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Tuesday 29 July 2014

Practical Demo of S R Latch using 7400 NAND Gate and Push Button Switches

by realfinetime  |  in S R Latch at  10:22

          S R latch is the basic Flip Flop and has an interesting property "memory". It can be set to a state which is retained until explicitly reset. Practically, S R latch can be demonstrated using 7400 NAND gate IC. Circuit is done as shown in the following diagram. Each 7400 has four NAND gates arranged as shown in the pinout diagram. 

          Two NAND gates are used in this circuit. Input is given to S and R through the pull up resistors. Output of the NAND gates are connected to LEDs through current limiting resistors. Output of both NAND gates are loop backed to one input of other NAND gate. 5V for the working of 7400 is given from Arduino or from a 5V regulator. Two push button switches, S1 and S2 controls the voltage level of S and R. 7400 is shown by a box in the circuit diagram. Dotted lines are to demonstrate the continuation of 7400 IC. A break is given to the drawing of 7400 IC by dotted lines. This is for drawing the loop back back connection easily. If the loop back connection is not given in such a way, it will become very difficult to understand the circuit from drawing.   

Pinout diagram of 7400

          Pinout diagram of 7400 is given below. Each 7400 has four NAND gates arranged as shown in the following diagram.
Why pull up resistors ?.

          5V is given to S and R through pull up resistors. Pull up resistors helps to pull the NAND gate input to a HIGH value always. When any of the push button switch turns on, corresponding NAND gate input get grounded through the push button switch. That NAND gate input will get LOW voltage.

          If the same circuit is done without connecting the pull up resistors of proper resistance, when any of the swith turns on, power supply get shorted through the switch. High current will flow through the switch due to low resistance of the path. This may damage the power supply and gate operation will not take place.

Why push button switches ?.

          Push button switches are connected in between the NAND gate inputs ( S and R ) and ground as shown in the circuit. Push button switch is most suitable for this purpose, because by default it will be in off state. A switch in off state means, corresponding NAND gate input (S and R) gets a HIGH voltage. When a switch is pressed, that switch turns on and corresponding NAND gate input will get grounded and get a LOW voltage. When press is released switch will automatically turn off. 

Truth Table of S R Latch

          From the truth table, it is clear that if S=R=1, output ( Q and Qbar ) will retains the previous state. ie if the output ( Q and Qbar ) was 0,1 previously, output will remain at 0,1 when S and R becomes 1. Similarly, if the output ( Q and Qbar ) was 1,0 previously, output will remain at 1,0 when S and R becomes 1. If S=R=0, that state is not allowed.
          Truth table can be pictorially represented as shown below. Working of the circuit is very clear from the animated image given below.

          When S1 turns ON and S2 turns OFF, S will be grounded and R will get HIGH voltage. That is, S will be 0 and R will be 1. If S=0 and R=1, Q will be 1 and Qbar will be 0. That is, first LED will turn ON and second LED will turn OFF. This is the SET condition. This state can be used as memory. Now, if both switches turn on, Q and Qbar will remain in the previous condition ( Q=1, Qbar=0 ).

          When S1 turns OFF and S2 turns ON, S will get HIGH voltage and R will get grounded. That is, S will be 1 and R will be 0. If S=1 and R=0, Q will be 0 and Qbar will be 1. That is first LED will turn OFF and second LED will turn ON. This is the RESET condition. Now, if both switches turn on, Q and Qbar will remain in the previous condition ( Q=0, Qbar=1 ). 


  1. can you demonstrate this on an ic series 7400?

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All the circuits, published in this blog is only after testing and getting proper results in my private lab. When you try these circuits, you should check the supply voltage, polarity of components, presence of childrens nearby and shorts in the circuits. This website will not be responsible for any harm happened to you or your components caused by your carelessness.

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