7-Segment
A seven-segment LED display forms numerals by lighting selected segments. Displays may be driven statically or multiplexed; a multi-digit design often shares segment connections and selects digits in turn.
A common-cathode display shares the LEDs' cathode connection, while a common-anode display shares their anodes. Suitable drivers establish current through selected segments. In a multiplexed design the controller repeats digit-selection intervals fast enough for the intended viewing conditions, while respecting peak-current and duty-cycle limits.
In plain terms
Eight LEDs arranged like the figure-eight on a calculator; light the right combination and any digit appears.
Why designers use it
- Providing a straightforward numeric readout.
- Displaying counts, time or measurement values without a full graphics display.
- Combining a compatible driver with a small numeric user interface.
Best for
- Clocks
- Scales
- Industrial readouts
Key specifications
- Configuration: Common anode or common cathode
- LED current and voltage: Check colour, construction and continuous/pulsed conditions
- Multiplexing: Digit count, scan timing and duty cycle
- Physical layout: Digit height, viewing properties and exact pinout
When not to use it
- When the interface needs detailed text or graphics.
- When the chosen display cannot meet viewing-angle, ambient-light or power requirements.
Common mistakes
- Mixing common-anode and common-cathode displays or drivers.
- Omitting current limiting or exceeding driver current limits.
- Treating multiplexed peak current as an allowed continuous current.
- Assuming a digit's package size determines its voltage or pinout.
Where you will find it
- The MAX7219/MAX7221 family can drive up to eight common-cathode numeric digits and includes multiplex scanning and display memory.
- A counter can update the driver's stored digit values rather than repeatedly transferring a full graphics framebuffer.
A short history
Introduced in 1972, the HP-35 was the first pocket calculator with transcendental functions and the first with RPN, according to the Museum of HP Calculators. Its LED display scanned individual digits and segments, using an inductive charge-discharge drive technique described in the later HP-21 design article. This was an important pocket-calculator example, not the first scientific calculator of any size or a claim about the invention of segmented displays.
Good to know
- The MAX7219/MAX7221 store display data and perform scanning internally.
- The manufacturer specifically identifies the MAX7221 as SPI/QSPI/MICROWIRE compatible; check the chosen driver's serial timing rather than assuming all similar parts behave identically.