Majority Detector Circuit

A majority detector is a combinational logic circuit which gives a logic-1 output if a given number of inputs are simultaneously active.

The following circuit gives a positive output when two or more inputs are enabled.

Worked Example 1

Design a 4-input combinational logic circuit which will produce a logic-1 output if any 3 inputs are simultaneously active.
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Assuming the output variable is X and the inputs are A, B, C and D, the Boolean expression may be written as X = B C D + A C D + A B D + A B C.

The truth table may be completed by setting the output to 1 whenever three or more inputs are active.

The equivalent combinational logic circuit is shown below.

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