Quote Originally Posted by Doddy View Post
Might work but not at 24V... or at least without spilling some smoke. Also CMOS tends to have a high switching voltage at high VSS - the 5V sensor voltage wouldn't cut it at 12V. Max current through a single channel is 10mA so rate the LED resistor accordingly.

Sorry!, not a dig!

OP: Why not use a common ground as a sense input, rather than a 5V input? (if you have 5V handy then ignore)
No - you are right, 15v abs max and 10mA. But I still can't see how the original circuit would work. I would need some resistor values then I could simulate it or even bread board it. Also it would be useful to know the impedance on the 12-24v.