Introduction: Radio Receivers: From Scratch - PART 2


In this segment, we will be adding a meter to measure the voltage and the amperage of the radio, and using it as a signal strength meter.

We will also build a variable capacitor, a "film" static capacitor, and a few sensitive point diodes (detectors) all except the meters will be from common household items.

As the complexity of the radio grows, so does it versatility.  More stations, and weaker stations will be able to be brought in and tuned to (believe me though, we are not going to be building a miniature  transistor radio from household parts;  this is merely to demonstrate how easy it is for  radio signals to be relieved, amplified, modulated, tuned, and listened to.

Step 1: Capacitors

A subject many of you are familiar with, so I will not belabor the point.

Step 2: Diode(s) - Detector


This is where I promised to demonstrate the construction of a home made diode out of household items.

This is NOT a typical diode however, and will work best with low power radio signals. It IS however, adjustable.

By adjustable, I mean the detector point can be moved about to find the most sensitive (responsive) area.

Taking a safety pin and clipping off the "point"  so as not to shove the thing through your finger while you work, and also taking a bit of pencil "lead" (graphite and clay is what it really is),  Hold it against one leg and wrap some stripped thin copper wire around it, lashing it to the metal leg.   Now you can solder the copper wires, and this will hold the whole thing together pretty well.

Make sure the graphite point sticks out beyond the metal pin...we are using the graphite/metal combo as a "detector pointer" or "cat's whisker" as they once were called.  

The other half of this crude diode, will be some semi-conductive material.   I have tried old corroded pennies (newer ones contain so little copper that they are almost useless here, but your rig may work with them...it doesn't hurt to try.   Another old time semiconductor was an old rusted razor blade. 
CAUTION:  RAZOR BLADES ARE VERY SHARP, AND WHEN RUSTY CAN HARBOR TETANUS VIRUSES. 

Gluing a double edged razor blade, one that has blue streaks running through it, is fine, so the blades are no longer exposed, and you have a  nice flat surface to move your "whisker" over.

One of the BEST "crystals" to use as a detector, is a Galena block that is broken.  In fact, that is what they normally supply one with, when one buys a kit.  I would picture mine, but I haven't a clue as where where the "cleaning lady" put it *sigh*. 

Another GOOD "crystal" is a bit of pyrite (I'd used this also as a youth, but haven't seen it lying around of late.  One could find it EVERYWHERE when I was much younger....now I have difficulties finding ONE PIECE.

Experiment though, somethings are semi-conductive and will work better then fully conductive bits.  The penny is a good example, if the outer copper coating has some greenish cupric chloride layer over it, the copper is rendered less conductive. 

Blue limestone, tarnished silver, carbon rods from old batteries, etc all make for useful testing.   Be careful though, cupric chloride is poisonous, as are the innards of any battery. 


Step 3: Putting It All Together...

Here I constructed one capacitor.  Not shown was the variable capacitor I made by placing a full sheet of aluminum foil between the pages of a book, and another sheet a page below that one.   Now, to vary the capacity, one need only slide one of the sheets part way out of the book to decrease capacitance, or push it back on for more.

The coil was made with 22 awg coated wire wrapped around an old Quaker Oats style tube / container, cut down to size.  

I made 16 taps (that is, after every 5-7 turns around the coil form, I'd pull out a small loop and glue or tape it in place for later, and continue to wind the coil.  Do NOT break  nor cut the wire at any time during this process.  Once fully wrapped, I taped the end down so they'd not move, and sanded the tap loops, so as to have bare wire.

I went all fancy and instead of using a simple slider type assembly as found in many crystal radio kits, with this one I attached 6 on/off switches to a panel and one rotary switch (10 position).  
IF you are thinking of just using a slider, no taps are needed....set up the slider to rub along the coil and use it to wear into the coating so you have contact all along the coil.

There is one more switch on my assembly.   Although no batteries nor power source is needed, I installed an on/off switch, so as not to bother anyone  nearby when I wasn't using it.

Two of the pictures show a umeter attached to it for demonstration purposes.  It shows that a few microvolts are all that is needed for a really CLOSE radio station. 


In case you missed it, the intro, i.e. part one is here.

 

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