**Amplified piezo disc touch sensor** This project started as the sensor for 3D-printer Z-axis minimum end-stop replacement. The piezodisc is supposed to be mounted directly on X-carriage and sense the backpressure from the hotend up when it contacts the printbed surface. The disc produces ~300 mV sensing the contact. The differential amplifier has a gain of ~4x and its output triggers the single +5V square pulse generator to report the surface contact to the printer. It works (I made it on a breadboard) and it can be used as intended, but this design has a drawback, which I neglected first. It requires the hotend to be mounted loosely to transfer some pressure to the disc. This design can be used and has its own positive features, but apparently I designed another solution which doesn't make me to change X-carriage construction and at the same time should give the better precision. So I went on to proceed with that design. 2017-01-11