Hacking With Gum

Almost anyone who has tinkered with electronics has built a crystal radio. I’ve always marveled not only at how amazing these little devices were, but also at the lack of descriptions regarding their inner workings. Descriptions of crystal radios almost invariably use some terminology synonymous with, “The detector changes the back and forth radio wave electricity into one way sound electricity…“; well, that’s nice, but how does it do that? Why does it work? What do we mean by “radio wave electricity” and “sound electricity”?

Before examining a crystal radio circuit, it is important to understand where crystal radios get their power from. One of the things that makes crystal radios so interesting is that they require no power source. Well, that’s not entirely true, as there has to be some energy coming from somewhere in order to make your speaker / headphones vibrate. Crystal radios get this energy from the radio waves themselves; in fact all radios use this same energy, although modern radios require additional power sources to amplify the relatively weak energy supplied by radio waves.

Radio waves are in many ways similar to the 60Hz AC power that you have in your house; both are forms of electrical energy, and both have currents that alternate direction. The electrons that are responsible for generating radio waves however, alternate direction much faster than the electricity in your power lines, typically switching direction millions of times per second. When we talk about electricity, we usually are referring to the electromagnetic fields that transfer power from one place (your electric company) to another (your computer, lights, phone charger, etc). That is exactly what radio waves are: electromagnetic fields. So, think of radio transmitters as small electric companies that send you very small amounts of electric energy, except in stead of sending it through wires, they send it through the air. Einstein’s description of radio is perhaps the most eloquent:

You see, wire telegraph is a kind of a very, very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? And radio operates exactly the same way: you send signals here, they receive them there. The only difference is that there is no cat.

Now, if you were to listen to a 60Hz AC signal, you’d find that it’s not much fun to listen to, just a constant humming sound. That’s because the 60Hz signal is switching directions back and forth at a constant rate and constant amplitude, which would cause your speakers to vibrate at that same constant rate and amplitude, which would produce a monotone humming sound. But crystal radios are usually designed to listen to AM radio stations, not 60Hz hums, so let’s take a look at how AM radio waves work.

AM stands for Amplitude Modulation, and as you might guess, it “encodes” radio waves with information by varying (modulating) the strength (amplitude) of the radio wave. It does this by using sound waves to control the strength of the radio waves generated by the transmitter. It is perhaps easier to understand how these currents are modulated by examining the following circuit of a simple AM transmitter (courtesy of SciToys.com). The audio input actually controls the power supply for the oscillator; obviously, if the oscillator is supplied with less power, its output will be weaker, and if it is supplied with more power its output will be stronger. The variations in the strength and frequency of the audio waves cause the amount of power supplied to the oscillator to vary, thus changing the amplitude of the oscillator’s output signal accordingly:

Simple AM Transmitter Circuit

Below you can see how the audio signal applied to the transformer in the schematic above changes the amplitude of the oscillator’s signal:

Amplitude Modulation

Notice that the transmitter’s frequency of oscillation is much higher than that of the audio signal; in the above image, one oscillation of the audio tone is spread out over twelve of the transmitter’s oscillations. This is important, because it allows variations in both the strength and the frequency of the audio tone to modulate the transmitter’s signal.

For example, if the audio tone were of a higher frequency, one audio cycle might only cause an amplitude change in five of the oscillator’s cycles. In other words, the amplitude change in the transmitted signal will be shorter (amplitude changes will occur more frequency) or longer (amplitude changes will occur less frequently) depending on the frequency of the imposed audio tone. When these time variations in amplitude are converted back into audio at the receiver, the audio wave will be respectively shorter or longer, thus re-producing the original audio tone.

But how do we convert these changes in amplitude back into audible sound waves? Let’s examine the simplest crystal radio circuit, which consists only of a diode and a pair of headphones:

Simplest Crystal Radio

When radio waves (electromagnetic energy) strike the antenna, they cause the electrons in the antenna to vibrate back and forth at the same frequency and relative amplitude as the electrons that produced the radio waves in the first place; this induces electric energy in the circuit. Remember that radio waves are generated by alternating current, which is just a bunch of electrons vibrating back and forth at a certain rate. This rate is called the frequency. The frequency of your AC line is 60Hz. The frequency of your local NPR station would likely be somewhere around 89MHz (mega-hertz).

The diode is of course the detector in this circuit. Germanium diodes are preferred over silicone diodes due to their lower turn-on voltage. Basically, the diode works to convert the AC current (induced by the radio waves) into DC current by only allowing electrons to flow in one direction through the speaker. This means that when the alternating current moves one way, the diode will block the flow of electrons, forcing them to go through the tuned circuit (path of least resistance, since the diode is blocking the flow of charges). When the alternating current is moving in the other direction however, radio signals at the desired frequency will move through the diode, and subsequently, through the speaker. This means that the electron flow moving through the speaker looks like this (courtesy CrystalRadio.net):

Pulsating DC In The Speaker

Anyone who has worked on DC power supplies will recognize that this diode is acting as a half-wave rectifier, simply converting AC into pulsing DC. The principle that you use to convert the electricity from your AC power lines into DC that you can use to charge your cell phone is the same principle used in a crystal radio; we simply convert the alternating current into a pulsating direct current (most power supplies are actually full-wave rectifiers, but the principle is the same). The amplitude of these pulsating direct currents is controlled by the audio input (i.e., your voice) to the AM transmitter, and as they change amplitude they force the speaker to vibrate up and down in a corresponding pattern. These speaker vibrations produce an identical copy of the original audio input.

