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6 dB/octave. The output of the low–pass filter/amplifier feeds a second order positive feedback bandpass filter
consisting of IC1B and IC1C. This is a very narrow–band filter having a 3 dB bandwidth of ±0.1 Hz. and is
tuned to the exact subaudible tone frequency by potentiometer R7. The subaudible tone selected by these
filters is rectified by diodes D4 and D5. This rectified tone is filtered by C7 and applied to voltage comparator
IC2. When this voltage exceeds the reference voltage setting of R16 the comparator output goes high,
turning on transistor Q1 and pulling in relay K1. Relay contacts are available at accessory connector J4 for low
current switching and signaling.
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