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EXAMPLE LOGIC CIRCUITS - 1

V. Ryan 2003 - 2017

 

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Below is the logic circuit for a simple house alarm. The alarm protects the front and back doors and six windows. Once the alarm is set if any of the doors or windows are opened the alarm will sound. OR gates have been used. The TIMER allows the house owner to enter the house by either the front or back door and turn of the alarm system before the alarm sounds.
The inputs for each of the gates representing the doors and windows can be connected to a vast range of sensors (eg. movement and magnetic sensors).

On the circuit below the input states of each of the sensors are 0 (false, low, off). This means that they have not detected an intruder. As a result the alarm does not sound.

   

   

 

   

The situation changes when local thug, Ed the Handyman forces window 3 open. Notice how the logic state of the input of GATE B changes from false to true. The output state of gate B changes to true, followed by the INPUT of gate E and its output. The input and output of gate G also change to true. This train of events leads to the alarm sounding. Because OR gates have been used, it only takes one input to change to true at the windows or doors to activate the alarm.

   

 
   

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