Page 87 - Electrician - TT (Volume 2)

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