I found this circuit on a December 1976 Radio Elettronica. The Italian magazine no longer exists, but with the replacement of some components the circuit can be easily assembled. For example, we replaced BC107 with BC547, BC548 or BC549 and 2N918 for a 2N2222. The sensor consists of a pickup coil or suction cup placed near the telephone handset. The coil L1 consists of 4 turns of 26 cm or 1 cm shaped wire without core. Without antenna the range is around 10 m, but a small antenna can be connected to the T2 collector to increase the range.
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