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.
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
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...