Using Breadboard Power Supply with Raspberry Pi and 74HC595

Question

I'm very new to electronics and I made this simple circuit using my Raspberry Pi, a 74HC595 (8-bit shift register) and 8 LEDs.

I wanted to use a HW-131 breadboard power supply to avoid drawing to much current from the RPi board directly, but I cannot get it to work.

This is how I wired it up:

The problem: this does not work. All kind of strange behaviors occur when I test it.

This second configuration, however, works. I'm simply using the RPi 5V pin to power the IC and the LEDs:

Is there a way to power my LEDs (and maybe the 74HC595) with the breadboard power supply?

By the way, the Raspberry Pi (Zero 2W) outputs a 3V3 signal (on the serial data pin, clock pin and latch pin connected to the 74HC595).

Answer 1

74HC595 has Vih at 70% of VCC, so when supplied with 5V, it's guaranteed to interpret 3.5V as logic high, but 3V3 is not guaranteed. Since its input logic high level is proportional to its VCC, if your breadboard power supply is a bit high, that may explain it.

Solution: use 74HCT instead, which will receive 3V3 logic levels correctly when powered with 5V. Or use HC, but power it from 3V3.

A decoupling capacitor on the chip would probably help, too.