External mic for phone timegrapher app

Checking the timing of a mechanical watch using a phone app is easy, but you need an external microphone.

RobG

Quantity		Component name
1	×	3D-printed main part of the holder see files for .stl
1	×	3D-printed sliding part of the holder see files for .stl
2	×	50-60mm length of 8mm diameter alu tube
1	×	rubber band
1	×	20mm diameter piezoelectric sensor Buy a few, you'll probably break some!
1	×	pre-amp circuit in a metal box