Some time ago I did tutorial on how to control 7 segment display with Arduino. We were using 7 pins of Arduino to do this.

This tutorial introduces shift registers that allow us to control multiple 7 segment displays with just 3 pins.

Please go to: https://youtu.be/QI1IJLB42G8

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Expanding Pin Architectures: The 74HC595 Shift Register

An Arduino Uno has severely limited digital pins. If you attempt to wire 8 individual LEDs or 8 Relays, your board is instantly consumed, leaving absolutely zero room for sensors or LCDs! The Shift Register completely eliminates this hardware bottleneck. By utilizing the legendary 74HC595 Integrated Circuit, you can aggressively push a massive 8-bit binary string (e.g., 11001010) entirely through just THREE Arduino pins! The chip acts as a physical hardware buffer, storing the bits sequentially and executing them simultaneously in parallel, expanding three pins into EIGHT!

The Synchronous Serial Interface (Data, Clock, Latch)

The 74HC595 is not an I2C or SPI device; it operates on a raw, brutal 3-wire synchronous serial protocol you execute natively using shiftOut()!

1. Clock Pin (SRCLK): Ticking rapidly, acting as the precise metronome synchronizing the Arduino to the IC matrix.
2. Data Pin (SER): The Arduino literally blasts HIGH or LOW down this single wire for each beat of the Clock.
3. Latch Pin (RCLK): When all 8 bits are perfectly loaded inside the chip's internal memory, the Arduino rips the Latch pin HIGH. The IC instantly dumps all 8 memory states directly out to the physical Output pins (QA-QH) simultaneously!

int dataPin = 11;
int clockPin = 12;
int latchPin = 8;

void loop() {
  // Let's turn on LEDs 1, 3, and 8! Binary: 10000101 (Hex: 0x85)
  byte ledMatrix = 0b10000101; 
  
  digitalWrite(latchPin, LOW); // Lock the output buffer heavily!
  
  // Blast the massive 8-bit byte out via native bit-banging algorithm!
  shiftOut(dataPin, clockPin, MSBFIRST, ledMatrix); 
  
  digitalWrite(latchPin, HIGH); // Release the dogs! All 8 LEDs update instantly!
  delay(1000);
}

Daisy-Chaining for Infinite Outputs!

The true power of the 74HC595 lies in its Q7' (Pin 9) Serial Out capability!

• If you wire the Q7' Output of Shift Register #1 perfectly into the Data Input of Shift Register #2...
• You can now blast shiftOut() TWICE using the exact same three Arduino pins!
• The first 8 bits overflow from Chip #1 directly into Chip #2! You have just achieved 16 Outputs with only 3 Pins. You can theoretically chain entirely up to literally hundreds of LEDs constructing massive 8x8 matrices without compromising the Arduino processor overhead!

Required Hardware Infrastructure

• Arduino Uno/Nano (Acting as the primary bitwise computational host).
• 74HC595 Shift Register IC (The literal workhorse bridging serial execution into parallel outputs).
• 8x Standard LEDs & 330-Ohm Resistors.
• Massive Breadboarding Arrays (Ensure decoupling capacitors of 0.1µF are tied exactly across the 74HC595 VCC/GND logic pins to explicitly prevent rapid frequency switching noise from totally corrupting the serial bitstream!).