This circuit was found in a 1992 documentation and has excellent sensitivity, using components that are still common in our market. The circuit generates a signal at or near 1 MHz depending on the crystal and the indicator is a microammeter. Coil L1 is wound inside a 12 mm diameter copper pipe forming a 28 cm circle. The coil may also be without the barrel, in a plastic form, for example. The coil consists of 17 turns of 26 wire. The circuit is powered by two 9 V batteries.
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