How does a crystal radio work?

Answer 1

This contributor finds it easiest to understand the following explanation.

Fasten your seat-belt; it may be a bumpy ride:

The basic principle to keep in mind is this:

When you pass more than one frequency through any non-linear device, the frequencies

get "mixed". What comes out the other end is the original frequencies PLUS the sum-

frequency and the difference-frequency of every possible combination of the original ones.

Remember this.

A non-linear device can be anything that conducts current better in one direction

than it does in the other direction. Like a semiconductor diode. Or a sharp-pointed

wire touching some varieties of naturally-occuring minerals, which are called "crystals"

when they're used in simple hobby radio kits.

Start at the radio transmitter:

-- The transmitter generates a "carrier" wave, at the frequency where it'll show up

on your radio dial.

-- The carrier wave and the voice or music waves are passed through some non-linear

device, where the frequencies get mixed.

-- What comes out is the (carrier frequency alone), (music frequency alone), sum of the

(carrier + music) frequencies, difference of the (carrier - music) frequencies.

-- The transmitter sends out the narrow cluster of (carrier alone), (carrier + music), and

(carrier - music) frequencies. The (music frequency alone) is filtered out and discarded.

Now go to the crystal radio:

-- The wire antenna catches some of the radio waves as they fly by, and conducts them

down to the "radio" set.

-- Flowing down the wire are separate signals at different frequencies. There

are the (carrier frequency alone), the sum (carrier + music) frequency, and the

difference (carrier - music) frequency.

-- All of them are sent through the "crystal", where the frequencies mix. The original

frequencies come out, along with the sum and difference of each pair.

-- Out from the crystal come these frequencies:

The originals: (carrier alone), (carrier + music), (carrier - music)

The sums: (carrier + carrier + music), (carrier + carrier - music), (2 x carrier)

The differences:

(carrier - carrier - music) = -music

(carrier - carrier + music) = music <==== ! ! !

(carrier + music - carrier + music) = 2 x music

Almost hidden in this pile of many electrical signals is one that's a copy of the original

music signal ... exactly what came from the microphone back at the studio, and got

mixed with the carrier for transmission.

All of the other signals that come out of the crystal are filtered out and discarded,

and just the "music" signal alone goes to the headphones ... "for your listening

and dancing pleasure", as they used to say in the days when the crystal radio was

about the best that everyone had.

Answer 2

Any radio set looks better in a neat container. A cabinet for your crystal set can easily be made from a square foot of either hardboard or three-ply wood. Cut the parts required -- that is, two sides, back, front, top and base-using the measurements shown on the next page. Then drill the five holes in their right positions.

Glue the sides together, and slip two or three strong elastic bands round this frame to hold it firmly in place. Glue the top in position and weight it down with something fairly heavy until the glue is dry.

When the glue is thoroughly dry, add the small corner supports which will later take the screws for securing the base. Cut out a semi-circular piece of paper, about three and a half inches in diameter, and paste it on the top of your cabinet just above the centre hole. This is the tuning dial on which you will mark the positions of the stations you receive.

The final appearance of the cabinet will be improved by a coat of varnish or wax polish.

You are now ready to make the crystal set itself.

STEP 1 -- MAKING AND FITTING THE INDUCTOR

All you need for the inductor is a core and some wire (see list of parts). The core must be fairly firm so that the wire can be wrapped tightly round it, and the cardboard centre of a toilet roll is ideal for this purpose. Trim it to the right length for the cabinet and cut a small notch at each end to allow your screwdriver to get inside when you fasten the inductor in position.

Make two small holes near one end of the core, and secure the end of the wire by passing it into one 1 hole and out of the other. Draw out enough wire to make a connection later.

Wind the wire neatly on to the core, taking about sixty turns and making sure they do not overlap. (By experimenting with the number of turns on the coil you may find you can vary the stations you receive.) Make two holes at the other end of the core, and secure the wire by threading it through them as before. Wrap a layer of transparent tape round the wire coil to hold it in position.Now fasten your inductor in the cabinet with two small screws as shown in the diagram.

STEP 2 -- FITTING THE CAPACITOR

Pass the spindle of the variable capacitor through its hole in the top of the cabinet and bolt it from the outside. Fit the tuning knob on top of the spindle. STEP 3 -- FITTING FOUR SOCKETS

Pass each socket through the hole provided for it, and bolt firmly from inside.

STEP 4 -- SOLDERING THE CONNECTIONS

Join up the various connections, soldering each joint as shown in the diagram. You can now screw the base on to the cabinet.

STEP 5 -- Solder plugs on to earphones.

STEP 6 -- Solder plugs on to aerial and earth leads (see Notes below). STEP 7 -- Insert aerial, earth and earphone plugs in their sockets, and adjust the tuning knob until a signal is heard. Mark the positions of the stations on your paper dial.

Answer 3

The single most important function required in order to strip the voice or music

off of an AM radio signal is:

- At some point in the receiver, push the current through some device that

conducts current easier in one direction and harder in the other direction.

A device that does this may be called a rectifier, a diode, a mixer, a non-linear device,

or other names. It can be done with a transistor, a vacuum tube, or a stack of layers

of different semiconductors or chemicals. In the simplest, cheapest device that can

possibly receive AM radio, it happens at the place where the sharp point of a wire

rests on the surface of a crystal of germanium or rochelle salt.

Answer 4

Detecting the radio signal is not the issue. Radio signals are inducing currents

through every wire and piece of metal in your house right now ... your mouse cord,

your stapler, your can opener etc. The trick is to lift the information off of those

currents.

In a crystal set, the point where the 'whisker' touches the crystal has the characteristic

that electric current can flow through it easier in one direction than in the other direction.

That right there is all you need to 'detect' AM radio ... separate the sound from the radio signal.

Answer 5

The crystal radio works by vibrating from the transmitted radio signal with the audio heard varies the frequency.