74HC595 shift register with Raspberry Pi Pico — 8 LEDs from 3 pins in MicroPython

The 74HC595 turns three Pico pins into eight outputs — and more if you chain chips. On the Raspberry Pi Pico it fits perfectly on SPI0: SCK on GP18 clocks the bits in, TX on GP19 carries the data, and GP17 latches them to the outputs.

Run it at 3V3 with an LED and 220 Ω resistor on each output. The script counts in binary from 0 to 255, then runs a Knight Rider sweep.

Wiring

Also needed: 8 LEDs, 8 × 220 Ω resistors, 100 nF capacitor across VCC/GND at the chip.

Every connection
Raspberry Pi Pico	74HC595 8-bit shift register (DIP-16)	Wire	Notes
—	Q0	—	To LED 1 + 220 Ω to GND (not drawn)
—	Q1	—	To LED 2 + 220 Ω to GND (not drawn)
—	Q2	—	To LED 3 + 220 Ω to GND (not drawn)
—	Q3	—	To LED 4 + 220 Ω to GND (not drawn)
—	Q4	—	To LED 5 + 220 Ω to GND (not drawn)
—	Q5	—	To LED 6 + 220 Ω to GND (not drawn)
—	Q6	—	To LED 7 + 220 Ω to GND (not drawn)
—	Q7	—	To LED 8 + 220 Ω to GND (not drawn)
3V3	VCC	red	Chip pin 16. Run the 595 at 3.3 V, the same as the Pico, so every logic level matches.
GND	GND	black	Chip pin 8.
GP19	DS	green	SPI0 TX, physical pin 25 → serial data (chip pin 14).
GP18	SH_CP	yellow	SPI0 SCK, physical pin 24 → shift clock (chip pin 11).
GP17	ST_CP	orange	Physical pin 22 → latch (chip pin 12).
GND	OE	black	Chip pin 13: outputs always enabled.
3V3	MR	red	Chip pin 10: never reset.

2–6 V part, 3.3 V board. Power it from the same voltage as your board: 3.3 V on ESP32 and Pico, 5 V on Uno. At 5 V the 74HC595 needs about 3.15 V for a HIGH, so a 3.3 V board driving a 5 V-powered chip is marginal — use a 74HCT595 at 5 V for that.

Board quirks

A shift register is exactly what SPI was made for, so the Pico’s hardware SPI0 clocks the byte out in microseconds: spi.write(bytes([b])), then pulse the latch.
At 3.3 V a 74HC595 output can supply a few mA per LED — fine with 220 Ω. For brighter LEDs or relays, follow it with a ULN2003.
Chain more 595s by wiring Q7S of the first to DS of the next and sharing SH_CP/ST_CP; then write 2, 3, … bytes before the latch pulse.
Don’t power the 595 from 5 V while driving it from the Pico: a 74HC595 at 5 V needs about 3.15 V for HIGH, which 3.3 V only just meets.

Code

main.pyMicroPython

# 74HC595 shift register driving 8 LEDs from a Raspberry Pi Pico (MicroPython, hardware SPI).
# Wiring: VCC -> 3V3 (pin 36), GND -> GND, DS -> GP19 (pin 25, SPI0 TX),
# SH_CP -> GP18 (pin 24, SPI0 SCK), ST_CP (latch) -> GP17 (pin 22), OE -> GND, MR -> 3V3.
# Q0-Q7 -> LED anode -> 220 Ohm -> GND (one resistor per LED).
from machine import Pin, SPI
import time

spi = SPI(0, baudrate=1_000_000, polarity=0, phase=0, sck=Pin(18), mosi=Pin(19), miso=Pin(16))
latch = Pin(17, Pin.OUT, value=0)


def show(byte):
    spi.write(bytes([byte]))  # 8 bits out MSB first: bit 7 ends up on Q7, bit 0 on Q0
    latch.value(1)            # rising edge copies the shifted bits to the outputs at once
    latch.value(0)


print("74HC595 on SPI0 (GP18/GP19), latch GP17")
while True:
    print("Binary counter 0-255")
    for n in range(256):
        show(n)
        time.sleep_ms(30)
    print("Knight Rider")
    for _ in range(3):
        for i in list(range(8)) + list(range(6, 0, -1)):
            show(1 << i)
            time.sleep_ms(70)
    show(0)
    time.sleep(1)

# 74HC595 shift register driving 8 LEDs from a Raspberry Pi Pico (MicroPython, hardware SPI).
# Wiring: VCC -> 3V3 (pin 36), GND -> GND, DS -> GP19 (pin 25, SPI0 TX),
# SH_CP -> GP18 (pin 24, SPI0 SCK), ST_CP (latch) -> GP17 (pin 22), OE -> GND, MR -> 3V3.
# Q0-Q7 -> LED anode -> 220 Ohm -> GND (one resistor per LED).
from machine import Pin, SPI
import time

spi = SPI(0, baudrate=1_000_000, polarity=0, phase=0, sck=Pin(18), mosi=Pin(19), miso=Pin(16))
latch = Pin(17, Pin.OUT, value=0)


def show(byte):
    spi.write(bytes([byte]))  # 8 bits out MSB first: bit 7 ends up on Q7, bit 0 on Q0
    latch.value(1)            # rising edge copies the shifted bits to the outputs at once
    latch.value(0)


print("74HC595 on SPI0 (GP18/GP19), latch GP17")
while True:
    print("Binary counter 0-255")
    for n in range(256):
        show(n)
        time.sleep_ms(30)
    print("Knight Rider")
    for _ in range(3):
        for i in list(range(8)) + list(range(6, 0, -1)):
            show(1 << i)
            time.sleep_ms(70)
    show(0)
    time.sleep(1)

Expected serial output

REPL / Thonny shellOutput

74HC595 on SPI0 (GP18/GP19), latch GP17
Binary counter 0-255
Knight Rider

74HC595 on SPI0 (GP18/GP19), latch GP17
Binary counter 0-255
Knight Rider

Common errors

All LEDs on or flickering randomly

MR or OE is floating. Tie MR to 3V3 and OE to GND.

Pattern is mirrored

SPI sends bit 7 first, so it lands on Q7. Reverse the LED order on the breadboard or reverse the bits in code.