Radio Receivers Part 1: From Scratch

Intro: Radio Receivers Part 1: From Scratch

This will be a collection of simple instructions on building an actual radio or two with household items.  It will further demonstrate building one's own capacitor(s), and detector diode...again, from household items.

Some care will need to be taken when soldering and when working with items like razor blades, etc.  but the careful person should have no difficulty in constructing these using the plans I will outline.

STEP 1:

We will build a few simple radios with this first in the series of Radio instructables.

The very first one, having only 2 parts to it.

Warning! This radio MAY NOT WORK in your area if there are no close by, strong stations.  If you have a really close AM station, you may be able to get this to work.  It is so simple though, that it is worth a shot.

First, find yourself a "piezoelectric" type earphone (not the magnetic and plate kind, they will not have the sensitivity needed).

Strip off the insulation (if there is a plug, it will need to be removed) from both wires.

Now, find yourself a 1N34A Germanium diode (low forward voltage, high sensitivity).   solder the one wire of the earphone to the diode as in the schematic. 

Now, tape (duct tape is good here) the other end of the diode to a water pipe (the facet at the sink is good), and grab the remaining free wire in your hand.  You have now become the antenna, and the faucet the ground.

IF you hear anything at all, it should be a faint AM station. 

There are 2 problems with this radio,  actually at least 4 problems:

#1: it is not tunable, you can't "pick" the station you wish to listen to,
#2: it is not loud, IF you hear anything at all,
#3: it is not portable at all
#4: IF  you have more then one station nearby, you will hear all of the strongest ones.

STEP 2: One Problem at a Time: Picking Up Weaker Signals: Resonance

One of the big problems of our first radio, if you got it work at all, was the faint signal you got, even with a very strong station signal very near by.

To solve THIS problem one can use "resonance". 

Envision a wave coming to shore.   If the wave is small it does very little.   And then comes another.  Still nothing, but if the waves come timed at just the right interval, the shoreline can take quite a battering, without increasing the size of the initial wave.  The "force" will build behind the repetitious wave form and increase the strength of the waves themselves.

This is resonance.

To use the simplest example, we must first do a little math:

divide  936 (feet) by the frequency desired in megahertz.  

For instance, lets say we wish to listen to station 770 on the dial.  This radio wave is oscillating at 770,000 (770 khz) times per second.  This gives us, in megahertz a figure of 0.770 mhz.

Plugging it into our equation we get: 936 / .77 = 1,215.58  or about 1,216 FEET. 

IF you wish to make this radio, you will have to take that length of wire, halved, attaching one half to each end of the diode.  Now attach the earphone wire to each end of the diode also.

Take the ends of the long wires and place them up in trees around 1216 feet apart.

Some problems arise with this setup of course:

#1: it isn't portable at all,
#2: it is hard to tune as the wire needs lengthened or shortened to get "other stations".

In the next step, we will solve both of these problems to some extent.

STEP 3: Taking the Next Step: a 3 Part Radio

In this last example for this Instructable, we will explain the make up of making the radio more portable and more easily tuned.

I will follow up with more instructables which I will place links in here and the others, so you can find them, that will give even more tune-ability to the radio, and more power.  We will also get into explaining the workings in more detail.

For now, the simple 3 piece radio. 

Note on the schematic, that 3 parts are the earphone, the diode and the coil which makes up part of the antenna.

I have seen MANY ways to "tap" into the coil.   One way is, while winding it (with a few thousand feet of magnet wire (called this as it is used in coils to make electromagnets), one pulls out a small loop now and then, and continues to wind the coil.   In this simplest of scenarios, one would need an alligator clip from the antenna to the coil so as to be able to move the settings from one "tap" to another.

Another way is to attach the antenna to a piece of metal (clamped or soldered) and attach it to a base with a single screw.  Now the "other end" can be swiveled.   Thus, placing the coil under that end, it can be slid up and down the coil (the biggest problem with this is that the coil needs to have the piece ran over it enough times to wear through the chemical coating on the side of the coil wires, but not so much that they "short out" against one another.

The "setup" shown at the beginning of this instructable, shows how this can be made even more complicated but much more convenient to "tune".  


As for an explanation for the coil, the need for the length of wire is for tuning into the proper resonance of the signal.  BUT it doesn't need to be stretched out straight as we did in the last step, but can be coiled in order to make it more portable.  Thus, moving the tap from one place to another shortens or lengthens the antenna in order to help tune in a different resonant frequency.


Eventually, we will be introducing how a capacitor helps in tuning, and how to make a simple capacitor, and also how to make a simple point sensitive diode from household materials.

See Part 2 for further  info on making the coil, the taps, and the diode detector...

