I have, for some unknown reason, always been interested in ancient technology, mainly electronics and mechanical things and I have always wanted to make a true old fashioned style crystal radio. A crystal radio, or crystal set as it is sometimes called, is a radio that only uses the power of the radio waves picked up by the antenna to generate the sound heard in the head phones. The reason it is called a crystal set is because they use a mineral crystal as a diode for the detector in the circuit.
Quite a few years ago I found a book in the library called "Radios That Work for Free" by K E Edwards and found a set of plans for a radio that I just had to make, unfortunately I did not have the resources to build it at the time so I made a photo copy of the pages from the book for future reference. Well I was poking around on the net the other day and found a site that sells copies of that old book, along with various other electronics books and parts related to crystal radios and decided to order a copy for myself along with two 365 pf variable capacitors needed for the circuit I copied from the book.
The radio that I am going to build is the second one shown in the book but I am going to make some changes to the original set. The three changes I am going to make to the set are that I am going to build the set into a box instead of just having the control panel attached to a base board. I am also going to make an antique style "Cats Whisker" style detector instead of using a Germanium diode and use a matching transformer so I can use a set of modern headphones instead of trying to find an antique high impedance set or a crappy little crystal earplug.
I have written this instructable assuming the reader has basic wood working and electronics skills like making boxes and soldering, explaining the more involved and specific tasks in detail, as in winding the coil.
Step 1: Circuit and Theory
Here is a scan of the circuit from the book. The circuit is actually pretty simple, just two variable capacitors(C1 & C2), a coil(L) with 8 taps and a tap switch(S), a detector(D) and a set of phones(P).
Without getting into the rather intensive math involved, I have taken classes on this stuff, here is in a nutshell how it all works.
Crystal radios mainly work on AM or Amplitude Modulation radio waves, there are some that well work on FM as well. AM radio waves are actually composed of two waves combined together, one is a high frequency wave called the carrier wave and the other is the lower frequency sound waves being transmitted. The carrier wave is on the order of 750 to 1500 kHz. and is used to transmit the sound wave. The way it works is that the two waves are added together, or modulated, producing a signal that is of the same frequency as the carrier wave but its amplitude is determined by the sound wave(see second picture).
The way a crystal radio actually works is pretty ingenious, it uses the coil L and variable tuning capacitor C1 as a low pass filter to only allow frequencies at the desired frequency of the carrier wave to be passed to the detector D. The detector then demodulates the signal by only letting the low frequency part of the signal to pass, ie the sound wave, to the headphones where they are then turned back into sound. There are no batteries, no transistors or amplifiers of any kind, the sound is produced solely by the energy of the filtered radio wave.