Bil Herd goes through a great Introduction to Analog to Digital Conversion. These days it is very simple to grab an analog reading and use it with a microcontroller with ease. This is because modern microcontrollers often have one or many ADC built in. Not that long ago a  would have been needed, this is still the case if a special ADC is needed for things such as high precision or very fast acquisition. Below you can see some items from the Microchip PIC 16 line of microcontrollers. This is just showing about a 5th of the available chips that can do ADC and it is sorted from cheapest to most expensive. Doesn’t take long looking at this chart to see that you can really get some big bang for the buck here. In volume the PIC 16F18323 will cost about $0.57 and this 14 pin count chip will give you 11 ADC inputs at a max resolution of 10 bits.