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.