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Infrared Proximity Detector/ Alarm

This circuit can be built from readily available low-cost components, some of which may even be hiding in your junkbox! The indicated value of 22 ΕΎ for resistor R1 causes an average current of about 65 mA through infrared emitter D1. Because the IRED is pulsed at a duty factor of about 50% through the action of T1 and IC1, a peak current of 128 mA flows during every half cycle. This may seem a lot but in fact is well within the safe specification of the LD274. The LM567 PLL IC is configured to supply a switching frequency of about 20 kHz.
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