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Kossel Plus FSR Holders
This FSR holder works quite well.
It is easy to modify it for different diameter or thickness glass beds.
You can place small (approximately 1mm thick 5mm x 5mm) squares of foam between the glass and the key piece which swivels to lock the glass in place. This pre-loads the FSR so that you can better control how sensitive it is.
I used a drop of rubber cement to attach the round plastic buttons to the bottom of the glass. This keeps the glass from being able to rotate.
The glass should lightly rest against the heads of the socket head cap screws so that it can easily move up and down. Tap on the glass to verify that the microcontroller triggers easily -- a red LED will light when it triggers.
The sensitive area is a triangle with the three FSR's at its corners.
I use Wing Wong & Ian Lee's code for the Adafruit Trinket, but you could use this with John SL's board. With a Trinket, wire all three FSR's in parallel, so that pressing on any one will trigger it. Note: the Trinket code is not limited by any license.
Someone else also made the Trinket wiring diagram. Note: I found I had to wire 5v from the RAMPS to USB+ on the Trinket. The BAT+ input is for more than 5v (for example from a battery), and the voltage regulator it is attached to seems to require a little more than 5v to deliver 5v to operate the Trinket.
I have been building Kossel Plus printers (my own design) using these. They allow the effector to be simpler, because it doesn't have to include a z-probe. I use RC's Marlin for auto calibration. Here is a link to my calibration instructions: http://www.maui-3d.com/kcZeroBacklashInstallation.php#calibration