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Here's an easy way to put a y-axis zeroing switch on your ShopBot, that works by simply closing an electrical circuit when your z-axis carriage makes contact with the head of a bolt. It will only work if your ShopBot is properly grounded, but that's something that needs to be done anyway.
A non-conducting block with a 3/16" hole drilled through it ( I used a scrap of wood ) is hot-glued to the top of my y-axis, so that it's at least 1.5" back when the y-axis is at the Y=0 position. A 10/24 x 3" machine screw is put through the hole, with it's head facing the Z-carriage and double nuts on each side. The wire from your input switch is connected to the bolt with a lock-washer and another nut.
All that's left to do is to zero your y-axis accurately, find a safe setback distance so that you can make small negative move with your Y-axis without wiping out the block ( I set mine to Y = -0.4 ), then adjusting the bolt with the nuts until it just barely makes contact and the indicator on the ShopBot screen shows that the input switch has closed. Tighten all the nuts so that they won't vibrate loose, test it a couple of times by manually rolling your carriage until it contacts and checking the indicator in the software, and you're finished. Modify the ShopBot ZZero program to work with the y-axis and you're ready to go.
[ Copy my y-switch program - Clamping table top ]