A combinational design begins with a clear requirement: the output depends only on the present inputs. Name the inputs, define what zero and one mean, list all combinations, and write the desired output for each. Then simplify the resulting function and draw the circuit.
Worked example
A fan should run when the system is powered and either temperature is high or manual override is active. Define P, T and M for those conditions. The expression is F = P AND (T OR M). With P = 0 the fan stays off for all T and M values; with P = 1 either request turns it on.
What to watch for
Resolve ambiguous requirements before simplifying. Real safety controls need suitable fail-safe and physical-interface design beyond this ideal logic exercise.
Try it yourself
What is F when P = 1, T = 0 and M = 1?
Answer: T OR M is one, and P is one, so F = 1. The manual request runs the fan in this simplified specification.
How to study this topic
Name each input and define its active level before drawing gates. Follow one data path at a time, then check the other combinations. A complete specification is the best starting point for a reliable circuit.
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