Crystal Radio
A crystal radio is a radio receiver that needs no battery or mains supply: its only components are an antenna and ground, a tuning circuit made of a coil and a capacitor, a crystal or other diode as a detector, and a high-impedance earphone. The audio in the earphone is produced entirely from energy collected from the radio wave itself. It is the simplest working demonstration of how a radio turns an invisible signal into sound, and a favorite hobby project for that reason.
The circuit has four jobs, and each part does exactly one. The antenna and ground intercept a tiny voltage from passing radio waves. The coil and variable capacitor form a resonant circuit, which responds strongly at one frequency given by f = 1 / (2π√(LC)) and weakly elsewhere, which is how a station is selected on the dial. The detector rectifies the signal so that only one half of each radio-frequency cycle survives, and the earphone, together with a small bypass capacitor, averages away the radio-frequency pulses and leaves the slow audio variation that rides on the carrier. In amplitude modulation, the carrier of roughly 530 to 1700 kilohertz is varied in strength to follow the sound, and the detector recovers that outline, the envelope.
The set is powered by the radio wave, and that sets hard limits on what it can do. The power an antenna gathers from a distant transmitter is on the order of microwatts to milliwatts, depending on how close the station is and how long the wire. An earphone of a couple of thousand ohms, such as a piezoelectric or sensitive magnetic type, can respond to such a feeble current, whereas an ordinary loudspeaker would not. For the same reason crystal sets work best near strong stations, with a long antenna and a good ground, and they select stations poorly, since a lone tuned circuit has no gain and cannot tell two neighboring stations apart as sharply as a modern receiver. A signal heard clearly in the earphone is therefore a small but direct conversion of antenna power into sound.
From 1900s experiments to the broadcast boom, the crystal set was the first mass receiver. Early wireless operators used cat-whisker detectors on galena or silicon to copy telegraph signals, replacing the coherer. When regular broadcasting began in the United States, with Pittsburgh station KDKA transmitting the November 1920 election returns, thousands of listeners built sets on cardboard oatmeal-box coils and listened with headphones. In the mid-1920s the vacuum-tube receiver, with its amplification and loudspeaker, took over the household market, but crystal sets continued as cheap hobby kits and as emergency receivers for decades.
The foxhole radio showed how little was needed. During World War II, soldiers and prisoners improvised receivers from a coil of wire, a razor blade and a pencil lead or safety pin as the detector, and an earphone. The surface of a blued steel razor blade formed a rectifying contact with the graphite point, a close cousin of the old crystal detector, and the antenna could be any long wire. These sets are often described as made from salvaged materials, which is true, but the surprise is that the oxide on the blade acted as a diode barrier, so even an improvised contact obeyed the same physics as the mineral crystal.
Common misconceptions are that crystal radios are powered by magic and that they have no modern relevance. They are powered by the incident wave, not by free energy, and a correct description is that the transmitter supplies all the power. The idea lives on in passive RFID tags and near-field communication cards, which harvest power from a reader's field to run a tiny chip, and in energy-harvesting sensors. Another trap is believing a better crystal gives louder sound: the volume is limited by the antenna power available, which is why a long antenna and a sharply tuned, low-loss coil matter more than the detector.
Read crystal radio as the whole of radio reception stripped to its logic: select a frequency, rectify it, listen, and let the wave itself pay the bill for the sound.