555_BASED_ALARM

Published:2009/6/14 22:43:00 Author:May | From:SeekIC

The alarm circuit has a single 555。scillator/timer(U1)performing double duty；serving both in the alarm-trigger circuit and the entry-delay circuit. In this application，the trigger mput of U1 at ptn 2 iS held high via R1，A normally-closed sensor switch，S1，supplies a positive voltage to the junction of R2 and C1，and lights LED1．With both ends of C1 tied high，there is no charge on C1 But when S1 opens，C1（initially acting as a short） momentarny pulls pm 2 of U1 low， triggering the timed delay circle At the beginning of the timing cycle，U1 produces a positive voltage at put 3，which charges FAUito near the positive voltage at put 3，which charges C4 to near the positive supply voltage Transistor Q1 is heavily biased on by R3，keeping its collector at near ground level With Qt on，SCR1’s gate lOclamped to ground，holding it off． When the delay circuit times out，pin 3 of U1 goes low and ties thepositive end of C4 to ground，That turns Q1 offWhen Q1 turns off，the voltage at the gate of SCR goes positive，turning on the SOP and sounding the alarm，The delay time is adjustable from just a few seconds(R6 set to its minimum resistance) to about one minute(R6 adjusted to its maximum resistance)



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