How do those safety table saws that stop when they sense fingers work?
There’s two basic parts of it: detecting fingers and stopping the blade.
In order to detect a finger, they use an electrical signal across the blade. Human flesh is more conductive than wood, so when flesh comes into contact with the blade the electrical signal changes in a noticeable way. This triggers the safety mechanism. (Note: this makes it incompatible with conductive materials like metal. Pretty much only works with wood.)
In order to stop the blade fast enough to prevent harm, it has a big chunk of metal that it hurls into the path of the blade (basically, something tough enough that the blade can’t cut through it and has to stop immediately). This piece has to be replaced after every activation, and the blade itself frequently does too. Pop over to /r/woodworking and you see the havoc this system wreaks on the blade and the stop block (although people are generally quite happy to sacrifice a blade to save a digit).
These saws have a sensor called a “flesh-detection system” that can detect when something soft, like a finger, touches the blade. This sensor works by sending out an electrical signal that creates an invisible field around the saw blade.
If something touches the blade, it disrupts the electrical field and triggers an automatic braking system. This system stops the blade from spinning and lowers it beneath the table, preventing any further injury.