My idea was to remove the lower cross-beam from between my two vertical posts.  Replace it with a thin guitar string,
 tightly strung horizontally between the two posts.  Then have the guitar string seated in a slot in the end of the b
oom. Since suggesting this though, I've realized this would be difficult to keep constant over time without dickering with it.&
nbsp; What about replacing the string with say a bicycle spoke? It's just an idea, I'm fairly new at this and you guys are the 
pros. I think I'm going to turn this one into the ball-bearing design, and then make something different for a second one. I'd 
be interested in seeing plans for a vertical sensor if anyone knows where some are. -dan

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