WINGONEER CNC 3D Printer Mechanical Optical Limit Switch Endstop with Cable for Ramps 1.4 Makerbot Prusa Mendel RepRap (Pack of 5pcs)

For my application, connecting to a Smoothieboard running grbl-lpc, I could not get these to work reliably until I removed the pull-up resistor (the top resistor in the first picture). With the resistor present, the board would detect the beam being blocked but would not then detect it being unblocked again.I don't know whether this is a quirk of my setup though - perhaps for other applications the pull-up resistor is helpful.One thing I did like is the way the indicator LED is arranged so that you can (by moving one resistor) have it indicate when blocked versus the default of indicating when unblocked.

Works like a charm