I built a distance logger using a Pro Mini 5V, ultrasound sensor, RTC, and Micro SD card module.
I've got everything working, however, I'm having trouble with the SD card module.
It's one of the cheap modules from Ebay: click here
The vendor claims it works on both 5v and 3.3v supplies.
I'm using a 5V Pro Mini so I connected SCK MOSI MISO CS GND to the right pins on the Pro mini, and the VCC from the module to the VCC of the Pro Mini (supplying 5v, checked with multimeter).
Now, everything works perfectly when the Pro Mini is powered via USB (usb-FTDI converter-Pro Mini).
It also works perfectly if I disconnect the USB, leave the FTDI module plugged in, and power from battery.
However if I power from battery without the FTDI module connected, it doesn't work. The problem seems to be communication with the SD module.
I had a good Google session, and while I can't pinpoint the problem I have a feeling it's related to logic levels.
Checking the SD modules I have, they do have a 3.3v regulator, and also what appears to be a level shifter (the SD modules I have are 100% identical to the picture in the link above).
I'm at a loss to be honest. Any help is appreciated at this stage.