Battery Powered One-Time-Only Alarm - Support Material

  
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The Cmos 4001 has four - independent - two-input NOR gates. The four gates are divided into two pairs. Gates 1 & 2 are wired together to form a type of latching circuit. When the alarm is triggered - they will latch - and sound the buzzer. Gates 3 & 4 are wired to form a monostable. When the entry delay expires - the monostable will sound the sire
Battery Powered One-Time-Only Alarm - Support Material - schematic

n for a fixed length of time. Then the siren will switch off - and stay off. = Cmos 4001 gates only have a high output when both inputs are low. See the truth table. When you switch the alarm on - C2 is in a discharged state. Because it`s discharged - the capacitor acts like a wire link connecting pins 2 & 13 to the positive line. This means that gates 1 & 4 each have one high input. These high inputs - force the output pins of gates 1 & 4 to remain low. And while the output pins of gates 1 & 4 are forced to remain low - both the latching circuit and the monostable are disabled. In other words - the alarm cannot be activated. = C2 will continue to hold pins 2 & 13 high - until it charges through R3. This is the exit delay. Depending on the setting of R3 - it will last for up to a minute or more. During this period - you may open the loop and leave the building without activating the alarm. = As C2 charges - the voltage on the negative terminal of the capacitor falls. When it falls to just below half the supply voltage - it takes pins 2 & 13 low. When pins 2 & 13 go low - the outputs of gates 1 & 4 are no longer forced to remain low. This means that both the latch and the monostable circuits are enabled. In other words - the exit delay is over - and it`s now possible to activate the alarm. = In standby mode - the normally closed loop connects pin 6 to ground. And the current through R1 is also diverted to ground through the...



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