The Useless Machine was a popular contraption in the 70s that is making a comeback. It’s a simple mechanism with a two-circuit switch that inverts the direction of motion of a DC motor in the box, enabling a finger in the box to switch itself off.
It’s sold as a kit, and the assembly is a pleasant weekend experience. It should not take more than an hour to put together.
I customized mine by ordering a halloween costume’s hand, and rebuilding the plastic to more closely resemble a human finger. The first time one see it pop out, it is actually quite startling!
I use mine in demonstrations to another team — to illustrate what tedious repetitive work is all about.