Hi, I have been adding photo interrupters to motor/gearboxes: http://forum.allaboutcircuits.com/showpost.php?p=750615&postcount=421 The interrupters are 180 degrees on a 9light 9dark film disk. It's not as straight forward as it might seem! First I tried one to get the best on/off balance by choosing resistors. Then built 4x of them (Perhaps a mistake, but it's done) Trying to read on/off times on an oscilloscope is giving me odd readings. Should it be set for 1 or 2 triggers? ac/dc? It give best result with 1x trigger, but my logic say it need 2xtriggers. Any tips welcome. Cheers, Camerart.