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Four: AM, FM, and Shortwave Crystal Radio Projects
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D1 germanium diode or galena crystal w/ cat s whisker C1 500 pF turning capacitor C2 001 F, 35 volt disk capacitor S1 single-pole 5-position rotary switch S2 single-pole 6-position rotary switch H1 crystal headphone or 2k headphones (Baldwin, Brandes) ANT 80 100 foot long wire antenna GND cold water pipe or ground rod Misc wood block, hardware, wire, clips, screws, Krylon, etc A crystal radio is the simplest form of AM (amplitude modulation) receiver ever invented It has great potential for experimentation and usually requires no source of power for its operation other than the radio signal itself, and costs little Most people do not realize that crystal radios can be built to pickup shortwave as well as FM radio signals Did you know that you can build very sensitive and selective crystal radios far better than most commercial AM radios! Ever thought of building a crystal radio Building a crystal radio will give you immense satisfaction, and the results are sure to please Crystal sets date back to the earliest days of wireless (pre-WWI) and an enormous variety of circuit designs have been produced over the years Their popularity has been variable as developments in other more elaborate forms of reception have taken place However, fascination with this crystal radio design, building and experimentation, is still very strong today with national and international organizations and clubs offering competitions So how does a crystal radio work An AM transmitter sends out its broadcast in the form of an electromagnetic
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wave that radiates from its transmitting antenna The AM transmitter sends out a fixed frequency carrier wave When sound is present in the program material, the strength, or amplitude, of these waves is made to vary in response to the audio content of the program The resulting wave is called an amplitude modulated wave See Figure 4-1 The purpose of a simple AM or crystal radio receiver is to pick up these AM waves and extract the audio signal so that it can be heard by the listener It does this by a process called detection The detection process utilizes a device called a detector which effectively strips off either the upper half or lower half of the AM wave It only remains to filter out the carrier wave to leave the audio signal and the job is done! The following diagram shows how the detector and filter work together to recover the original audio signal The strength of the recovered audio signal is small but, given the right conditions, it can be large enough to drive a pair of headphones; all without any power source other than the signal itself The diagram shown in Figure 4-2 illustrates a basic crystal radio In its simplest form, a crystal radio is composed of just our main components A good antenna, a detector, a filter capacitor and pair of headphones The basic diagram shows the input section at (A), the rectifier section at (B), the filter section at (C) and finally the headphones at section (D) The antenna consists of a length of wire suspended above the ground, while the ground or earth connection could be a metal spike driven into the ground When a
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Figure 4-1 Amplitude modulation
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Four: AM, FM, and Shortwave Crystal Radio Projects
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Figure 4-2 Basic crystal radio
capacitor is connected in parallel with an inductor and an alternating voltage applied across the combination, alternating current will flow The amount of current that flows depends upon the frequency of the applied voltage At a particular frequency, called the resonant frequency, almost no current flows For frequencies above or below the resonant frequency, significant current will flow In Figure 4-2, capacitor C1 plus the aerial capacitance forms the capacitor of our tuned circuit, while L1 forms the inductor At the resonant frequency of the tuned circuit, almost no current flows to earth through L1 or C1, leaving virtually all of it free to flow through the detector Alternating currents at broadcast frequencies either above or below the resonant frequency will tend to flow through C1 or L1 to earth Thus we have added selectivity to the receiver Notice that C1 is adjustable, as signified by the arrow By varying C1 we can tune the receiver to select specific broadcast frequencies The main requirement for a detector is that it should act as a non-return valve or one-way switch to the alternating currents of the AM wave If you could look at the alternating current in a circuit you would see it flowing back and forth, first one way and then the other A detector placed in such a circuit allows the alternating current to flow easily in one direction but not in the other Certain naturally occurring minerals were found to have this property and these became some of the earliest forms of detectors One such mineral, the crystalline form of galena (lead sulphide), was found
to be particularly good at detection, and became very popular for building crystal radios The detector consists of galena in a small cup or tin A small coil of wire, with one free end was called the cat s whisker which was used to make contact with the crystal The remaining uni-directional current is filtered by the combined effect of the headphone impedance and the capacitor at C2 and only the desired audio signal current passes on through to the headphones at H1 One drawback with the very simple circuit shown is that it has little selectivity That is, it picks up all AM broadcast signals with similar efficiency and often close together So as time passed more advanced, more sensitive and more selective receivers were born Let s construct a simple crystal radio In order to build the crystal radio shown in Figure 4-3, you will need to secure some tools and supplies First you will need to find a clean well lit work bench or table to spread out all your tools, charts, diagrams and components You will also need to secure a soldering iron, some 60/40 rosin core solder and a small jar of Tip Tinner, a soldering iron tip cleaner/dresser, obtainable from your local Radio Shack store You will also want to locate some small tools such a pair of end-cutters, a pair of needlenose pliers, a magnifying glass and a set of Phillips and flat-blade screwdrivers for this project Grab the crystal radio schematic, see Figure 4-2, and the resistor and capacitor identification charts, and place them in front of you, so we can get started building the project The resistor identification chart in Table 4-1 will help you
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