Seven Segment Decoder
Enter a Hex digit (0-F) to see it light up.
Active Segments
| a | b | c | d | e | f | g | dp |
|---|
How it Works
A Seven Segment Display is composed of 7 LEDs arranged in a figure-8 pattern. By lighting specific combinations, we can display any digit from 0-9 and letters A-F.
In digital logic, we use a BCD-to-7-Segment Decoder (like the 7447 or 7448 chip) to convert a 4-bit binary number into the 7 output signals (a through g).
Common Anode vs Common Cathode
Common Cathode (CC): All negative pins (cathodes) are tied to Ground. The control signals must be positive (High) to light a segment.
Common Anode (CA): All positive pins (anodes) are tied to VCC. The control signals must be grounded (Low) to light a segment.
Truth Table (0-9)
- 0: a,b,c,d,e,f (0x3F)
- 1: b,c (0x06)
- 2: a,b,d,e,g (0x5B)
- 3: a,b,c,d,g (0x4F)
- 4: b,c,f,g (0x66)
- 5: a,c,d,f,g (0x6D)
- 6: a,c,d,e,f,g (0x7D)
- 7: a,b,c (0x07)
- 8: all (0x7F)
- 9: a,b,c,d,f,g (0x6F)
FAQ
Do I need resistors?
YES! Each segment is an LED. You need a current limiting resistor (typically 220Ω or 330Ω) for each segment line. If you use just one resistor on the common pin, the brightness will change depending on how many segments are lit (the number "8" would be dim, "1" would be bright).
What is Multiplexing?
To drive a 4-digit clock, you would need 28 wires! Multiplexing solves this by wiring all segment pins together but controlling the Common pins separately. You flash each digit one by one very fast (>60Hz). Your eye sees them all on due to "Persistence of Vision."
What is the "dp" segment?
It stands for Decimal Point. It is the 8th segment used to display floating point numbers (e.g., 3.14). It is usually controlled independently.