I need a 3V ( 2 1.5V batteries) for a RC car transmitter and I have a Raspberry Pi Model B giving out 3.3V. How can I convert the 3.3V from Raspberry Pi to use it in my RC car Transmitter ? Basically I'm trying to run the RC car with the Raspberry Pi GPIO pins 1, 3V3OUT, and Pin 39 GND.
I think the additional 0.3V won't bite.
Should you really be concerned about this you can get a Schottky rectifier like
1N5819 to drop the voltage by 0.2V.
By the way
1N5819 are so cheap that you can get boxes of thousand for a few bucks - you can get a few hundred of those and start replacing
1N4001's in your circuits with those, and in some circuitry its lower forward drop means better efficiency.
The voltage specifications for the RaspberryPi GPIO don't appear to be available, but this website has done some comparisons to similar parts. Specifically, the section "Input and output pin electrical specifications" shows that comparable parts have a minimum Input High Voltage (VIH) under 2.5V. As long as your part has a minimum output high voltage (VOH) greater than 2.5V, you're probably going to be fine. You didn't provide a part number for your radio transmitter, so make sure you check the spec for that to verify that it can handle 3.3V -- as others have suggested, it shouldn't cause any problems.
The voltage of an alkaline cell can vary from ~1.65V when brand new under no load to ~0.9V when discharged (or even lower if under heavy load).(Alkaline Battery - Wikipedia)
The RC car transmitter should have been designed to operate over this range so powering it directly from the RaspberryPi is a good way to go.
As pointed out by Chris many products do not get the maximum life out of the batteries and may stop functioning before the 0.9V per cell limit is reached.
What is probably more important is that any logic signal you connect to the device is consistent with its power supply - for example powering it from a lower voltage but using the 3.3V GPIO may cause problems.