Proximity sensor is <M30 - 12mm, 6V~30V>
I used just 1K ohm resistor for adapt to RAMPS 5V.

but good pull-down schematics is at the last image !
and this is from here:
http://forums.reprap.org/addon.php?219,module=embed_images,url=http%3A%2F%2Fthingiverse-production.s3.amazonaws.com%2Frenders%2F32%2Fb2%2Ff0%2F54%2Fbd%2Fdiagram_preview_featured.jpg