it has a shell that's held together with screws. Remove all the screws and put them aside for a moment (don't lose 'em!).

2) Remove the audio tape, the rollers and all of the other moving parts from inside the cassette. Throw them away.

3) Measure off about 2 feet of enamel-covered copper wire of about 30 gauge. If you don't have any suitable wire, Radio Shack sells a 200 foot roll of "magnet wire" (Cat. #278-1345) for $4. Remove the enamel from all but two inches on both ends of the wire by scraping it off with the edge of a knife blade.

4) Scrape off about a half inch from the ends of the remaining two inch enamel-covered end pieces you left on in step 3.

5) Wrap the central scraped section around the two plastic V-shaped pins found in the tape playback head area on the cassette (where the tape used to meet the tape head of the cassette player). Wrap the wire about 10 times around the pins so that you've created an oblong wire coil (see image below).

6) Cut the wires on the headset just above where they "Y" off on their way to each earphone. Strip off the insulation from a half inch on each of the cut ends. Connect these loose ends to the corresponding ends of the coil you just made. It's best to solder the connections, but if you don't have a soldering tool, very small wire nuts can be used (as long as you're sure they'll fit inside the closed cassette shell). The point is that you must have a solid mechanical and electrical connection. (If you don't have an old headset to cannibalize, you can get a 6 foot shielded audio cable with mini plug attached from the Shack (RS Cat. #42-2387) for $4. Clip off one of the ends and strip back the insulation from the wires. The shielding on the cable will help prevent unwanted electrical noise emanating from engine components from getting into the car's stereo.)

7) Before you screw the cassette shell back together, you have to determine which way to bring the cable out of the shell. If you have a front loading cassette player, bring it out of the back. If you have a side loading player, bring it out one of the cassette shell's sides (it doesn't matter which one). To make the hole for the cable, simply notch a small "V" in both shell halves with a hobby knife.

The completed assembly. The copper coil can be seen at the bottom center
of the cassette shell, wrapped around the two pins.

If you've been able to scrounge all the parts, congratulations, you just saved yourself $20 and "home-brewed" a useful electronic gizmo. Even if you had to buy parts from the Shack, it only set you back $4-8, and hopefully you've had a little DIY fun in the process.

So, how does your adapter work? The coil of wire you wound around the cassette pins is an inductor. When energy from your CD player's output is fed into the coil, a magnetic field is generated. The energy in the coil corresponds to the energy coming out of the CD player's audio output and it is picked up by the cassette player's playback head without even touching the coil. It doesn't have to touch because the magnetic field is big enough to encompass the tape head.

The best part is that you can use this adapter for any audio source. Try running the audio out from a portable shortwave radio and listen to the BBC World Service on your way to work. Having this adapter on a long car trip makes it possible to listen to news and music from many different sources. The neat thing about listening to a shortwave radio station is that you're never out of range of big powerhouse broadcasters like the BBC, the Voice of Russia, or Deutsche Welle. Another fun radio to use in your car is one that picks up the TV band so you can listen to your favorite TV shows on the road.

Regardless of the device you're using, you'll have to play with the volume controls on both the radio and your car stereo. You want to have enough power to generate the magnetic field but not too much to "overdrive" the car stereo, which will make the CD player or shortwave radio sound distorted.

- Ken Reitz [2/2/98]