7447 BCD-to-seven-segment decoder: which segments light for the digit “1”? For a 7447 code converter driving a common-anode display, which segments are active to display the numeral 1?
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ASegment a is active.
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BSegment b is active.
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CSegments a and b are active.
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DSegments b and c are active.
Answer
Correct Answer: Segments b and c are active.
Explanation
Introduction / Context:Understanding standard seven-segment coding is essential for designing front-panel displays, debugging instrument readouts, and writing HDL driving logic. The TTL 7447 is a classic BCD-to-seven-segment decoder for common-anode LEDs.
Given Data / Assumptions:
- IC: 7447 BCD-to-seven-segment decoder/driver.
- Target digit: decimal 1.
- Common-anode display (7447 outputs are active-LOW).
Concept / Approach:A seven-segment display uses segments a through g. For the digit “1,” only segments b and c should illuminate. With the 7447 and a common-anode LED, the decoder pulls b and c LOW (active-LOW), while other outputs remain HIGH (off).
Step-by-Step Solution:
Recall the standard encoding: “1” → segments b and c on; a, d, e, f, g off.Map to the 7447: outputs for b and c go LOW; others stay HIGH.Therefore, the correct description is “segments b and c are active.”Verification / Alternative check:Check a segment truth table from a datasheet or memorize common patterns: 0 (a,b,c,d,e,f), 1 (b,c), 2 (a,b,d,e,g), 3 (a,b,c,d,g), etc.
Why Other Options Are Wrong:
- a only / b only / a and b: None match the standard “1” pattern, which always requires b and c together.
Common Pitfalls:
- Mistaking common-anode for common-cathode: the 7447 uses active-LOW outputs, but the lit segments for a given digit are the same logical set (polarity just inverts the drive).
Final Answer:Segments b and c are active.