I'm working on designing a simple thermometer powered by the Nano. The JBtek BMP180 will be used for temperature and pressure. The Adafuit H2UD21A-F will give me relative humidity (it has a redundant temperature sensor as well). For the display, I'm going with the Frently I2C 2004 Serial Yellow Green Backlight LCD Module with 20x4 display (this device: http://www.amazon.com/gp/product/B018V1UA68). My understanding is that I can power the Nano with a 9v battery hooked to Vin and Gnd (https://www.arduino.cc/en/Main/ArduinoBoardNano).
The data sheets for the BMP180 and HTUD21A-F both say they take 3.3v. The LCD takes 5v. All of them do I2C which I am going to use for communications using the Wire Library. I'm concerned about the different voltages. Do the break out boards have voltage regulators so that they can run with 5v and all will be well? If not, can 5v and 3.3v devices share an I2C bus?
If none of the above, what do I need to do to make this work? I was hoping this would be plug and play with just a relatively simple sketch to write in the Arduino IDE.
I found my answers. The JBTek claims to have a 3.3v regulator on the break out board. Same deal with the Adafruit. According to each company, I can hook to the 5v logic and SLC/A no problem. Adafruit has libraries and example code for both.
So it looks like I'm cool.
Sorry to waste space. Perhaps someone else has the same question at some point.
If a module has a 3.3V regulator on the board, the SDA and SCL may not connect to a 5V I2C bus. If for example a 5V module has pullup resistors on the SDA and SCL to 5V, then current from the 5V will go into the 3.3V chips via SDA and SCL, possibly damaging the chip.
If the display has no pullup resistors, the display might work with a 3.3V I2C bus. But then I would like to know the pullup resistors on the 3.3V modules.
If you think that the 3.3V sensors can connect to a 5V I2C bus, that is not true. Unless they are Adafruit modules with onboard level shifting.
Perhaps you should buy a I2C level shifter, that solves all the I2C level problems.
A 9V battery is not enough to power the backlight of the display. A 9V battery is good for a smoke detector, and that's about it.