Operation and Types
– Crystal detector acts as a rectifier, conducting current in one direction and resisting current in the other direction
– Connected between the tuned circuit and the earphone in a crystal radio
– Functions as a demodulator, converting the radio signal from AC to pulsing DC
– Extracts the audio signal from the radio frequency carrier wave
– Audio signal passes through the earphone, causing it to vibrate and produce sound waves
– Crystal detector consists of an electrical contact between a semiconducting crystalline mineral and a metal or another crystal
– Construction of the detector depends on the type of crystal used
– Different minerals require varying contact area and pressure for a sensitive rectifying contact
– Crystals like galena require light pressure and are used with a wire cat whisker contact
– Silicon and silicon carbide can tolerate heavier pressure and are used with point contacts
Cat Whisker Detector
– Popular type of crystal detector
– Uses a wire cat whisker contact on a crystal surface, often made of galena or iron pyrite
– Contact point is adjustable to find the sensitive spot on the crystal surface
– Commonly used in portable radios, with the crystal protected inside a glass tube
– Provides a rectifying contact to extract the audio signal from the radio frequency carrier wave
Sensitivity and Reception Range
– Sensitivity of the crystal detector is a major factor in determining the receiver’s sensitivity and reception range
– Crystal radios have no amplifying components, so the sound power comes solely from the radio waves
– Research focused on finding sensitive detectors to improve reception
– Crystal detectors were also used in scientific experiments and test instruments
– The DC output current of the detector was registered by a sensitive galvanometer
Historical Significance
– Crystal detectors were the first type of semiconductor diode and one of the first semiconductor electronic devices
– Crystals were first used as radio wave detectors in 1894 by Jagadish Chandra Bose
– G. W. Pickard played a key role in developing the crystal detector into a practical radio component
– Crystal radios were the first type of radio receiver used by the general public
– Crystal radios became obsolete with the development of vacuum tube receivers in the 1920s, but continue to be used as educational projects today.
Invention and Function
– Patented by Karl Ferdinand Braun and Greenleaf Whittier Pickard in 1906
– Most common type of crystal detector
– Used with galena and other crystals
– Consisted of a pea-size piece of crystalline mineral in a metal holder
– Surface of the crystal touched by a fine metal wire or needle
– Formed a crude unstable point-contact metal-semiconductor junction
– Wire whisker served as the anode
– Crystal served as the cathode
– Current could flow from the wire into the crystal but not in the other direction
– Only certain sites on the crystal surface functioned as rectifying junctions
– Device highly sensitive to the geometry and pressure of contact
– Contact could be disrupted by the slightest vibration
– Usable point of contact had to be found by trial and error before each use
– Wire suspended from a moveable arm and dragged across the crystal face
– Required skill and patience to adjust the cat whisker for proper functioning
– User would tune the radio to a strong local station if possible
– Adjust the cat whisker until the station or radio noise was heard in the earphones
– Static hissing noise indicated radio reception
– Skill and patience required for adjustment
– Alternative method of adjustment was to use a battery-operated device
– Crystal detector played a crucial role in early radio technology
– Provided a means of detecting and receiving radio signals
– Paved the way for the development of more advanced radio technology
– Demonstrated the principle of point-contact metal-semiconductor junctions
– Contributed to the progress of telecommunications and broadcasting Source: https://en.wikipedia.org/wiki/Cat%27s-whisker_detector
A crystal detector is an obsolete electronic component used in some early 20th century radio receivers that consists of a piece of crystalline mineral which rectifies the alternating current radio signal. It was employed as a detector (demodulator) to extract the audio modulation signal from the modulated carrier, to produce the sound in the earphones. It was the first type of semiconductor diode, and one of the first semiconductor electronic devices. The most common type was the so-called cat's whisker detector, which consisted of a piece of crystalline mineral, usually galena (lead sulfide), with a fine wire touching its surface.
The "asymmetric conduction" of electric current across electrical contacts between a crystal and a metal was discovered in 1874 by Karl Ferdinand Braun. Crystals were first used as radio wave detectors in 1894 by Jagadish Chandra Bose in his microwave experiments. Bose first patented a crystal detector in 1901. The crystal detector was developed into a practical radio component mainly by G. W. Pickard, who began research on detector materials in 1902 and found hundreds of substances that could be used in forming rectifying junctions. The physical principles by which they worked were not understood at the time they were used, but subsequent research into these primitive point contact semiconductor junctions in the 1930s and 1940s led to the development of modern semiconductor electronics.
The unamplified radio receivers that used crystal detectors are called crystal radios. The crystal radio was the first type of radio receiver that was used by the general public, and became the most widely used type of radio until the 1920s. It became obsolete with the development of vacuum tube receivers around 1920, but continued to be used until World War II and remains a common educational project today thanks to its simple design.
