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Digital Circuit System Lab Experiments

To design R-S Flip Flop


To design R-S Flip Flop

Apparatus Required:

Digital trainer kit, IC 7400, IC 7402, IC 7408, Connecting wires


The logic circuits that incorporate memory cells are called sequential logic circuits; their output depends not only upon the present value of the input but also upon the previous values. Sequential logic circuits often require a timing generator (a clock) for their operation. Flip flops are actually an application of logic gates. With the help of Boolean logic you can create memory with them. Flip flops can also be considered as the most basic idea of a Random Access Memory [RAM]. When a certain input value is given to them, they will be remembered and executed, if the logic gates are designed correctly. Usually there are two outputs, Q and its complementaryvalue.

R-S Flip-Flop

The fundamental latch is the simple SR latch, where S and R stand for set and reset respectively. It can be constructed from a pair of cross-coupled NOR logic gates. The stored bit is present on the output marked Q. Normally, in storage mode, the S and R inputs are both low, & feedback maintains the outputs in a constant state, with Q and the complement of Q. If S (Set) is given with high while R is held low, then the Q output is forced high; similarly, if R (Reset) is given with high while S is held low, then the Q output is forced low. For S & R both high, output is invalid. It is an invalid state because the values of both Q and Q’ are 0. They are supposed to be compliments of each other. Normally, this state must be avoided.


A clock pulse [CLK] is given to the inputs of the AND Gate. When the value of the clock pulse is ‘0’, the outputs of both the AND Gates remain ‘0’. As soon as a pulse is given the value of CLK turns ‘1’. This makes the values at S and R to pass through the NOR Gate flip flop.


  1. Place the breadboard gently on the observationtable.
  2. Fix the IC which is under observation between the half shadow lineof breadboard, so there is no shortage ofvoltage.
  3. Connect the wire to the main voltage source (Vcc) whose other end is connected to last pin of the IC (14 place from thenotch).
  4. Connect the ground of IC (7th place from the notch) to the ground terminal provided on the digital labkit.
  5. Give the input at the gate of the ICs by using connectingwires
  6. Connect output pins to the led on digital lab kit
  7. Switch on the powersupply.
  8. If led glows then output is true, if it doesn’t glow output is false, which is numerically denoted as 1 and 0respectively.
  9. The values of the outputs aretabulated


  1. All ICs should be checked before starting theexperiment.
  2. All the connection should betight.
  3. Always connect ground first and then connectVcc.
  4. Suitable type wire should be used for different types ofcircuit.
  5. The kit should be off before changing theconnections.
  6. After the completion of experiment, switch off the supply of theapparatus

Result:S-R flip-flop is designed and its truth table is verified.


Q1:What is alatch?

Ans:Storage elements that operate with signal levels are referred to as latches.

Q2:What is a flipflop?

Ans:Storage elements controlled by a clock transitions are called flip flop.

Q3:Differentiate between a latch and a flipflop.

Ans:A latch checks all its inputs continuously and changes its outputs accordingly at anytime. Flip flop samples its inputs and changes its outputs only at a time as determined by a clocking signal.

Q4:Differentiate between combinational and sequentialcircuits.

Sequential Circuits: *Dependent only on the inputs at that instant * It depends on present and past history of the inputs * requires the Memory Elements