Simple Magnetic Contact Alarm Project

The circuit shown below is a basic latching circuit consisting of a quad 2-input NAND Gate IC. It is configured such that the flip flop circuit U1A and U1B has both its input held at logic `1`. Pin 1 is held at logic `1` by the magnetic contact and pin 6 is held at logic `1` by resistor R1. S1 switch is a normally open push-button switch which is used as a reset button. When S1 is pressed, the input pin 6 will be at logic `0`, hence the output of U1B will go to logic `1`. This logic is inverted by U1C and U1D to logic `0`. The transistor Q1 will be OFF and buzzer will not sound.
Simple Magnetic Contact Alarm Project - schematic

Simple Magnetic Contact Alarm Project - img1

When the magnetic contact sensor is opened, the input to pin 1 will be at logic "0", giving an output of logic "1" at pin 3 of U1A. Pin 5 of U1B will go "1" and pin 6 will be "1" as well giving an output at pin 4 a logic "0". This will be inverted and a logic "1" will be generated at the base of transistor Q1. The transistor Q1 will turn ON and cause the buzzer to sound continuously until the reset switch S1 is pressed.

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