What are Logic Gates
Logic Gates are the building blocks of Digital Systems. They are basically used to perform logical operations by acting as a switch. Boolean Functions are implemented using Logic Gates. A Logic Gate accepts the input and operates on a required condition. If a certain condition is true, it turns “ON” and goes “OFF” when the condition is false.
Function of a Logic Gate is expressed using Truth Table. It displays all possible combination of the inputs and a column against each row shows the corresponding output value. Any number of inputs provided results in a single output with a time delay.
Fig.2 below shows an example of Logic Gate circuit which has three inputs A, B, C and single output Y.
Types of Logic Gates
-
NOT Gate (Inverter)
-
AND Gate (Multiplication)
-
OR Gate (Addition)
-
NAND Gate
-
NOR Gate
-
XOR Gate
-
XNOR Gate
NOT Gate (Inverter)
The output of NOT Gate is ‘High’ if the input is ‘Low’ and Vice versa.
AND Gate (Multiplication)
The output of AND Gate is ‘High’ if both the inputs are High and the output is ‘Low’ if both or either of the inputs is ‘Low’.
OR Gate (Addition)
The output of OR Gate is ‘High’ if both the inputs are ‘High’ or one of the inputs is ‘High’. The output is ‘Low’ if both the inputs are ‘Low’.
NAND Gate
The output of NAND Gate is ‘High’ if both the inputs are ‘Low’ and if either of the input is ‘Low’. The output is ‘Low’ if both the inputs are ‘High’
NOR Gate
The output of NOR Gate is ‘High’ if both the inputs are ‘Low’. The output is ‘Low’ if both the inputs are ‘High’ or either of the input is ‘High’
XOR Gate
The output of XOR Gate is ‘High’ if either of the input is ‘High’. The output is ‘Low’ if both the inputs are ‘High’ or if both the inputs are ‘Low’.
XNOR Gate
The output of XNOR Gate is ‘High’ if both the inputs are ‘High’ or if both the inputs are ‘Low’. The output is ‘Low’ if either of the input is ‘Low’
Working Principle of Logic Gates
To understand the working principle of Logic Gates, let us consider the example of Street Lighting System. Fig below shows the Circuit Diagram of Street Lighting System which mainly consists of Logic Gate (s) (NOT and OR Gate), Switch and Light Sensor. The operational condition for the street light to function is as follows:
-
Switch: On = 1, Off = 0
-
Light Sensor: Dark = 0, Bright = 1
-
Street Light: On = 1, Off = 0
Based on this, we can prepare a Truth Table as shown in Fig. 10
Applications of Logic Gates
-
NAND Gates are used in Burglar alarms and buzzers.
-
They are basically used in circuits involving computation and processing.
-
They are also used in push button switches. E.g. Door Bell.
-
They are used in the functioning of street lights.
-
AND Gates are used to enable/inhibit the data transfer function.
-
They are also used in TTL (Transistor Transistor Logic) and CMOS circuitry.
Advantages of Logic Gates
-
Logical Operations are performed using Boolean Algebra which makes the circuit design more economical and simple.
-
Logic ‘1’ and Logic ‘0’ can be easily distinguished.
Disadvantages of Logic Gates
-
Operating Voltage is limited.
-
Time delay occurs between input and output.
