Touch sensor

Touch sensor project, based on a design by Rick Sparber (https://rick.sparber.org/rctf.pdf).  I have added a trimpot to make it easier to move the sensor from tool to tool, and an option to implement a 4-wire resistance measurement technique.  This gets rid of any sensitivity to the connector wires' resistance.  Each force wire is connected to its sense wire at the machine-contact end (magnetic hold down, clip, etc).

For proper operation, the resistance of the "machine current path" must be equal to or greater than about 2 ohms.  This is mostly determined by the spindle bearing(s).  If the resistance is too low, it usually can be increased by hand rotating the spindle.

For more circuit information, please see comments on the schematic.