A code converter maps one representation to another using a truth table. A seven-segment decoder converts a digit code into controls for segments a through g. Each segment is its own Boolean output, so its on/off column can be simplified independently.
Worked example
Using active-high segment controls, digit 1 lights b and c while a, d, e, f and g remain off. Digit 0 lights a, b, c, d, e and f while g stays off. A BCD decoder can define patterns for decimal digits 0 through 9.
What to watch for
Common-anode displays often require active-low segment drive. Physical displays need appropriate current limiting and drivers; logic-level examples are not wiring instructions.
Try it yourself
Which segments display digit 7 under the usual seven-segment naming?
Answer: a, b and c. The top bar and the two right-hand bars are lit.
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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Digital Electronics