Crystal Receiver
Hand-Made Crystal Radio Components
Okay, so what electronics hobbyist hasn't considered building a crystal radio set at one time or another? I found myself with a chunk of free time during the summer of 2007 and decided that my time had come. I rooted through my old mineral collection and pulled out a few chunks of galena and iron pyrite and measured the voltgage drop on several random signal diodes I pulled out of equipment. Supposedly germanium diodes are the best for crystal set work, namely the very classic 1n34. It has a voltage drop of about 0.4V as I recall, and in general germanium diodes will have a smaller forward voltage drop than silicon. However, the rectification diodes in a few computer power supplies that I've salvaged have an amazing 0.2V forward drop, so I'ev fooled around with using them as well.
So far NO LUCK. This thing doesn't seem to work. I don't know why, but to rule out the problem being a lack of strong local AM station I built a super simple AM transmitter (see link at left), but I have not gotten around to using it to test this thing just yet.
Sliding Tuning Coil, #26 copper magnet wire on an enameled form
This originally was an old variable 50 Ohm rheostat, but has been repurposed. Isn't it pretty? I worry that the metal form may be too lossy for this to work well as a tank circuit/filter circuit inductor.
Following directions from H. P. Friedrichs' book "The Voice of the Crystal", I made a high impedance earphone/speaker. There is a magnetically biased coil armature made from many individual strands of soft iron or steel wire (I'm not sure which, it was an old spool), and the diamphram/sound box is a little steel tin that originally held packets of expensive tea. The coil was wound with wire taken from an old cheap guitar pickup that I bought for $3. The wire broke several times during the winding process, but I carefully resoldered each connection and tested for continuity and 1kHz impedance as I went along. I am confident that there are no shorts. At 1kHz it has an impedance of 64.6k Ohms, at 120Hz 5.892k Ohms. I can clearly hear the 1kHz and 120Hz signals comming from it when testing. If I ground one terminal and hold the other in my hand, I can hear the 60Hz hum from the house electrical wiring.
