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5.1 Using the 74HC595 Shift Register

In this lesson, we’ll learn how to use the 74HC595 shift register to control multiple LEDs with just a few GPIO pins on the Raspberry Pi Pico 2. The 74HC595 is an integrated circuit (IC) that allows you to expand the number of digital outputs using a serial input. This is incredibly useful when you want to control many outputs but have limited GPIO pins available.

What You’ll Need

In this project, we need the following components.

It’s definitely convenient to buy a whole kit, here’s the link:

Name	ITEMS IN THIS KIT	LINK
Newton Lab Kit	450+	Newton Lab Kit

You can also buy them separately from the links below.

SN	COMPONENT	QUANTITY	LINK
1	Raspberry Pi Pico 2	1	BUY
2	Micro USB Cable	1
3	Breadboard	1	BUY
4	Jumper Wires	Several	BUY
5	Resistor	8(220Ω)	BUY
6	LED	8	BUY
7	74HC595	1	BUY

Understanding the 74HC595 Shift Register

The 74HC595 is an 8-bit serial-in, parallel-out shift register with output latches. It has the ability to take serial data input and convert it into parallel output, allowing you to control 8 outputs using only 3 GPIO pins from the Pico.

Key Pins on the 74HC595:

img_74jc595_pin

DS (Pin 14): Serial Data Input
SHCP (Pin 11): Shift Register Clock Input
STCP (Pin 12): Storage Register Clock Input (Latch Pin)
OE (Pin 13): Output Enable (Active Low, connect to GND)
MR (Pin 10): Master Reset (Active Low, connect to 3.3V)
Q0-Q7 (Pins 15, 1-7): Parallel Outputs
VCC (Pin 16): Connect to 3.3V
GND (Pin 8): Connect to GND

Circuit Diagram

sch_74hc_led

Wiring Diagram

Writing the Code

Now, let’s write a MicroPython program to control the LEDs through the 74HC595 shift register.

Note

Open the 5.1_microchip_74hc595.py from newton-lab-kit/micropython or copy the code into Thonny, then click “Run” or press F5.
Ensure the correct interpreter is selected: MicroPython (Raspberry Pi Pico).COMxx.

import machine
import utime

# Define the pins connected to the 74HC595
SDI = machine.Pin(0, machine.Pin.OUT)   # Serial Data Input (DS)
RCLK = machine.Pin(1, machine.Pin.OUT)  # Register Clock (STCP)
SRCLK = machine.Pin(2, machine.Pin.OUT) # Shift Register Clock (SHCP)

# Function to send data to 74HC595
def shift_out(data):
    for bit in range(8):
        # Extract the highest bit and send it first
        bit_val = (data & 0x80) >> 7
        SDI.value(bit_val)
        # Pulse the Shift Register Clock
        SRCLK.high()
        utime.sleep_us(1)
        SRCLK.low()
        utime.sleep_us(1)
        # Shift data left by 1 for the next bit
        data = data << 1
    # Pulse the Register Clock to latch the data
    RCLK.high()
    utime.sleep_us(1)
    RCLK.low()
    utime.sleep_us(1)

# Main loop to demonstrate shifting patterns
while True:
    # Light up LEDs one by one from Q0 to Q7
    for i in range(8):
        data = 1 << i
        shift_out(data)
        utime.sleep(0.2)
    # Light up LEDs one by one from Q7 to Q0
    for i in range(7, -1, -1):
        data = 1 << i
        shift_out(data)
        utime.sleep(0.2)
    # Create a moving bar effect
    for i in range(9):
        data = (1 << i) - 1
        shift_out(data)
        utime.sleep(0.2)
    # Turn off all LEDs
    shift_out(0x00)
    utime.sleep(0.5)

When you run the code, the LEDs connected to the 74HC595 shift register will display dynamic light patterns:

First Sequence: LEDs light up one after another from left to right. Each LED turns on in sequence, creating the effect of a light moving across the row.
Second Sequence: LEDs light up one after another from right to left, reversing the direction of the movement.
Third Sequence: LEDs create a growing bar effect, where LEDs turn on cumulatively from left to right until all LEDs are lit.
Final Step: All LEDs turn off briefly before the entire sequence repeats.

This results in an eye-catching display of lights moving back and forth and a bar growing across the LEDs, looping continuously.

Understanding the Code

Import Modules:
- machine: Provides access to GPIO pins.
- utime: Contains time-related functions.

Define Control Pins:

We define the GPIO pins connected to the 74HC595.

SDI = machine.Pin(0, machine.Pin.OUT)   # Data Input
RCLK = machine.Pin(1, machine.Pin.OUT)  # Latch Clock
SRCLK = machine.Pin(2, machine.Pin.OUT) # Shift Clock

Shift Out Function:

This function sends 8 bits of data to the shift register.
It sends the most significant bit (MSB) first.
Pulses the shift register clock (SRCLK) to shift in each bit.
After all bits are shifted in, it pulses the register clock (RCLK) to latch the data to the outputs.

def shift_out(data):
    for bit in range(8):
        bit_val = (data & 0x80) >> 7
        SDI.value(bit_val)
        SRCLK.high()
        utime.sleep_us(1)
        SRCLK.low()
        utime.sleep_us(1)
        data = data << 1
    RCLK.high()
    utime.sleep_us(1)
    RCLK.low()
    utime.sleep_us(1)

Main Loop:

Lights up each LED one by one from Q0 to Q7.

for i in range(8):
    data = 1 << i
    shift_out(data)
    utime.sleep(0.2)

Lights up each LED one by one from Q7 to Q0.

for i in range(7, -1, -1):
    data = 1 << i
    shift_out(data)
    utime.sleep(0.2)

Gradually lights up LEDs to create a bar that grows from Q0 to Q7.

for i in range(9):
    data = (1 << i) - 1
    shift_out(data)
    utime.sleep(0.2)

Sends 0x00 to turn off all LEDs.

shift_out(0x00)
utime.sleep(0.5)

Experimenting Further

Create Custom Patterns:

Modify the data sent to create different LED patterns. For example, to blink alternate LEDs:
```
shift_out(0b10101010)
```
Control More LEDs:

Chain multiple 74HC595 chips together to control more outputs. Connect the Q7’ (Pin 9) of the first chip to DS (Pin 14) of the second chip.
Integrate with Sensors:

Use inputs from sensors or buttons to change the LED patterns dynamically.

Conclusion

In this lesson, you’ve learned how to use the 74HC595 shift register to expand the output capabilities of your Raspberry Pi Pico 2. This technique is invaluable when working with projects that require controlling many outputs with limited GPIO pins.