Foxhole_radio_WW2

Photo of a "Foxhole radio" used on the Italian Front in World War 2. It uses a pencil lead attached to a safety pin pressing against a razor blade for a detector. - from https://en.wikipedia.org/wiki/Crystal_radio

What is a Crystal Radio ?

Crystal radios are usually little radios made out of mostly junk or spare electronic parts. It allows anyone to receive AM radios signals utilising a crystal earpiece placed in one's ear.  The signal filtering is done using  LC Tank circuit  (ie. a coil of enamelled wire wound round a non-conductive core in parallel with a variable capacitor). By changing the inductance or capacitance of the tuned circuit, it allows anyone to tune to a particular AM radio channel. The audio detection is done simple Germanium diode (1N34) or it can be a 'Cat whisker' poking a Galena crystal or a pencil scrubbing on a razor blade.

During WW2, the GIs in Europe and the Pacific used crystal radio to tune into civilian radio in the field. Today, boy scouts who wanted to earn a Electricity badge will be required to show that they can build a BOY SCOUT RADIO (basically a simple crystal radio with a razor blade detector).


crystal-radio

A crystal radio with 1N34 Diode as detector

CatWhisker

The Cat-whisker and the Galena detector

OLYMPUS DIGITAL CAMERA

The razor blade & pencil detector

Why do I build Crystal Radios?

Basically, during the Summer of 2019, I wanted to introduce to my son, the strange and interesting world of electronics. So I began researching and gathering the parts to make one. Sadly, after completing my first crystal radio, I quickly realized that he is more interested in his Roblox game development than actually tinkering with electronics. (Incidentally, I did build a simple diode crystal radio during my Electronics 1 class in the mid-80s when I was at Tech College).

 

CR#1 : My First Crystal Radio in the 21st Century

My first crystal radio was build based on the video tutorial from Rimstar.org's Steve. His video is very easy to follow and it involves actually building your own paper towel core variable capacitor. You can find his video here. Steve also made a few troubleshooting videos and I find them very helpful in getting my crystal radio working. I build it with the paper capacitor and later i replaced it with a Variable Air Capacitor bought from Amazon.

    

 

The CR#1 Tests

CR#2 : My  2nd Crystal Radio (tabbed coil)

My 2nd crystal radio was build based on the video tutorial from Squarewave2 video at https://www.youtube.com/watch?v=UiRCxpVpHgw&t=414s . So this crystal radio has a coil with tabs that allow me to change the inductance by changing the tab connections. The components are mounted on a honeycombed carboard box I took from Costco.

   

The CR#2 Tests