Microcontrollers are digital devices at heart. They can do fancy things like convert analog signals into a digital value but going the other direction is a bit tougher. Pulse-Width Modulation is used to approximate an analog output but what you’re actually doing is turning the operating voltage on and off very quickly to achieve an average value somewhere in between. This is the method most commonly used to dim an LED. But generating a smooth voltage in this way takes just a few more parts.
[Scott Daniels] spent some time discussing the process of smoothing a PWM output by using a low pass filter. This is a compilation of digital and analog circuitry to produce a smoother signal than PWM can achieve on its own. As you can see above, the low pass filter is made up of one resistor and one capacitor. The theory is not hard to understand, and with [Scott's] help you’ll become much more comfortable with choosing the component values for you own filters. His examples center around an Arduino using the analogWrite() function but the techniques can be applied universally.