Using Logic Gates to Make a Light Activated Burglar Alarm

Simple circuits made of up the different types of logic gates – NOT, OR, AND NOR, NAND, XOR, XNOR – can be used to perform simple and useful tasks. Because the circuits are simple and the logic gates are reliable and mass produced, the circuits are reliable.

One such – a light activated burglar alarm – is shown below.

NAND GATES require both inputs to be 1 before a zero output is produced. Since the buzzer will sound when there is a voltage across it, this requires the upper input to the buzzer to be at a positive voltage. When there is no light, the resistance of the LDR is infinite so the input 1 is zero. The output 3 is then 1, so if the switch is set to A the bottom input to the bottom NAND GATE is 1 and the output from the bottom NAND gate is 0. The buzzer will stay silent.

When light falls on the LDR, the voltage across it will fall so input 1 can rise to 1. This trips the switch to B, so that input 4 is now 0, input 3 is 1 and output 2 is 1. The buzzer sounds and keeps sounding until the switch is reset to out. The various outputs and inputs are shown in the table below.

Light On/Off

Switch A or B

Input 1

Input 2

Input 3

Input 4

Buzzer

On

A

1

0

1

1

Off

On

A

1

1

0

1

On

Off

B

0

1

1

0

On

On

B

1

1

0

0

On

Off

A

0

0

1

1

Off

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