Atributos relevantes e irrelevantes En la definición curricular de Media, se determinan los siguientes atributos relevantes para :

Simple Crystal Radio

L1 150 turns of 24 ga.

enameled wire on

4” × 2” dia tube

C1 365 pF tuning

capacitor (old radio)

D1 1N34 germanium diode

(not a silicon diode)

C2 .005 µF, 35 volt

disk capacitor

H1 high impedance

headphones (crystal or 2000k type)

ANT 80–100 foot long

wire antenna

GND cold water pipe or

ground rod

Misc wood block, coil

tube, hardware, screws, clips,

washers, Krylon spray, wire, etc.

FM Crystal Radio Set

R1 47k ohm 1_/ 4 watt, 5% resistor C1 82 pF capacitor C2 80 pF air variable capacitor C3 18 pF capacitor D1 1N34 or rock crystal diode L1 5 turns AWG #18

bare copper or silver wire, 12 mm inside diameter, tapped at 2.5 turns

H1 high impedance

headphones (crystal type of 2k ohm type)

Ant antenna- 7” of

#18 bare copper wire

Misc wood block, wire,

hardware, clips, etc.

Supersensitive AM/Shortwave Crystal Radio L1 11 turns, 22 ga. enameled wire on 27_/ 8” cardboard form L2 (AM) 54 turns,

22 ga. enameled wire on 3” air-core form Taps are brought out at 40T, 27T, 15T and 6T

L2 (SW) 15 turns,

22 ga. enameled wire on 3” air-core form Taps are brought out at 12T, 9T, 6T and 3T

D1 germanium diode or galena crystal w/”cat’s whisker” C1 500 pF turning capacitor C2 .001 µF, 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-W.W.I) 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

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

Chapter Four: AM, FM, and Shortwave Crystal Radio Projects

Modulation Envelope Amplitude Modulation of a Carrier Wave Time Carrier Wave Amplitude 0 + −

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 needle- nose 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

Chapter Four: AM, FM, and Shortwave Crystal Radio Projects

identify resistors needed for the some of the projects in this chapter. Each resistor will have three or four colored bands on the body of the resistor. These colors identify the resistor’s value. The color bands start at one end of the resistor, the first colored band is the first digit of the resistor’s value, while the second resistor represents

the resistor’s second digit of the value. The third colored band is the resistor’s multiplier value. For example, a resistor whose first color band is yellow represents a digit four (4), while a second color band which is violet is seven (7). If the third color band is orange, then the multiplier would be (000) or one thousand, so the value would be 47,000 or 47k ohms. Resistors usually have a fourth colored band which represents the resistor’s tolerance value. A silver band denotes a 10% tolerance value, while a gold band denotes a 5% tolerance resistor. No color band represents a tolerance value of 20%.

Our crystal radio projects in this chapter all utilize capacitors of some sort. Capacitors usually are available in two major classifications, polarized and non-

polarized. The project in this chapter will utilize only non-polarized capacitors. Non-polarized capacitors can often be physically very small and not have their actual value printed on the body of the capacitor, so a chart was developed to help identify a capacitor using a three-digit code. See Table 4-2.

As this project will not require a circuit board, we will build the crystal radio on a 6′′ ×6′′ ×3_⁄

4′′wood

block base. You can elect to use an additional piece of wood at a right angle to the base as a front panel to secure the tuning capacitor as shown in Figure 4-3.

Chapter Four: AM, FM, and Shortwave Crystal Radio Projects

Table 4-1

Resistor color code chart

Color Band 1st Digit 2nd Digit Multiplier Tolerance

Black 0 0 1 Brown 1 1 10 1% Red 2 2 100 2% Orange 3 3 1,000 (K) 3% Yellow 4 4 10,000 4% Green 5 5 100,000 Blue 6 6 1,000,000 (M) Violet 7 7 10,000,000 Gray 8 8 100,000,000 White 9 9 1,000,000,000 Gold 0.1 5% Silver 0.01 10% No color 20%