Shift Registers

A shift register converts serial data (one bit at a time) into parallel outputs (all bits at once). The 74HC595 is a Serial-In Parallel-Out (SIPO) device: you clock 8 bits in over a single data wire, then latch them simultaneously to 8 output pins. Multiple chips can be daisy-chained to produce 16, 24, or more outputs while still using only 3 GPIO pins.

Key pins

Arduino's built-in shiftOut() function handles the bit-banging. Pull LATCH low before shifting, then high after to atomically update all outputs simultaneously — this prevents partial flicker during the shift.

const int DATA_PIN  = 23;   // DS  — serial data
const int CLOCK_PIN = 22;   // SH_CP — shift clock
const int LATCH_PIN = 21;   // ST_CP — storage (latch) clock

// Push 8 bits into the 595 and latch them to the outputs
void write595(uint8_t value) {
    digitalWrite(LATCH_PIN, LOW);
    shiftOut(DATA_PIN, CLOCK_PIN, MSBFIRST, value);
    digitalWrite(LATCH_PIN, HIGH);
}

void setup() {
    pinMode(DATA_PIN,  OUTPUT);
    pinMode(CLOCK_PIN, OUTPUT);
    pinMode(LATCH_PIN, OUTPUT);

    write595(0b00000000);   // all LEDs off on start
}

void loop() {
    write595(0b01010101);   // LEDs 0, 2, 4, 6 on
    delay(500);
    write595(0b10101010);   // LEDs 1, 3, 5, 7 on
    delay(500);
}

Connect Q7S (pin 9) of the first chip to DS (pin 14) of the second. Share SH_CP and ST_CP across all chips. Shift data for the farthest chip first — it travels through each Q7S→DS link on its way to the final position.

// Two 74HC595 daisy-chained — 16 outputs, same 3 GPIO pins
// Data for the far chip is shifted out first (MSBFIRST order)

const int DATA_PIN  = 23;
const int CLOCK_PIN = 22;
const int LATCH_PIN = 21;

void write595x2(uint8_t chip2, uint8_t chip1) {
    digitalWrite(LATCH_PIN, LOW);
    shiftOut(DATA_PIN, CLOCK_PIN, MSBFIRST, chip2);   // far chip
    shiftOut(DATA_PIN, CLOCK_PIN, MSBFIRST, chip1);   // near chip
    digitalWrite(LATCH_PIN, HIGH);
}

void setup() {
    pinMode(DATA_PIN,  OUTPUT);
    pinMode(CLOCK_PIN, OUTPUT);
    pinMode(LATCH_PIN, OUTPUT);
    write595x2(0x00, 0x00);
}

void loop() {
    // Annunciator panel: overhead lights chip1, engine chip2
    write595x2(0b00000011, 0b11111100);
    delay(1000);
    write595x2(0b00000000, 0b00000000);
    delay(1000);
}

Map each output bit to a named indicator. Keep a uint8_t state variable and toggle individual bits when the Bridge delivers simulator dataref updates.

// Cockpit annunciator driven by simulator datarefs
// Call this from your event handler whenever simulator state changes

const int DATA_PIN  = 23;
const int CLOCK_PIN = 22;
const int LATCH_PIN = 21;

// Annunciator bit map (one bit per indicator LED)
#define LED_GND_SPOILER   0
#define LED_DOOR_FWD      1
#define LED_DOOR_AFT      2
#define LED_SEATBELT      3
#define LED_NO_SMOKING    4
#define LED_MASTER_CAUTION 5
#define LED_FIRE_WARN     6
#define LED_OVHT_DET      7

uint8_t annunciatorState = 0;

void setIndicator(uint8_t led, bool on) {
    if (on) {
        annunciatorState |=  (1 << led);
    } else {
        annunciatorState &= ~(1 << led);
    }
    // Latch new state immediately
    digitalWrite(LATCH_PIN, LOW);
    shiftOut(DATA_PIN, CLOCK_PIN, MSBFIRST, annunciatorState);
    digitalWrite(LATCH_PIN, HIGH);
}

Call setIndicator() from your WebSocket message handler whenever the X-Plane dataref changes — no polling required.