A retractable probe using an over centering design that toggles to deploy the switch.

Music wire of .031" diameter
The switch used is http://www.digikey.com/product-detail/en/AV404461/255-3401-ND/646763

Please check out the instructions for more info.

Credit: Kossel Mini J-Head Retainer (white part) is shown in the first photo.

The switch is bonded to the flipper with cyanoacrylate glue. Some care is needed to keep the adhesive away from the switch mechanics.

The jaws of the pliers are 8.3mm wide and this defines the width of the folded spring. Folding it as shown will provide enough spring action when stretched in place. The long leg is ~20mm, but that is not critical. It does need to be long enough to extend past the retracted switch by some 3-4mm. The wire diameter is 0.8mm [.031"]. The same music wire is used for both the hinge pin and the spring.

Install the spring first and insert the hinge pin second.

One of the endstop screws is replaced by a nut and a long screw whose head was cut off to create a smooth stud.

it is possible to drive the effector to a position where the spring vertex is just above or below this stud and lift up or push down. Deploying or retracting the switch.