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2014-11-28 19:15  
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Q1 (2N3904) is an oscillator tuned to a quiet spot in the FM broadcast band. D1 through D4 (1N914) ensure proper polarity. Dc from the line powers the bug. Audio on the line causes incidental FM, which can be heard on an FM receiver tuned to the frequency of the oscillator. Warning: Use of this device for certain purposes could violate federal and/or state laws and subject the violator to prosecution.