🔢 7 Segment Display Code Generator

What is a 7 Segment Display Code Generator?

A 7 segment display is a simple readout made of seven LED bars (segments) arranged in a figure-8 plus an optional decimal point (dp), used to show digits and a few letters. This 7 segment display code generator works out which segments must light up for the character you pick and returns that as a single byte in binary, hexadecimal (hex) and decimal. It also builds an Arduino code line you can drop straight into a segments[] array in your 7 segment Arduino projects. All processing happens in your browser; no data is ever sent to a server.

Segment naming: a-g and dp

The seven segments are conventionally named a, b, c, d, e, f, g. Segment a is the top horizontal bar, b is top right, c is bottom right, d is the bottom bar, e is bottom left, f is top left and g is the middle bar. The decimal point is labelled dp. When we pack these eight LEDs into a single 8-bit byte, the arrangement that says which bit maps to which segment is called the bit order (segment map).

Bit/byte map — default dp-g-f-e-d-c-b-a

This tool uses dp g f e d c b a by default: the least significant bit (bit 0) is segment a, bit 6 is segment g, and the most significant bit (bit 7) is the decimal point (dp). So the byte is laid out as bit7=dp, bit6=g, bit5=f, bit4=e, bit3=d, bit2=c, bit1=b, bit0=a. For example, the character 0 lights every segment except g (a, b, c, d, e, f), which gives 0b00111111 = 0x3F = 63 on a common cathode display. If your segments are wired to different pins, switch the bit order to "a b c d e f g dp".

Common cathode or common anode?

There are two types of 7 segment display. In a common cathode part all LED cathodes share a common GND, so you light a segment by driving its pin 1 (HIGH). In a common anode part the anodes share a common +5V, so a segment lights with 0 (LOW). Because of this, the common anode byte is the bitwise NOT of the common cathode byte: anode = ~cathode. Use the "Display type" selector to flip between the two values instantly.

Example Arduino wiring and code

In a typical setup you connect the 8 segment pins (a-g + dp) through current-limiting resistors to 8 digital pins. Put the generated bytes in an array and write them to the pins in a loop:

// Common cathode, dp-g-f-e-d-c-b-a order
byte segments[10] = {
  0x3F, 0x06, 0x5B, 0x4F, 0x66,
  0x6D, 0x7D, 0x07, 0x7F, 0x6F
};
int pins[8] = {2,3,4,5,6,7,8,9}; // a..g, dp

void showDigit(int n) {
  for (int i = 0; i < 8; i++)
    digitalWrite(pins[i], bitRead(segments[n], i));
}

Tips

• Use current-limiting resistors: every segment is an LED, so a 220-470Ω resistor per pin protects both the LED and the microcontroller pin.
• Consider multiplexing: for multiple digits you rapidly flash each digit in turn; the eye sees them as continuously lit and you save many pins.
• Save pins with a shift register: with a 74HC595 shift register you can push the whole byte at once using shiftOut() and use just 3 pins.