For designing a half adder, we will follow steps described follows –

  1. As we know that half adder is used to add two bits, from the statement of the problem it is clear that we have to add two bits in binary.
  2. As two bits have to be added, number of input variables will be 2. Now, the result when value for AB = 11 consists of two digits out of which higher significant bits is known as carry, we have to use two output variables, one of which is used for sum and another for carry.
  3. We give names A, B to input variables and c and s to output as carry and sum respectively.
  4. We know the required output value for given set of input values, so the truth table for half adder can be designed as follows –
Half Adder Truth Table

5. From above truth table, we can obtain Boolean function for both the output variables.

C = AB , S = A’ B + AB’ , =A+B

Logical Diagram of Half Adder

The main disadvantage of this circuit is that it can only add two inputs and if there is any carry it is neglected.

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