How to output a true analog voltage at output pin
You basically have three options:


Switch to an Arduino Due which has a built-in DAC which outputs a real voltage.
Add an external DAC chip (such as the MCP4821/2) to create the voltage for you
Use a low-pass filter (R-C network) on a PWM pin.


Of the three options I usually use an MCP4822 since it gives the best results and doesn't cost as much as using a Due.