Due to popular demand, I've assembled building instructions for the fourth graders crystal radio.
I'm short on time, so you'll need to print out and read over the links below to develop a parts list. I don't think the following is in the text, so I'll add it here.
I used 1/2" wood screws and finishing washers to connect the wires. The finishing washers have a sharp edge that bites into the wire wonderfully.
The crystal is a germanium crystal diode. As of 2003, Radio Shack no longer carries them, but I found this web site that might be able to help: http://www.web-tronics.com/1n34.html. I've not contacted them, so I don't know if you will meet with success there. You can use silicon diodes that Radio Shack sells, but they don't work well. The transformer is Radio Shack 273-1380. The phone jacks are Radio Shack 274-249. Connect each of the 8 ohm outside (not center tap) leads of the transformer to each of the two terminals on the back side of the phone jack.
In the original Mystery radio, which works well with a 25 foot antenna, the short coil (second winding) goes to the diode. The first winding, the long one, goes to the antenna and ground. If you connect it this way, for best performance, you will need to experiment switching the wires of one winding once you have the radio and antenna set up.
You can switch the connections of the short coil and long coil, so that the long coil goes to the diode. This often works better with a longer antenna, and may require an antenna tuner, mentioned in the text. It can work quite well this way, perhaps better than the original.
Why do I say "can" and "may?" Because with this simple circuit, many external variables control its performance.
The transformer middle lead (black wire, the center tap) is not connected to anything. The blue and green wires (1000 ohm side) go to the diode and ground; it doesn't matter if the blue and green are switched. The red and white wires (8 ohm side) go to the phone jack. If your transformer wire color codes don't sound like this, and you can't tell which is which, you can measure the resistance of the coil with an ohmmeter; high resistance goes to diode area, low resistance goes to headphone jack. Your headphone jack has three terminals. For walkman phones, if the wrong two are used, you get only one ear. Remember to use the terminals on the back side of the headphone jack, not the terminal nearest the hole where you insert the plug.
This is a very fun project. I've tried to be as precise and clear as possible. If something is unclear, feel free to email me, and please understand when I cannot give long explanations or answers. Some people are really good at putting this stuff on the web very quickly. It took me HOURS! So if you appreciate it, I'd enjoy hearing from you. If you think I'm a jerk for not telling you everything you need to know, sorry 'bout that.
You should be aware of safety considerations when installing antenna and ground systems. People have fallen to their death and have been electrocuted while installing antennas. I am not qualified to advise you regarding electrical safety. If you don't know about safety practices, consult an electrician or someone who is qualified. Safety first, then have fun!
Winding the Coil
Building a Coke Can Capacitor
Antenna and Grounding Instructions
Wiring Diagram and Top Plate (this sheet gets pasted to the wood base)
Coil Winding Guide (paper strips)
Miscellaneous Q & A
Counter added May 1, 2001
A: You are observant, and correct that I reversed the primary and secondary. Each configuration will give different performance. I suggest you start yours as with the original Mystery plans and then experiment later. I can't stress enough that crystal radio performance is affected by the whole system. In the case of the fourth grader's radio with the antenna and ground we used, the radio worked well across the band with the coils as shown on my pages. It is interesting that most crystal radio antenna coils are the small ones, the detector coils being the larger ones. That's part of the wonderful Mystery, I guess.
Q: The original Mystery plans show a .001 capacitor connected across the headphones. Why do you not use it?
A: The .001 cap was not necessary, particularly in conjucntion with the transformer. I've never been able to tell the difference on any of my radios using a capacitor across the headphones, although some swear by it. There are advanced configurations using a calculated matched value of a resistor and capacitor that help very weak stations, and that goes beyond the scope of the simplicity of this little jewel of a radio.
Q: The use of the audio transformer and modern headphones vs. a typical crystal earphone (the $3.00 variety) - do you find much loss with the transformer, or is it acceptable given the relative difficulty of finding an off-the-shelf item?
A: The transformer does introduce losses; however most 2000 ohm magnetic earphones are lossy, too. In my experience, the efficiency and quality of Walkman headphones more than make up for the transformer losses. Experimenting with a better transformer like the one on my advanced radio might yield amazing results. It has the RS part number listed on the schematic.