Piezo + FFC Extruder Head Cable PCB

~~##Rev.E is incoming!~~

[*I've started over from scratch and built my own electrical design from the ground up. The new project can be found here*](https://www.thingiverse.com/thing:3343928)
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The new version has an onboard microcontroller to auto-calibrate on the fly, and a much more stable circuit with better filtering to prevent false positives.

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I'll be posting it in it's own listing, as Rev.E is less universal, and more geared to "Mendel" style Cartesian printers like the i3 clones. The Rev.E print head board has only one change at this time which is using actual 0402 footprints and some better silkscreening as a result.

The Rev.D board for the print head is still compatible with the Rev.E Control/X-Car board chain as well. I'm sure eventually I'll figure out a more intuitive way of naming all of these boards eventually.

In future I may add circuitry for alternate temperature sensors. Let me know what you think about adding an RTD PT1