7-Segment Display
Optoelectronic
A display device shaped like a '8' that shows numbers by lighting up individual segments. Common in digital clocks, calculators, and meters.
What it looks like
7-Segment Display
A typical optoelectronic — exact shape, colour and markings vary between manufacturers.
Types & variants
Key specs
Forward voltage
Volt (V)Voltage drop across each segment when lit (typically 1.8-2.2V for red, higher for other colors).
Forward current
mACurrent needed per segment for proper brightness (usually 5-30mA).
Segment intensity
mcdLuminous intensity per segment (higher = brighter).
Common type
Common cathode (all cathodes tied together) or common anode (all anodes tied together).
Digit count
Number of digits in the display (1, 4, 8 are common).
Laws that govern it
- Ohm's lawV = I × R
Every segment is an LED needing its own series resistor — sharing one on the common pin makes brightness change with the digit displayed.
- Kirchhoff's current law (KCL)ΣI(in) = ΣI(out)
All eight segment currents meet at the common pin, so an all-8s digit can draw 8 × 20 mA — well past a microcontroller pin's limit.
- Watt's lawP = V × I = I²R = V²/R
Multiplexed displays are on only 1/N of the time, so you drive them harder for the same average power and the eye sees them as steady.
Each law is stated in full — with its diagram, variables and worked meaning — on the electrical laws page.
Markings
Usually marked with manufacturer logo, part number, and pin 1 indicator (dot, notch, or beveled edge). Multi-digit displays have pin 1 marked.
Standard values
No electrical values like resistors. Common sizes: 0.3", 0.5", 0.56", 0.8", 1.0" digit height. Colors: red, green, blue, yellow, white.
How to choose
1) Choose common anode/cathode to match your driver. 2) Pick digit count for your application. 3) Select size/color for visibility. 4) Check brightness requirements. 5) Ensure current draw matches your driver capability.
Pinout & package
Segment names (single digit)
Check first: Segment letters are standard but the pin numbering is not — and the display is either common-anode or common-cathode. Find the common pin with a multimeter in diode mode, then always drive each segment through its own resistor.
Single digit: 10 pins (8 segments + common + decimal point). Multi-digit: 1 pin per digit + segment pins. Common packages: DIP (through-hole), SMD (various sizes). Pin 1 is usually marked with a dot or notch.
Example circuits
- Basic counter with 74LS47 decoder
- Arduino multiplexed display
- Digital clock circuit
- Voltmeter display with microcontroller
- Scoreboard for a game
Common failures
Segments stop lighting (burned LED), ghosting (incorrect multiplexing), dim segments (aging LEDs), or complete failure (short circuit). Physical damage to segments or pins.
How to test
1) Set multimeter to diode test. 2) For common cathode: connect black lead to common pin, red lead to segment pins (should light). 3) For common anode: connect red lead to common pin, black lead to segment pins. 4) Test decimal point separately.
Substitutes
LCD displays (lower power), OLED displays (better contrast), graphical LCDs (more flexible), or individual LEDs (more flexible but complex wiring).