I developed this sensor interface board during my ongoing effort to create a really great open source PLC.
It's an 8 channel interface board that allows you to use industrial sensors with your Arduino or other 5V microcontroller. You can use both NPN (sinking) and PNP (sourcing) sensors from 5 to 30VDC.
You can configure each channel to interface with either one of the 2 sensor types. That means you can use any combination of 8 NPN or PNP sensors on any channel you'd like and then reconfigure it on the fly if you need to.
You can use multiple sensors of different voltages at the same time (as long as they have a common ground), such as a 5V, a 12V, and a 24V sensor.
The board electrically isolates your microcontroller from your sensors up to 1kVDC, and has 8 red LEDs to indicate the status of each channel (on / off).
Its design is completely open source and you can find all the project files on GitHub.