# How should I alter my PISO shift register circuit so that it works with 3.3v instead of 5v?

I've followed this Arduino guide to build a circuit using a CD4021BE PISO shift register, and 10k pulldown resistors. It works perfectly fine with the Arduino's 5v supply.

I then attempted to use this circuit with my Raspberry Pi's GPIO, which only supplies 3.3v, and I was getting very unreliable readings from the data pin. Increasing the clock delay to 1ms improved it a bit, but it's still very unreliable. (Some bits read high every few seconds, when everything should be low.)

I assume that running a CD4021BE shift register at 3.3v requires a different value for the pulldown resistors, so which value would you recommend? I have seen this answer to a similar question about pullup/pulldown resistors, but would like to know if there is any 'rule of thumb' for pulldown resistors in a 3.3v circuit. I would guess that a 5k resistor should solve the problem, but I wanted to get some advice before buying resistors and desoldering the existing ones.

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ti.com/product/cd74hct165 but the pinout is different. –  Ignacio Vazquez-Abrams Jun 18 '13 at 1:48
Christ, as usual, the Arduino docs are sloppy and full of errors. You need a 0.1 uF bypass capacitor as close as you can get it to the power connections of each CD4021. –  Connor Wolf Jun 18 '13 at 3:29
The fact that the tutorial doesn't include bypass capacitors tells you: A - it was not written by a person who is very familiar with electronics, and B - the arduino people apparently don't have any actual engineers or even people familiar with electronics proofreading their documents. –  Connor Wolf Jun 18 '13 at 3:33
@AnindoGhosh - Bullshit - The url is www.arduino.cc, I certainly don't see any disclaimers that the content is crowd-sourced (hell, the URL doesn't even have "wiki" or similar in it. it's www.arduino.cc/en/Tutorial/ShiftIn). If they want to run a wiki, and have it include random, poorly written tutorials, they need to be pretty damn sure that it is CLEAR that said wiki and it's content is not directly affiliated with their brand. They have entirely failed to do this. –  Connor Wolf Jun 19 '13 at 2:16
Also, their schematics are f**king horrible. –  Connor Wolf Jun 19 '13 at 6:09
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## protected by markrages♦Jun 18 '13 at 6:41

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