Page 87 - Electrician - TT (Volume 2)
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ELECTRICIAN - CITS





                     Fig 1                                              Fig 2












           The logic expression for the inverter is as follows: Let the input variable be ‘A’ and the output variable be Y, then
           the output Y = A  .

           OR and AND gate circuits
           OR  Gate : The output of an OR will be in 1 state if one or more of the inputs is in 1 state. Only when all the inputs
           are in 0-state, the output will go to 0-state. Fig 3 shows the schematic Symbol of an OR Gate :

           The boolean expression for OR gate is Y=A+B.
           The equation is to be read as Y equals A ORed B. Two-input truth table given below is equivalent to the definition
           of the OR operation.
                                            Truth table for OR gate
                                 A                  B             Y=A + B
                                 0                  0             0
                                 1                  0             1

                                 0                  1             1
                                 1                  1             1
           Electrical equivalent circuit
           The Fig 4a shows the electrical equivalent circuit of an OR gate. It is evident that if any one of the switch is closed,
           there will be output.

               Fig 3                                              Fig 4
















           2 in-put OR gate using diode
           The Fig  4b shows one way to build a 2-input OR gate, using diodes. The inputs are labeled as A and B, while
           the output is Y.
           Assume        logic 0 = 0V (low)
                         logic 1 = +5V (high)

           Since this is a 2 input OR gate, there are only  four possible cases,
           Case 1: A is low and B is low. With both the input voltage low, both the diodes are not conducting. Therefore the
           output Y is in low level.





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                                    CITS : Power - Electrician & Wireman - Lesson 60-69
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