Here is a link to a .pdf of the design: http://dl.dropboxusercontent.com/u/13355828/Tach/Layout1.pdf
This design is based on this I found on the Interwebs: http://myplace.frontier.com/~wgmumaw/Sun%20Tach/Sun%20Tach.html
And a video showing the results:
I suspect that at the higher RPMs in the test the drill runs a bit slower because of the added load. Of course I have no way of knowing how far off the drill is since I don't have tachometer to test the speed. I have yet to install the controller and head but I don't expect any problems.