To control my simple Beacon, I used PWM and as simple (but slow) RC filter. That worked good enough for a test, but I really wanted the voltage changes to be more rapid. What alternatives are there? Well, I actually ordered a $5 I2C DAC board from Sparkfun, but it hasn’t arrived. The obvious low-tech alternative is to use a R-2R resistor ladder. Somewhat coincidently, hackaday had a tutorial on them just a few days ago.

Your first Digital to Analog Converter build – Hack a Day.

Addendum: This page talks about the need for a buffer amplifier to be used in concert with the D/A converter. That’s because any load (resistance) you place on the output works in concert with the resistor network to change the output voltage. If that load varies, it will also cause your output voltage. By buffering through a buffer amplifier (in this case, a rather cheap op amp) you prevent all that, and the DAC works more reliably.