Grumpy_Mike:
You need to use the much neglected comparator in the arduino chip.Gammon Forum : Electronics : Microprocessors : Using the Arduino Analog Comparator
I tried this technique and got lots of spurios interrupts from a slightly noisy source .. to use you must filter and clean the voltage in ..
I went for the read every x milli secs interrupt appproach coupled with a first order filter in the software to smooth the resultand a/d convert counts...