In the Playground is a link to using the Arduino to sense voltages above 5V. Arduino Playground - HomePage
When wired to either circuit, the Arduino reports 1023 when there is no
or little voltage on the sensor side. As the voltage on the sensor side is increased, the Arduino number drops.
My assumption on both circuits is that as the sensor voltage is increased, this turns on the transistor/optocoupler so that the 5V is going to ground instead of to the Arduino. Is that a correct assumption? And this would also be true if you used clamping diodes, too, correct?
In effect, the Arduino is not reading the voltage on the sensor side directly, but indirectly. Am I close?