Voltage confusion, USB, FTDI cables etc...(noob!)

I've been reading just about everything I can about the *duino boards for the last week or so. I'm awaiting some boards and parts to be delivered.

[I understand your usual (Arduino, Seeduino, Free....) have built in serial of some sort (RS232, USB etc.) I'm looking to use some very small boards in a device I am developing that lack the expensive and space eating components for serial communications. Once programmed these are sealed in a box destined to just pass numbers to a display for ever and ever, amen. The board I intend to use runs on a 3V supply (2 x 1.5V battery pack).]

One thing I keep finding myself lost on is voltage. I see references to 3.3V and 5V with regularity.

From what I can tell this has to do with both the operating voltage of the board AND the TTL level voltages between something like an FTDI based connection to the TTL serial I/O on the AVR.

I see some FTDI breakout boards can do both voltages, some are set to one to the other. Are these voltages 'supply' power that can run the *duino board or do these voltages refer to the signal levels to the I/O on the AVR or perhaps this relates to both?

Needless to say this is a bit perplexing.

Could somebody explain this with some level of conciseness and clarity?

Please be gentle. I'm still wet behind me ears! ::)

Links to relevant reading are of course welcome.

Thank you.

M.S.

Hi Richard, thank you. Yes, I should have provided links. Pretty simple tachometer project.

I've seen the RBBB boards. right in line with my needs.

My intention is (now was?) to use the Anarduino board: http://www.anatools.com/anarduino/ along with a Hitachi HD44780 based 16x2 display with a Fairchild QRB1134 IR Photoreflector for input.

[insert grumple here] I need to check the operating voltage of the QRB1134.. Foo. I -knew- I'd overlook something...

Thanks.

-- M.S.