7-Segment Displays

A 7-segment display has seven LED segments (a–g) plus an optional decimal point. Driving them directly from an ESP32 requires 8 pins per digit — impractical for multi-digit readouts. Driver ICs handle the multiplexing internally and communicate over a 2- or 4-wire bus.

Segment map

Basic number display

// Library Manager → search "TM1637Display" by Avishkar Mishra / Arduino TM1637
#include <TM1637Display.h>

const int CLK_PIN = 22;
const int DIO_PIN = 23;

TM1637Display display(CLK_PIN, DIO_PIN);

void setup() {
    display.setBrightness(5);        // 0 (dim) – 7 (bright)
    display.showNumberDec(0, true);  // show "0000" with leading zeros
}

void loop() {
    // Show current altitude from X-Plane (example: 35000 ft)
    int altitude = 35000;
    display.showNumberDec(altitude);
    delay(100);
}

Colons and custom segments

// Colons and individual segments
#include <TM1637Display.h>

TM1637Display display(22, 23);

void showTime(int hours, int minutes) {
    // showNumberDecEx: value, dots mask, leading zeros, length, position
    // dots mask 0x40 = colon on (centre dots)
    display.showNumberDecEx(hours * 100 + minutes, 0x40, true);
}

void showLabel() {
    // Custom segments for text — e.g. "AL t" (altitude label)
    const uint8_t SEG_A = 0b01110111;   // A
    const uint8_t SEG_L = 0b00111000;   // L
    const uint8_t SEG_T = 0b01111000;   // t
    uint8_t data[] = { SEG_A, SEG_L, 0, SEG_T };
    display.setSegments(data);
}

RSET — segment current resistor

Both ICs require an external resistor between ISET (pin 18) and V+ to set the peak segment current. The setIntensity() call in software then scales brightness from 0 to 15 within that ceiling.

The LedControl library works with both MAX7219 and MAX7221 without any code changes — the constructor and all method calls are identical.

// Library Manager → search "LedControl" by Eberhard Fahle
#include <LedControl.h>

// LedControl(DIN, CLK, CS, numDevices)
LedControl lc = LedControl(23, 18, 5, 1);

void setup() {
    lc.shutdown(0, false);       // wake up MAX7219 (starts in power-down mode)
    lc.setIntensity(0, 8);       // brightness 0–15
    lc.clearDisplay(0);
}

void showAltitude(int feet) {
    // Write individual digits right-to-left (position 0 = rightmost)
    for (int pos = 0; pos < 8; pos++) {
        lc.setDigit(0, pos, feet % 10, false);
        feet /= 10;
        if (feet == 0) { break; }
    }
}

void loop() {
    showAltitude(35000);
    delay(100);
}

To drive multiple chips, set numDevices to the chain length and pass the device index (0-based) to each lc.* call.

The MAX7221 is a drop-in replacement for the MAX7219 with the same pinout, register map, and software interface. Two hardware differences matter for cockpit builds.

When to choose the MAX7221

SPI bus sharing. The MAX7219's DOUT is permanently driven, which corrupts the MISO line for any other SPI device sharing the bus. The MAX7221 tri-states DOUT whenever CS̄ is high, so it coexists cleanly with other SPI peripherals (SD cards, OLED displays, accelerometers) on the same three-wire bus.

Cockpit radio-frequency environment. The MAX7221's slew-rate-limited segment drivers reduce high-frequency switching noise on the display lines. In a cockpit with COM / NAV receivers, lower EMI from display drivers lowers the risk of interference with sensitive radio equipment — a meaningful practical advantage even if the effect is small on a home simulator.