Finally got a little time to do a quick test to get a “feel” for just how sensitive these piezo transducers and the board really are…
The answer Very 
I really only needed to very lightly touch it with the end of a [plastic] pen to trigger a read. In fact I couldn’t “place” the pen on the disk without triggering a read, no matter how gently I tried. Maybe it’s a bit too sensitive right now but that can be adjusted on the board.
My first project with these will be on my Anet A8 printer, under the bed. When I start on my Hevo build in the new year I will probably go the hot end route.
The amplifier board came from https://www.precisionpiezo.co.uk
