Mini 1 Z axis minimum endstop triggered


When doing the bed levelling and trying to make contact with washers, the Z min endstop is forever triggered, which means that the firmware assumes that it’s already touched the washers, and will then move onto testing the other three washers, which again, it thinks it’s already touched. It them proceeds to extrude filament into mid-air, and the bed levelling is incredibly messed up. moving the X axis for instance has a very pronounced slant in mid-air.

We’ve got two Lulzbot Mini 1’s, and the exact same issue happened to the other one, and we had to replace the entire Mini Rambo board to get it working again.

On looking at the schematics for the Mini Rambo, and the Lulzbot mini’s wiring, some things spring to mind as to what might be causing this:

Clearly, we want to get to the bottom of this, so we’re not forever replacing entire Mini Rambo controllers:

Most of the limit switches are just that, limit switches; microswitches. The wires are not exposed, and they don’t fail.
The Z min endstop is electrical contact between the extruder nozzle and the washers; washers wired to the input. There is no input protection on the Atmega2560 microcontroller input.
M119 always reports Z min as triggered.
Static electricity zaps applied to the washers?
Hotend power accidentally applied to the nozzle?

So, what would be the best way to mitigate against these failures?


We usually see this issue when the hot end is touched with something metallic. Either a user will clean the nozzle with a wire brush or touch the nozzle with tweezers when removing filament. This will send a short via the zero-sense wire back to the RAMBo.

The best way that we know of to mitigate these issues is to use a Scotch Brite Pad when cleaning the nozzle. Just preheat the nozzle up to operating temperature and give the nozzle a good scrubbing. Also avoid touching the nozzle with anything metallic.