Comments

[i couldnt think of a name]

oh ok thanks

[i couldnt think of a name]

does it matter what size of wire i use on this?

[Goodhart]

Only when it comes to to coil and the antenna. With the coil, you'll not want to ues very heavy wire (coated, so called Magnet wire is suffciant for the coil). The antenna depends on the length. IF you do not use a standard antenna as I tried here, a long 40 foot piece of magnet wire. does well (if you have the room for it).

[electronicz]

Where does the 936 feet in the equation come from?

[Goodhart]

Here is a better description of the reason than I could ever give.

[electronicz]

Ok, thanks

[nutsandbolts_64]

No close stations you say? Maybe I can modify the coil and tune it to airport frequencies (assuming I can actually get one to work with FM, if I can...)

[Goodhart]

The plans here, and the ones I will soon add to this (by means of a link) are pretty low frequency (and FM is in a bit of a higher range). The use of a "crystal diode" which will be illustrated in the next ible, will help tune in a station, but you probably will not be able to get anything on "the FM dial", without a lot of modulation.

I have, inadvertently picked up AM stations on many devices though, not meant to receive such a signal, from digital recorders to home made buglar alarm systems....sometimes it is comical, but mostly it is just annoying :-)

[nutsandbolts_64]

Speaking of tuning, can't a sliding mechanism slide a screw back and forth then tighten it to make contact with the coil. It would have a small, thin conductive object (a needle?) that would handle precision. My friend made one a few weeks ago and both of us had no idea what may have caused the difference in signal in the ff. situation:

My friend set-up the radio inside his house, and he got random signals from some random station which I can clearly hear when he stuck the earpiece up to the microphone on the telephone. He then set it up outside the house, but he surprisingly received weaker signals. I hypothesized the surrounding buildings may have somehow affected the signal, but we wouldn't know unless we demolished the entire block.

[Goodhart]

On one of the earliest "crystal" sets I ever build (decades ago) the coil was not tapped as mine is here, but rather a piece of metal was bolted to the wood base, using two washers, so it could be moved back and forth, and bent an angle up closer to the coil, so only a "cut out point" would touch the coil. Of course, this meant running the metal piece back and forth over the chemically coated wires until they were worn through and contact could be made but that was a very simple method for making a nearly infinite amount of variability.

[nutsandbolts_64]

Not exactly what I meant... But it appears I went a bit off course from what you meant. In whichever case, has anyone considered using aluminum wire? Galvanized wire perhaps? Any metal that doesn't need a chemical coating in order to reduce its corrosion. Although yes galvanized wire is coated, what it's coated with is still electrically conductive. I don't know how this would affect resonance, but it's worth a shot -- wow I just got an idea. Interchangeable coils anyone? I did instantly realize the pros and cons.

Pros:
-No need to fine tune into desired radio station

Cons:
-Carrying a box full of coils isn't really viable for portable sets, unless you really want to

Thoughts?

[Goodhart]

The coil would have to be wound VERY carefully if not coated, so that NONE of the wires TOUCHED each other. That would constitute a short, and the coil wouldn't be a coil anymore.


I have a box of coils, most of them are not really as useful as one might think.  Since we are looking at the need for such a BIG coil, carrying a bunch of those around would be cumbersome (at very least ;-)  

[nutsandbolts_64]

Now I realized how off course I really am... How about you remove the coating on one side, and physically separate the exposed surfaces with some sort of dielectric? And here we are exploring all the options on how to reinvent the coil (reinventing the wheel is a bit too far-fetched).

[Goodhart]

Sounds painfully hard to do, to be honest. Using the coated wire, one only need run the blade of the adjuster bar (more like a then bent piece of metal) a few times to wear through a ridge for contact. My "tapping out the coil" method actually complicates the original crystal set I had, and maybe unnecessarily so.

[nutsandbolts_64]

Agreed... Maybe then there is no need to reinvent the wheel (ahem, coil). Then it looks like the tried and true methods are the best techniques for use on crystal sets. It all comes down on experience, and that's where most people are lacking (of which also includes me).

[Goodhart]

Yeah, I was kind of thrown into it head first as a youngin, some 38 - 39 years ago or so.....was cool then to "listen to a radio without any electricity" :-)

[nutsandbolts_64]

Speaking of the radio, would magnet wire from a burnt-out coil from an electric buzzer suffice? (the electric buzzer is from a door bell)

[Goodhart]

as long is "burnt out" doesn't mean the insulation was "burnt off" (a continuaty meter clipped on both one end and drawn along the length of the insulation, should tell you).

[Infinitevortex]

Nice. Looking forward to the next episodes!

[Goodhart]

I am having difficulties finding the time....hopefully this weekend.