But why do we need the diode in the first place? If the amplitude changes drive the speaker, shouldn’t we be able to connect the antenna directly to the speaker and achieve the same effect? To understand why, you must first think about what a speaker is made of. It is basically a magnet, an inductor, and a diaphragm (some piece of light, flexible material). A simple speaker can be made by wrapping some wire around a tube of paper and placing a magnet in the center of the paper tube; connect the ends of the inductor (the wire coil) to an audio source, and you have a very low quality, but working, speaker (video by CalcProgrammer1):

Electricity and magnetism are closely linked, and as electric power moves through the speaker coil, a magnetic field is produced. Everyone knows that if you hold two magnets next to each other, they will attract one another if the opposite poles are facing, and repel one another if the same poles are facing. By placing a magnet inside of the speaker coil, the coil will actually move back and forth as electric power generates a magnetic field of varying strength in the coil: the magnetic fields in a speaker work on the same principle of attraction / repulsion that all magnetic fields exhibit.

However, the above speaker is very weak; in order to improve the response of a speaker, most speaker coils (aka, voice coils) are wrapped around iron or some other magnetic metal. This improves the quality of the speaker, however, it also introduces a slight problem for radio work: as the AC audio signals in the speaker coil switch back and forth, they create a magnetic field that causes the polarity of the metal core to switch back and forth as well. This is fine at low frequencies, but as the AC begins switching back and forth very rapidly, you encounter a phenomenon called hysteresis. Basically, the core’s polarity can only switch back and forth so fast, and once you exceed this maximum frequency, the core’s polarity stops switching altogether. Audio is fine, but radio frequencies are much higher in frequency and will induce hysteresis. By using the diode to convert the alternating current to direct current, the magnetic polarity never has to switch back and forth because the flow of electrons through the speaker are always going in the same direction.

The final piece to this circuit is ground. Everyone will tell you that a good ground makes for a better receiver; but why? Well, quite simply, electrons need a place to go. Electrons aren’t produced by radio waves, but already exist throughout the crystal radio’s wires and components; the radio waves just make them move back and forth. But they need a place to move to, which in most cases is provided by an earth ground.

To demonstrate, I have built both the simple crystal radio as well as the AM transmitter described above. The receiver is poor and the transmitter is weak, so instead of connecting the receiver to an antenna as depicted in the schematic, I’ve simply connected the output of the transmitter directly to the diode in the crystal radio. In order to actually hear the output I have also connected an amplified speaker in place of the headphones, although the audio is still rather weak. Note the loud humming noise that comes out of the speaker with no ground connected; you will hear a dramatic difference as soon as the ground wire is connected, as well as some faint audio from the transmitter:

In reality however, this crystal radio is not very useful because it is picking up all frequencies at once and has no way of selecting out one particular signal. So when you connect it to an antenna all you hear is a lot of noise. This is much like sitting in a loud auditorium full of people talking – it would be very difficult to pick out any one conversation unless they were yelling right in your ear, just like connecting the transmitter’s output directly to the crystal receiver’s input. Here is what the radio sounds like when connected to an antenna:

In order to provide some selectivity, an inductor (coil) can be added to the circuit. Without getting too in depth into the subject of reactance, inductors will generally impede changes in frequency; thus, signals with lower frequencies will pass through inductors more easily than those with higher frequencies. By connecting an inductor between the antenna and ground, we can create a very rudimentary filter that essentially short-circuits low frequency signals between the antenna and ground, because they will see the inductor as a low-resistance path between the antenna (power source) and ground. Higher frequencies will still pass through the diode because they will see the inductor as a high-resistance path between the antenna and ground:

Crystal Radio With Inductor

After connecting the inductor to the circuit much of the low frequency hum from electrical appliances and the like are filtered out, and a local AM station can be faintly heard:

The problem with this circuit is that there is still not much selectivity. It works if there is only one loud AM station, but what if there are two or three or more? In order to provide more selectivity, a capacitor can be added in parallel with the inductor:

Crystal Radio With Tuned Circuit

Like inductors, capacitors also have reactance, but capacitors tend to “pass” higher frequency signals and “block” lower frequency signals. The inductor and variable capacitor in the circuit above together act as a filter; this filter is tuned to allow only a certain range of radio frequencies to reach the detector. Tuning can be accomplished by changing the value of the capacitor or inductor; variable capacitors are convenient to use because their values are easily changed.

For example, say that we want to receive radio waves with a frequency of 1000kHz; the inductor and variable capacitor would be selected so that they act as low-resistance paths to AC signals that are above and below the desired 1000kHz frequency. Since electrons prefer the path of least resistance, radio signals at these frequencies will pass directly between the antenna and ground, bypassing the detector. The inductor and variable capacitor also appear as high-resistance paths to AC signals that vibrate at the desired frequency of 1,000,000 times per second, so these signals will prefer to travel through the detector. Of course, in real life, such filters are not so precise, and will allow a range of frequencies to travel through the detector. However, the electromagnetic spectrum is quite large, so they still serve to filter out the vast majority of undesired signals.

As an example, I’ve added a variable capacitor to the circuit and it is selective enough so that both the local AM station and my small AM transmitter can be individually tuned in and listened to:

You will also often see a capacitor placed in parallel with the headphones in a crystal radio. This capacitor acts as a smoothing capacitor, just like one that you would find in an AC to DC power converter. This smooths out the high-frequency pulses that were rectified by the diode:

Crystal Radio With Smoothing Capacitor

The signal differences with and without the smoothing capacitor can be clearly seen in the following pictures; the unfiltered signal looks much noisier:

Signal With Smoothing Capacitor

Signal Without Smoothing Capacitor

And that is the magic of crystal radio: it’s just a long-range, wireless power supply. Now if only we could figure out where that cat went…