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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 ž 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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