Electronic – Is interfacing the 5V TTL UART with 3.3V microcontroller via voltage divider possible

I think the easiest circuit that does what you want would use a N channel MOSFET like the bss138 and two pull-ups, hooked in the following way:

Tell me if it doesnt fit your needs, and why.

Having said this, I have to recommend you to take a look at the MAX3000 series of level converters. I have used them before and they are great bidireccional converters that "simply work" (in apple terms).

Hope it helps.

edit: It seems like the image appears broken with some browsers, you can see it here

A voltage divider is not a good idea because the output voltage will depend too much on current draw, or it will always draw lots of current from the supply. A better idea is to linearly regulate the 3.3 V supply down to the other voltages you need. That will be somewhat wasteful of power, but if the arduino supply can handle it, it will be the easiest solution.

Whenever you say you need to power something at a particular voltage, you should also mention the current requirement. Since you didn't, there is little more specific advice to give. If it's only a few 10s of mA, then linear regulators are a no brainer. If it's 1 A then it gets more complicated, and quite possibly the existing power supply doesn't have enough extra to provide that power.