SonomaProducts.com wrote:
That demo was a trick.

OK, it is an electrical circut completion that triggers the safety
brake right?

What if you have no contact with the table top but just the blade?
Notice his hand is resting on the table top.
What if you are using a wooden sled? Does it stil work?
What if you are standing on a rubber mat?

Just wondering.
....

OK, somewhere I saved one of the Patents...ah, here's the pertinent
section from the Disclosure section--

"The detection subsystem includes a sensor assembly, such as contact
detection plates 44 and 46 , capacitively coupled to blade 40 to
detect any contact between the user's body and the blade. Typically, the
blade, or some larger portion of cutting tool 14 is electrically
isolated from the remainder of miter saw 10 . Alternatively, detection
subsystem 22 may include a different sensor assembly configured to
detect contact in other ways, such as optically, resistively, etc. In
any event, the detection subsystem is adapted to transmit a signal to
control subsystem 26 when contact between the user and the blade is
detected. ..."

AFAIK, the only ones on the market use the capacitively-coupled
embodiment rather than resistive or optical.

Primarily the physical causative factor of the body capacitance
instigating trip is owing to the water content of flesh; one of the
features of the saw (at least initially, I presume still is altho I've
not looked at one in detail since shortly after initial introduction) is
a bypass switch so it won't be triggered falsely when cutting, say,
construction treated tubafores or similar that are wet. Of course,
using that is a conundrum since it prevents a real trip if one were
required as well...

That's probably more than I know...

I _think_ all are based on passive capacitance change as somebody else
similar to the lamp switch effect. There are active capacitative
proximity sensors, but they're measuring a field change.

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