Posted on Oct 27, 2012

Al and A2 are two sections of a quad comparator. The first, Al, functions as an amplifier and detector. Resistors R~and R6 set the gain at 100; the output of Al is an open collector to negative-peak-rectify the output with a decay time constant determined by R9 and C3. This de output is then compared with the reference level selected by RB.

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A2 triggers switch Ql, and an LED inserted in the base drive of Ql gives visual indication of switch closure. The standby battery drain is 2 mA. Use potentiometer R8 to select the desired sensitivity.

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