Assume that you need to log some of your readings from the input pins of your Arduino Uno (or MEGA, etc.) dev board which needs to work standalone. The proper way is to store the log in an SD card ...

I believe blue 5V Arduino SD modules work with 3v3 devices with a simple trick. These feature a 74LVC125A and an AMS1117 3v3 LDO. If you bypass the LDO (connect the 3v3 supply to the ldo's 3v3 pin), ...