eli5. How do table saws with an auto stop tell the difference between wood and a finger?
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The safety feature detects an electric signal. Human bodies are electrical conductors, so when we touch the blade, we create an electrical circuit. The machine detects this electrical difference, and initiates the blade jammer when it does. A piece of wood is not conductive, and so it does not create a circuit with the blade.
It’s similar to those lamps or even your smart phone screen. You operated them by making contact with your skin. They detect the electrical impulse of your skin. If you tried to active these with a thick glove, it would detect no signal and not turn on.
Electricity.
The saws that can detect that stuff have a tiny current constantly flowing through the running sawblade. Wood, being wood, does not conduct electricity very well, and so there’s not much change in that current when wood contacts the saw. Thumbs, being sweaty and made of meat, conduct electricity a bit better than wood, and so they change the current in the sawblade more.
When the saw detects a change in the current, it fires a mechanism that punches an aluminum block into the saw, stopping it immediately.
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