This circuit found in Popular Electronics from July 1974 consists of an oscillator that sends its signals to a nearby AM radio. In the presence of metal near the sensor coil, it changes frequency, escaping the tuning of the radio, which can be easily perceived. The transistor can be MPF102 or BF245 and the sensor coil consists of 4 or 5 turns of common wire in a plastic form, 10 to 20 cm in diameter. Capacitor C2 is a variable obtained from an unused AM radio.
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