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Information Junky
Join Date: Mar 2001
Location: an island, upper left coast, USA
Posts: 73,167
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they usually have one light (per axis) . . .and then the sensor has two pick-ups, slightly off-set (for direction)
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Everyone you meet knows something you don't. - - - and a whole bunch of crap that is wrong.
Disclaimer: the above was 2¢ worth.
More information is available as my professional opinion, which is provided for an exorbitant fee. 
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