Lesson 132: Multiplying Outputs - Using the 74HC595 Shift Register

Your Arduino Uno has 14 digital pins. That seems like a lot until you try to build an LED cube or a digital clock, at which point you realize you are hopelessly out of pins and completely stuck. Instead of throwing money at a bigger Arduino, you can cheat the system using a magical little IC called a Shift Register. It takes three pins and turns them into eight. It’s like a power strip for your microcontroller outputs. A shift register acts like an expansion module, turning a few control pins into many output pins.

What is a Shift Register?

The 74HC595 is one of the most famous shift registers. It takes serial data (data sent one bit at a time, sequentially down a single wire) and converts it into parallel data (multiple bits output simultaneously across multiple wires).

In practical terms: you use just three pins on your Arduino to talk to the 74HC595, and the 74HC595 gives you eight new digital output pins! Even better, you can “daisy-chain” multiple shift registers together. Connect two 74HC595 chips, and you get 16 outputs, still using those same original three Arduino pins.

What You’ll Need

• An Arduino board (like the ELEGOO UNO R3)
• A 74HC595 Shift Register IC (Available here)
• 8x LEDs (Any color)
• 8x 220-ohm Resistors
• Jumper Wires and a Breadboard

Understanding the Pins

The 74HC595 has 16 pins. The three critical pins connected to the Arduino are:

1. Data Pin (DS, Pin 14): This is where the Arduino sends the 1s and 0s.
2. Clock Pin (SHCP, Pin 11): The Arduino pulses this pin to tell the shift register, “I just put a new bit on the Data Pin, read it now!”
3. Latch Pin (STCP, Pin 12): Once all 8 bits are loaded in, the Arduino pulses the latch pin to say, “Push those 8 bits to the actual output pins all at once.”

Wiring It Up

Wiring 8 LEDs can be messy, so take your time!

1. Place the 74HC595 across the center trench of your breadboard. The small notch or dot on one end indicates the top (Pin 1 is top-left).
2. Connect Pin 8 (GND) and Pin 13 (Output Enable) to the breadboard’s ground rail.
3. Connect Pin 16 (VCC) and Pin 10 (Master Reclear) to the breadboard’s 5V rail.
4. Connect the Arduino control pins:
   • Arduino Pin 11 to 74HC595 Pin 14 (Data)
   • Arduino Pin 12 to 74HC595 Pin 11 (Clock)
   • Arduino Pin 8 to 74HC595 Pin 12 (Latch)
5. Connect the 8 output pins (Q0 through Q7) to the positive legs of your 8 LEDs, with a 220-ohm resistor in series for each.

The Code: Shifting Bits

Here is the code to make the LEDs light up in a sequence using the shiftOut() function built into Arduino.

// Define the pins connected to the 74HC595
const int latchPin = 8;  // STCP
const int clockPin = 12; // SHCP
const int dataPin = 11;  // DS

void setup() {
  pinMode(latchPin, OUTPUT);
  pinMode(clockPin, OUTPUT);
  pinMode(dataPin, OUTPUT);
}

void loop() {
  // Loop through all 8 LEDs
  for (int i = 0; i < 8; i++) {
    // Ground the latch pin so the LEDs don't flicker while transmitting
    digitalWrite(latchPin, LOW);
    
    // Shift out a byte (8 bits). 
    // bit(i) creates a byte with only the i-th bit set to 1.
    shiftOut(dataPin, clockPin, MSBFIRST, bit(i));  
    
    // Pulse the latch pin high to push the data to the outputs
    digitalWrite(latchPin, HIGH);
    
    delay(200); // Wait so we can see the LED
  }
}

By mastering the shift register, you unlock the ability to build massive LED displays and complex indicators without worrying about running out of pins!