Dave's Sunday Morning Special #59 Crystal Radio
Here is a little project that I built in a few hours. A customer wanted a set that would be simple, have a detector stand and was cheap. When people think of cheap, they always come to me!
This set is to be used for a classroom demonstration at a college. My idea was to make a simple set where none of the wiring is hidden. It is sometimes hard to understand a set if half the wiring is on top and the other half on the bottom.
The circuit is pretty much straight forward. A 2 gang variable capacitor is needed. I used a 3 gang as it was in my experimental parts box. The 3/8 inch shaft makes it not so useful in most of my designs, but it is fine here.
The coil is on a 3 inch form (7,62 cm). There are 35 turns of wire on this coil, with a tap at the 25 turn mark. This tap is for the detector. I used 40/44 litz wire as it is my favorite wire for the smaller sets. It is what I started with. Winding this coil only took a few minutes.
I designed this set for use with a crystal earphone. They are plentiful and work well in these radios. A 47k ohm resistor and a 1000 pf capacitor shunt the earphone.
The detector is a piece of pyrite that was cast in woods metal. Woods metal has a low melting point so the heat doesn't destroy the detector material. The clip grasps the potted pyrite very well. I suppose that with one of these clips, the potting isn't really necessary. But even my simple sets go first class.
Because the variable capacitor is fairly large, this radio tends to work the best with a shorter antenna. It also works well with my long wire.
Good DX! Dave - N2DS