74HC595 shift register with Arduino Uno — 8 LEDs from 3 pins

The 74HC595 turns three Uno pins into eight outputs. The Uno has only about 18 usable I/O pins, so this chip is how LED bar graphs, 7-segment displays and large relay banks fit on one board. Several chips can be chained on the same three wires.

The Uno shifts a byte in one bit at a time on DS (D11) with clock pulses on SH_CP (D12). A pulse on the latch, ST_CP (D8), then copies all eight bits to the outputs at once. Two control pins need fixed levels: OE to GND keeps the outputs on, and MR to 5V keeps the register out of reset.

The sketch runs a light back and forth across the eight LEDs. It then counts in binary, printing each pattern so you can check it against the LEDs.

Wiring

Board quirks

• The chip is rated 70 mA total through VCC/GND. With 470 Ω resistors each LED takes about 6 mA, so all 8 on is about 50 mA. Smaller resistors look brighter but overheat the chip when every output is on.
• shiftOut() works on any pins. The data and clock here are D11 and D12; D13 is left alone because the on-board LED flickers with the clock. For speed you can use hardware SPI (D11 + D13) instead.
• Chain more chips by wiring Q7S (pin 9) of the first to DS of the next, sharing clock and latch. Then send one byte per chip, last chip first.
• Pin names vary by maker: NXP prints DS/SH_CP/ST_CP/Q7S, TI prints SER/SRCLK/RCLK/QH′. They are the same pins.

Code

// 74HC595 shift register on an Arduino Uno R3: 8 LEDs from 3 pins.
// DS → D11, ST_CP (latch) → D8, SH_CP (clock) → D12. OE → GND, MR → 5V, VCC → 5V, GND → GND.
// Q0–Q7 → LED + 470 Ω resistor each → GND.
const uint8_t DATA_PIN = 11, LATCH_PIN = 8, CLOCK_PIN = 12;

void writeByte(uint8_t value) {
  digitalWrite(LATCH_PIN, LOW);
  shiftOut(DATA_PIN, CLOCK_PIN, MSBFIRST, value);  // Q7 gets the first bit sent
  digitalWrite(LATCH_PIN, HIGH);                   // outputs change together on this edge
}

void setup() {
  pinMode(DATA_PIN, OUTPUT);
  pinMode(LATCH_PIN, OUTPUT);
  pinMode(CLOCK_PIN, OUTPUT);
  writeByte(0);
  Serial.begin(9600);
  Serial.println(F("74HC595: running light, then a binary counter"));
}

void loop() {
  for (uint8_t i = 0; i < 8; i++) {  // Q0 → Q7
    writeByte(1 << i);
    delay(80);
  }
  for (int8_t i = 6; i > 0; i--) {   // and back
    writeByte(1 << i);
    delay(80);
  }
  for (uint16_t n = 0; n < 256; n += 17) {
    writeByte(n);
    Serial.print(F("Binary "));
    for (int8_t b = 7; b >= 0; b--) Serial.print((n >> b) & 1);
    Serial.print(F(" = "));
    Serial.println(n);
    delay(300);
  }
}

// 74HC595 shift register on an Arduino Uno R3: 8 LEDs from 3 pins.
// DS → D11, ST_CP (latch) → D8, SH_CP (clock) → D12. OE → GND, MR → 5V, VCC → 5V, GND → GND.
// Q0–Q7 → LED + 470 Ω resistor each → GND.
const uint8_t DATA_PIN = 11, LATCH_PIN = 8, CLOCK_PIN = 12;

void writeByte(uint8_t value) {
  digitalWrite(LATCH_PIN, LOW);
  shiftOut(DATA_PIN, CLOCK_PIN, MSBFIRST, value);  // Q7 gets the first bit sent
  digitalWrite(LATCH_PIN, HIGH);                   // outputs change together on this edge
}

void setup() {
  pinMode(DATA_PIN, OUTPUT);
  pinMode(LATCH_PIN, OUTPUT);
  pinMode(CLOCK_PIN, OUTPUT);
  writeByte(0);
  Serial.begin(9600);
  Serial.println(F("74HC595: running light, then a binary counter"));
}

void loop() {
  for (uint8_t i = 0; i < 8; i++) {  // Q0 → Q7
    writeByte(1 << i);
    delay(80);
  }
  for (int8_t i = 6; i > 0; i--) {   // and back
    writeByte(1 << i);
    delay(80);
  }
  for (uint16_t n = 0; n < 256; n += 17) {
    writeByte(n);
    Serial.print(F("Binary "));
    for (int8_t b = 7; b >= 0; b--) Serial.print((n >> b) & 1);
    Serial.print(F(" = "));
    Serial.println(n);
    delay(300);
  }
}

Expected serial output

74HC595: running light, then a binary counter
Binary 00000000 = 0
Binary 00010001 = 17
Binary 00100010 = 34

74HC595: running light, then a binary counter
Binary 00000000 = 0
Binary 00010001 = 17
Binary 00100010 = 34

Common errors