Ukulele Radio

A crystal radio can decode an AM signal and only requires a few basic electrical components to assemble:

1) an inductor (a coil of wire)

2) a variable AC capacitor (variable plate capacitor is ideal)

3) 30ft of copper wire (enameled copper wire, also called magnet wire)

4) a speaker element (in my case it goes through an amplifier and then a speaker)

5) a crystal diode (I used a 1N34A germanium diode) 

The working principle of a crystal radio is the ability to "tune" a circuit to resonate at a certain frequency. That is, when a signal at a specific frequency is detected, this circuit will resonate and oscillate the electrons in the wires in the same way. Below, you can see the electrons oscillating back and forth at the resonant frequency of the circuit.

The equation that models this is:

L is inductance and C is capacitance. This equation tells us is that if you can change one or both of these properties of the circuit, you can change the frequency you're tuned to.

Among other properties of the coil, inductance is proportional to the number of coils of wire. To increase the inductance of a coil, increase the number of turns. One factor to note about the inductor is that it is just a long piece of wire wrapped in a coil shape with one end connected to the antenna, and the other connected to ground (I will explain this further later on). If you theoretically wanted a shorter coil, you could effectively do this by connecting anywhere in the middle of the coil to ground. 

Grounding the coil earlier reduces the number of coils the current has to flow through, reducing the inductance and increasing the tuned frequency.

The other option for tuning is changing the capacitance. Sound waves are AC signals, meaning they are represented by an alternating current, and only certain kinds of capacitors can be used here. A ceramic capacitor or a parallel plate capacitor are perfect options here and are exactly what I used. I specifically did not use an electrolytic capacitor because they are polarized and ONLY work for DC applications. 

In a parallel plate capacitor, the capacitance is inversely proportional to the distance between the plates. We're working with AC power, so the charge is constantly flip-flopping back and forth as shown below.

The closer the plates get, the higher the capacitance becomes.

Now that the circuit can be tuned, it needs to be decoded into sound. As it is, the circuit is producing an AC signal that interferes with itself because the wave has equal and opposite shape, going up as much as it goes down.

If you tried to play this through a speaker, nothing would play because the upper half of the signal would be canceled out by the bottom half before any sound could be heard.

In order to obtain the signal we want (the top of the wave form), we need a way to eliminate the bottom of the wave. This can be done in a very easy way with a diode. 

In it's simplest form, a diode is an electrical component that allows current to flow in only one direction, but not the other. 

If you imagine that everything below the horizontal center line is a negative voltage, the diode will effectively remove this half of the graph (to an extent, but that's beyond the extent of this project site) leaving only the positive part of the graph. 

Now we have a signal that can be run through a speaker element of some sort, albeit very quietly. For the best results of a system without external power, using a piezo-electric speaker is recommended. For a powered system, which I have, you would need a low power amplifier which can increase the gain of very low power signals to make them audible.

In my case, I'm utilizing a pocket amplifier I made in a previous project that happens to be perfect for this application. Due to the fact that this system is externally powered, the negative terminal of the battery acts as the ground for the system.