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Logic Gates
A digital circuit which one or more input signals but only one output signal is called Logical gates.
Since a logic gate is a switching circuit (i.e. a digital circuit), its outputs can have only one of the two possible states viz. either a high voltage (1) or a low voltage(0) - it is either ON or OFF. Whether the output voltage of logic gate is high (1) or low (0) will depend upon the condition at its input. Fig. shows the basic idea of a logic gate using switches.
(I) When S1 and S2 are open, the bulb is OFF.
(II) When S1 is open and S2 closed, the bulb is OFF.
(III) When S2 is open and S1 closed, the bulb is OFF.
(IV) When both S1 and S2 are closed, the bulb is ON.
Note: The output (OFF or ON) depends upon the conditions at the input.
S1
S2
Bulb

open
open
OFF
open
closed
OFF
closed
open
OFF
closed
closed
ON
S1 S2 Bulb

0
0
0
0
1
0
1
0
0
1
1
1
The four possible combination of switches S1 and S2 are shown in the table below. It is clear that when either of the switches (S1 or S2)or both are open, the bulb is OFF. In binary language , when either of the inputs or both the inputs are low (0), the output is low. When both switches are closed, the bulb is ON, It is a usual practice to show the conditions at the input and output of a logic gate in the binary form as shown in the table above. Such a table is calls truth table.
The term "logic" is usually used to refer to a decision-marking process. A logic gate makes logic decision regarding the existence of output depending upon the nature of the input. Hence such circuits are called logic circuit.
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