Posted on Jan 7, 2013

A resonance-tuned narrow-band amplifier reduces this optical object detector`s sensitivity to stray light. C1 and L1 in IC2A`s feedback loop cause tbe op amp to pass only those frequencies at or near the LED`s 5-kHz modulation rate.

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IC2B"s output increases when the received signal is sufficient to drop the negative voltage across C2 below the reference set by R2.

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