Scott Harden writes:
I recently had the need to carefully measure a voltage with a microcontroller which lacks an analog-to-digital converter (ADC), and I hacked together a quick and dirty method to do just this using a comparator, two transistors, and a few passives. The purpose of this project is to make a crystal oven controller at absolute minimal cost with minimal complexity. Absolute voltage accuracy is not of high concern (i.e., holding temperature to 50.00 C) but precision is the primary goal (i.e., hold it within 0.01 C of an arbitrary target I set somewhere around 50 C).
More details at Scott Harden’s blog.