I had a Basic Stamp project that needed to measure a nominal 12 volt battery, and I wanted a simple solution. This is the simplest I could come up with. The 555 timer will put out positive pulses. The pulse width is inversely proportional to the difference in voltage between the voltage at `ANALOG IN` and the voltage of the 4.7uF capacitor(let`s say 2.5 volts). To calibrate this circuit, hook it up to a Basic Stamp measuring positive pulses, and give the circuit a known voltage.