The crystal radio receiver (also known as a crystal set) was first built circa 1900 by Greenleaf Whittier Pickard, who used crystalline minerals to detect radio signals.
People first built and used simple and inexpensive crystal radio sets without batteries or electrical power. Even though vacuum tube radios were common following World War I, crystal radios remained popular, especially among beginning amateur radio enthusiasts, Boy Scouts and school children, who built crystal radios to learn basic electronics and communication.
Early wireless telegraphy used spark gap plasma arc transmitters powered by alternating current generators at frequencies of 400 - 1000 hertz. When these intermittent telegraph keyed dot/dash signals were received, the tone or pitch of the generator was clearly heard varying the strength of the radio wave. Varying the strength in this way is called amplitude modulation. As wireless telegraphy became more and more widely used, significant development of radio transmitters using amplifiers produced more power with a purer signal that did not occupy unnecessary bandwidth, causing interference.
The first radio telephone transmitters also used amplitude modulation to carry speech and music on a radio wave. The crystal set received these signals almost as well as wireless telegraph signals. AM radio stations today still use amplitude modulation at power levels up to 50,000 watts on the U.S. broadcast band, and 1,000,000 watts on shortwave bands. Crystal sets with long wire antennas can still be used today, but they work best within 20 miles or so of a high power transmitter.
1920s and 1930s
When radio broadcasting became popular in the 1920s, many amateur experimenters bought or constructed crystal sets, often with the tuner inductor coil wound on a tubular box or a drinking glass. This led to a series of adventure novels, the Radio Boys books, similar in kind to the Hardy Boys books.
When electronic amplifiers and oscillators were invented, they were almost immediately put into service in radios, first in the form of vacuum tubes and much later in the form of transistors and integrated circuits replacing crystal sets with vastly more sensitive and selective receivers such as the regenerative and the superheterodyne.
During the Great Depression parents would build a crystal radio detector from inexpensive galena crystal and a safety pin. After this detector was connected to iron bedsprings (which doubled as an antenna) and grounded to household cold-water pipes, a youngster needed only inexpensive headphones.
GIs during World War II constructed similar radios from rusty razor blades and pencil lead, the iron oxide crystals of the rust replacing the galena crystal and the graphite of the pencil lead substituting for the safety-pin wire. These crude, but functional, radios were nicknamed foxhole radios.
Crystal sets were the most common form of radio listening device for non-professionals in the early 1920s. A hundred years after their first use, hobbyists still build and tinker with – and listen to – crystal radios constructed from just a few parts.
The most common modern design uses a coil for a tuner, and a semiconductor diode instead of a crystal. The output is usually from an earplug. A lengthy antenna wire (15m (40ft) or more) is still helpful to get good sound. A modern design for a "trash radio" is constructed from a tin can and some wire.
A crystal set is the simplest radio receiver, consisting of a long-wire antenna, a tuner to select the desired radio signal by frequency, and a detector consisting of a diode demodulator usually consisting of a sharp wire pressing against a sensitive point on a galena mineral crystal in a holder.
A semiconductive mineral crystal, usually lead sulfide (galena) or cadmium sulfide was fixed inside a brass cup and the radio operator found the loudest signal by touching the wire, called a "cat's whisker", to various points on the surface of the crystal. The radio was entirely powered from the radio waves it received. Often the most expensive part was the large antenna required.
The detector extracts the amplitude modulation from the radio signal and provides an audio output in proportion to the strength of the signal coming from the antenna. The entire set is passive, requiring no external power. Because no electrical amplification is used, sensitive earphones are required. While the original models did not have a way to increase the volume, the later models used transistors and vacuum tubes for that purpose.
Components that are used in a modern crystal set include:
A variable condenser across the coil will help separate stations.
It has been suggested that crystal radios may still be in use by spies. Crystal sets have no oscillator so a counter-espionage organisation cannot determine the frequency being listened to.