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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:

second schematic

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:

first schematic

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).

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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.

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  • \$\begingroup\$ Yeah Thank you ! Now I understand the problem :) (after measuring, the Raspberry Pi is even giving a 4.73V while the power supply gives a big 5.04V. That's not helping) I read that the 75HC595 Vcc is +5V in specs. can I power it with 3V3 though ? \$\endgroup\$
    – Stijn B
    Mar 17 at 16:17
  • \$\begingroup\$ Oh well, I found it in the datasheet (operating voltage: 2V ~ 6V). Thanks \$\endgroup\$
    – Stijn B
    Mar 17 at 16:20
  • \$\begingroup\$ 70% of 4.73V is 3.31V so that's bang on the edge of working! That explains it. HC works fine on 2-6V, however the point of HCT is the input thresholds, and these will be wrong if not used at 5V, so HCT is usually specced for 5V only. \$\endgroup\$
    – bobflux
    Mar 17 at 16:57

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