Eli5: how do modern cutting tools with an automatic stop know when a finger is about to get cut?

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It is based on the capacitance of the finger. The technology is quite similar to modern touch screen technology which is only able to detect fingers but not gloves. Wood is usually too dry for the sensors to trigger and the metal is too small for it to trigger. This is of course something the designers have to take into consideration. Possibly the hardest thing you can put in them is soaking wet wood for example if you are sawing raw lumber out in the rain but they have apparently been able to set the sensors to handle this as well. However as the SawStop patent is about to expire and competing products have already been shown to the market as being much cheaper to trigger there might be some advantage to triggering on metal as well protecting the blade.

Gnonthgol

There’s an electric current running through them. Your body conducts electricity, wood kinda doesn’t. As soon as it makes contact with you, the electric signal pulls the blade away. As fast as electricity can react, which is why in the ads you see people will barely have a scratch on them

RedAftermath

The saw blade carries an electrical current, and the braking system monitors this. When your hand touches the blade, your *electrical* resistance changes that circuit and triggers the emergency stop. The stops are quite violent, so don’t test this.

Lithuim

Seeing a lot of not quites in this thread. My freshman year of electrical engineering degree that I never finished, the guy who invented saw stop came and demoed his saw and gave an explanation of how it works. It’s not about current. The cartridge monitors the capacitance of the blade. When the capacitance of the blade (and whatever its touching with relatively low resistance) drastically rises, it triggers the brake. When you touch the blade, the cartridge sees more capacitance and triggers. This is why wet wood or wood with nails can trigger it. The wet wood can hold charge much better than dry wood, so it is able

hairysandvich