Common crystal radio circuit, used in inexpensive crystal radios sold today.  The tuning coil L and capacitor C1 together make up the tuned circuit; it selects the radio signal to be received, out of all the signals picked up by the antenna.  C1 is adjustable, and is used to tune in different stations.   The crystal detector D rectifies the radio frequency current from the tuned circuit to pulsing direct current to extract the audio modulation from the radio frequency carrier wave.  The earphone E converts the audio current to sound waves.
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Chetvorno