seven-segment display and truth table
1. What a Seven-Segment Display Is
Seven-segment display = 7 LEDs (segments a–g, plus dp for decimal point) arranged as "8" shape to display decimal digits 0–9 (and some letters) by selectively lighting segments.
Layout:
aAAA
f b
f b
gGGG
e c
e c
dDDD (+ dp dot)
Segments: a (top), b (top right), c (bottom right), d (bottom), e (bottom left), f (top left), g (middle)
Types:
- Common Anode (CA): all LED anodes common to Vcc, cathode of each segment driven low (0) to light (active low).
- Common Cathode (CC): all cathodes common to GND, anode driven high (1) to light (active high). Resistors limit current.
2. Truth Table (active high CC, 1=on, 0=off — for CA invert)
| Digit | a | b | c | d | e | f | g | Pattern (active high CC) |
|---|---|---|---|---|---|---|---|---|
| 0 | 1 | 1 | 1 | 1 | 1 | 1 | 0 | all but g |
| 1 | 0 | 1 | 1 | 0 | 0 | 0 | 0 | b,c |
| 2 | 1 | 1 | 0 | 1 | 1 | 0 | 1 | a,b,g,e,d |
| 3 | 1 | 1 | 1 | 1 | 0 | 0 | 1 | a,b,c,d,g |
| 4 | 0 | 1 | 1 | 0 | 0 | 1 | 1 | f,g,b,c |
| 5 | 1 | 0 | 1 | 1 | 0 | 1 | 1 | a,f,g,c,d |
| 6 | 1 | 0 | 1 | 1 | 1 | 1 | 1 | a,f,g,c,d,e |
| 7 | 1 | 1 | 1 | 0 | 0 | 0 | 0 | a,b,c |
| 8 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | all |
| 9 | 1 | 1 | 1 | 1 | 0 | 1 | 1 | a,b,c,d,f,g |
For CA, invert bits (0=on). DP separate.
3. Driver
BCD to 7-seg decoder (e.g., IC 7447 for CA, 7448 for CC) takes 4-bit BCD (0–9) and outputs 7 segment lines per truth table; includes ripple blanking etc.
Alternatively, microcontroller drives via current-limiting resistors (e.g., 330Ω per segment at 5V ⇒ ~10mA/segment).
4. Worked Example
Need to display "5" on CC: a=1,b=0,c=1,d=1,e=0,f=1,g=1 ⇒ segments a,f,g,c,d light ⇒ pattern looks like 5 ✓
Need to display "1" on CA: invert CC pattern 0110000 ⇒ 1001111 active low.
5. Exam Points
- Draw 7-seg layout with segments a–g — 5 marks, very frequent (almost certain to ask diagram).
- Give truth table for digits 0–9 (or at least 0,1,2,5,8) — 7 marks, very frequent (the most common 7-seg question).
- Distinguish common anode (active low) vs common cathode (active high) — 5 marks, very frequent.
- Mention decoder IC 7447/7448 or resistor — 3-5 marks, frequent.