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SR flip flop or SR latch is the most essential and widely used flip flop. It is also known as SET-RESET flip flop. 

SR flip flop has two inputs S and R. S is used to set the flip flop and R is used to reset the flip flop and two outputs Q and Q(NOT)  in this one is complement of another. When flip flop is storing 1 we can said that it is set and while reset it is storing 0. Flip flop states are shown below in table. 

Flip flop	Output
State	Q	Q(NOT)
SET	1	0
RESET	0	1

There are two input named S and R as shown in circuit diagram. To understand the working one must know the truth table of NOR gate. In NOR gate we will get output as 1 only if both the inputs are low and if any of the input is high we will receive logic 0. Working of SR flip is very simple. 

Suppose we have applied S=0 and R=0 at the input of the flip flop the flip flop will not change its state and it will remain in its present state. 

Suppose  S=0 and R=1, the output of  upper NOR gate become low. Thus both the inputs of lower NOR gate is low, Output will be high. Therefore  we will receive 1 at reset input it will switch the flip flop to stable state where Q=0 and Q(NOT)=1. In this state flip flop will reset itself and at this time red LED will glow. 

Now  consider S=1 and R=0, now output of lower NOR gate become low making both the inputs of upper NOR gate low ,Output will be high. Hence now we can say that a  1 at S input sets the flip flop and flip flop switches to a stable state where Q=1 and Q(NOT)=0. In this state flip flop will set itself and at this time yellow LED will glow. 

Now again consider both S=1 and R=1 at this time output of both the NOR gates to become low, both the output will be Q=0 and Q(NOT)=0 and this violates the definition of flip flop which says that both the outputs of a flip flop are complement of each other. Hence practically this condition is not possible in this both LED will glow. 

The above working of NOR gate as SR flip flop can be summarized in table as shown below- 

S	R	Q	State
0	0	Previous State	No change
0	1	0	Reset
1	0	1	Set
1	1	?	Forbidden

 

Circuit Diagrams

Project Components

Project Video

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Filed Under: Electronic Projects
Tagged With: flip flop, nor gate, sr flip flop
 

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