This is a remix/redesign of a great design from Bruno Porto.
I just squeezed the parts to fit 3.7g micro servo instead of 9g.
It is intended to use 2mm rod (bike wheel spike) threaded on the end. The length of the rod should be 30 - 35mm depending on the setup in your printer.
I relay on gravity to drop the probe. A ring + spring setup should be added if you feel you want a reliable and always positive drop action.
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The reaction arm of the servo may require a bit scaling/boring depending on your printer precision as the hole is really small and needs a tight fit with the servo.
My angles are 70 /180 deg to deploy/stow the probe.
The micro servo is from Banggood:https://www.banggood.com/3_7g-Micro-Analog-Servo-GH-S37A-For-RC-Airplane-Helicopter-p-1116559.html?rmmds=search&p=UN1003814241201410X7
I also used a brass M3 inserts assembled with soldering iron to mount it from top:https://www.banggood.com/100pcs-M3x5x5mm-Metric-Threaded-Brass-Knurl-Round-Insert-Nuts-p-1050182.html?rmmds=search&p=UN1003814241201410X7