PHOTOELECTRIC_SENSOR

Published:2009/6/17 2:37:00 Author:May | From:SeekIC

The circuit can be used as a sensor that can trigger an alarm without direct contact being madeby the intruder In this circuit，a visible or invisible tight Source radiates on the sensor，keeping thedetection loop In what could essentially be called a normally closed condition As long as the light source striking R5 remalns uninterrupted，the switch remalns closed But ifan intruder passes between the light Source and the sensor，the circuit goes from closed to open，andtriggers the alarm. A light-dependent resistor(LDR)， whose resistance varles inversely in with the amount of lighthitting its sensitive surface，is used A bright light aimed at R5 causes its internal resistance to dropas low as a few hundred ohms；In total darkness，the unit’s resistance can rlse to several megohmsThe light-dependent resistor(R5) is conrtected between the ＋V supply and the base of Q1．As longas R5 detects light，it supplies ample base current to cause Q1's collector to saturate to near groundlevel That also pulls the base of Q2(a 2N3906 genera-purpose pnp transistor) to near ground level，turning it on and clamping its collector to the＋V rail.



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