7-Segment Displays

A 7-segment display is seven bar-shaped LEDs (plus a decimal point) arranged so that lighting the right combination forms the digits 0–9. de:volt ships both wiring polarities: the common-cathode display seg7-cc and the common-anode display seg7-ca. They share the same segment layout and the same per-segment electrical limits; only the shared-pin polarity differs.

The segment map

The seven bars are named a through g, with dp for the decimal point:

     a
   ─────
 f│     │b
  │  g  │
   ─────
 e│     │c
  │     │
   ─────  • dp
     d

Each segment is an ordinary LED. To show a digit you light the subset that draws it — a b c d e f for “0”, b c for “1”, a b d e g for “2”, and so on. Because every segment is an LED, each segment needs its own series current-limiting resistor; one shared resistor would let the lit segments steal current from each other and change brightness as the digit changes.

Both displays share these per-segment specs:

A series resistor of roughly 220–330 Ω at 5 V keeps each segment around 10 mA, well under the 20 mA ceiling. These per-instance values are visible in the Inspector.

Common cathode — seg7-cc

In a common-cathode display all seven segment cathodes (and the decimal point’s) join to the shared COM pins. You tie COM to GND and drive each segment pin HIGH through its resistor to light it — a HIGH segment pin pushes current down through that LED to the shared ground.

Pinout

The two COM pins are the same internal node — real displays expose the common on two pins for layout convenience. Tie both to GND.

Common anode — seg7-ca

A common-anode display is the mirror image: all the segment anodes join at COM. You tie COM to +5 V and pull each segment pin LOW through its resistor to light it — current flows from the shared anode, through the lit LED, into whatever is sinking the segment pin.

Pinout

As with the common-cathode part, both COM pins are one internal node; tie both to the positive rail.

Driving digits with a decoder

You rarely toggle all seven segments by hand. The usual build feeds a 4-bit BCD value into a BCD-to-7-segment decoder, which maps 0–9 to the correct segment pattern automatically. The polarity of the display dictates the chip:

• Common cathode pairs with the CD4511 — push-pull, active-HIGH outputs that drive each segment HIGH.
• Common anode pairs with the 74LS47 — open-collector, active-LOW outputs that pull each segment LOW.

The decoders are documented under Logic ICs, which also walks through chaining a counter into a decoder so a running count appears as digits. Whichever decoder you use, keep one current-limit resistor per segment between the decoder output and the display.

Example: common cathode

4-bit BCD ─── CD4511 ─── a–g ─── 7× 330 Ω ─── seg7-cc (a–g)
                                               seg7-cc COM ─── GND
                                               +5V ─── CD4511 VCC

Related parts

• LEDs — the single LEDs each segment is built from, with resistor sizing.
• Logic ICs — the CD4511 and 74LS47 decoders, plus counters that feed them.

New to placing and wiring parts? Start with Getting started.