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I need to make up a device which will warn the operator of a combine if a pulley stops turning. I was thinking of using a couple of inductive prox sensors to do this, (2 so that if the pulley should stop in front of one sensor it will not think its still spinning, because the second sensor will be positioned so its not possible for both sensors to be sensing at the same time.) I will then use a timer arrangement to switch a relay etc but this I can design later. What I am wondering is if I need a PNP sensor or NPN. I studied electronics a few years ago but this has got me thinking, I cant remember it all that well! Can someone explain the difference to me again please. Also How quick will a prox sensor detect an object in front of it, bearing in mind the pulley will be spinning at about 800 rpm!? Thanks for your input. Chris