Digital Display

Seven-Segment Construction

A seven-segment display contains seven individually controlled LED bars, labelled $a$ through $g$, arranged as a figure eight. An optional decimal point is labelled $dp$.

Seven-segment lettering and package concept.

Each segment is an LED and requires correct polarity and current limiting. Segment names are standardized; package pin numbers, forward voltage and current ratings are not, so use the actual data sheet.

Digit	BCD	$a$	$b$	$c$	$d$	$e$	$f$	$g$

Decimal digit patterns (1 means illuminated).

This table describes light, not electrical pin level. Common-anode displays require the complemented electrical commands.

Common Cathode and Common Anode

One illuminated segment in common-cathode and common-anode displays.

Feature	Common cathode (CC)	Common anode (CA)
Common terminal	Cathodes to ground	Anodes to $V_{CC}$
Segment ON command	HIGH at segment anode	LOW at segment cathode
Driver action	Sources current	Sinks current
Electrical polarity	Active HIGH	Active LOW
Classical decoder
Digit selector	Low-side NPN/NMOS	High-side PNP/PMOS

Common-cathode and common-anode displays.

Use one resistor in each segment line. A resistor only in the common lead makes current and brightness depend on how many segments are illuminated.

Segment Driving

For a static segment drive,

$$\boxed{R_S=\frac{V_{CC}-V_F-V_{driver}}{I_{segment}}}.$$

For a sinking output, use its LOW voltage $V_{OL}$ for $V_{driver}$. For a sourcing output, use the actual loss between $V_{CC}$ and the guaranteed HIGH level. Choose the next larger standard resistance, then check $P_R=I^2R$ and the driver’s per-pin and total-current limits.

Note

Example — Segment resistor

A red common-anode display uses $V_{CC}=5\,V$, $V_F=1.9\,V$, $V_{OL}=0.2\,V$ and desired current $I=8\,mA$.

$$R_S=\frac{5-1.9-0.2}{8\times10^{-3}}=362.5\,\Omega.$$

Choose $390\,\Omega$, giving approximately $I=(5-1.9-0.2)/390=7.44\,mA$.

Use a transistor or driver IC when one logic output cannot safely provide the required segment current. Never exceed the LED’s continuous or pulsed-current rating.

BCD-to-Seven-Segment Decoding

A decoder accepts BCD input $D C B A$, with $D$ the most-significant bit, and generates seven segment commands. For example, $0101_2=5$ selects $a,c,d,f,g$.

BCD decoder/driver signal chain and auxiliary controls.

Device	Output/display	Important point
	Active-LOW open-collector outputs for common anode	Sinks current; external segment resistors required
	Active-HIGH drive for common cathode	Source capability depends on the exact family version

Classical BCD-to-seven-segment drivers.

The usual auxiliary controls are active LOW:

• $\overline{LT}$: lamp test turns all segments on;

• $\overline{BI}$: blanking input turns all segments off;

• $\overline{RBI}$ and $\overline{RBO}$: suppress leading or trailing zeros across several digits.

Codes 1010–1111 are invalid BCD inputs; their display pattern is unspecified unless the chosen decoder defines or blanks it.

Multiplexed Multi-Digit Display

Multiplexing shares the seven segment buses and enables one digit common at a time. An $n$-digit display then uses about $7+n$ principal control lines instead of $7n$.

Four-digit multiplexing with a shared segment bus and one-hot scan.

For equal slots,

$$\boxed{D_{digit}\approx\frac1n},\qquad \boxed{f_{slot}=nf_{frame}},\qquad \boxed{I_{avg}\approx D_{digit}I_{peak}}.$$

The frame rate must avoid visible flicker. Peak current must stay within LED, segment-driver and digit-driver pulse ratings. Use break-before-make scanning: disable the old digit, change segment data, then enable the new digit. This prevents ghosting.

Note

Key Point — Seven-segment checks

Label $a$–$g$, state common-anode or common-cathode, use the correct active polarity, place one resistor per segment, and check source/sink current. For a multiplexed display also verify duty cycle, frame rate, peak current and a blanking interval between digits.