PCB heated bed Mk3

This is a derivative of Prusa's excellent PCB heatbed. It has two major additions.

First, it has a place to solder a surface mount thermistor with pads on the side to connect to the electronics. Hopefully we won't have to tape a thermistor under the bed anymore.

Second, there has a spot for a surface mount right-angle LED on the side of the bed instead of soldering a normal LED facing down like the Mk1.