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Recently i saw a video about Nest Protect, an smart smoke detector, that can be operated by waving your hand under it. Since there is no visible holes on the lower part i suppose they are not using any light/acoustic sensors, like IR or ultrasonic, so it could be an RF type sensor, but i have no idea what specific sensor, do you know how they detect the waving of the hand with such a precision?, i mean, i can easily detect movement with a PIR sensor, but i have no idea how to reduce the range (if possible) in order to detect only near objects movements.

EDIT: I found this picture of the sensor, they are actually two, one vertical and the other one inclined, seeing the picture can you tell what sensor type is?

enter image description here

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    \$\begingroup\$ Look like US (Utra Sone == high frequency sound) to me. \$\endgroup\$ – Wouter van Ooijen Dec 27 '13 at 23:12
  • \$\begingroup\$ but they are behind the plastic enclosure, do they still work? \$\endgroup\$ – DomingoSL Dec 28 '13 at 0:37
  • \$\begingroup\$ Dunno. Or maybe the twins are just beepers, and that bigger thingy with 'FAST' on it is the handwaving detector. \$\endgroup\$ – Wouter van Ooijen Dec 28 '13 at 8:13
  • \$\begingroup\$ Not gonna read past the sloppy writing. \$\endgroup\$ – Olin Lathrop Dec 29 '17 at 15:47
  • \$\begingroup\$ Teardown says ultrasound: learn.sparkfun.com/tutorials/nest-protect-teardown. And not an 8-bit micro but three 32-bit ARM-based devices. \$\endgroup\$ – pjc50 Dec 29 '17 at 16:34
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Those two are definitely ultrasonic transducers. One for Tx one for Rx. My guess is that only a hand wave close enough (because of the tilt angle) will trigger this. It could detect a simple echo, or might even do a doppler analysis to detect movements. More advanced method would be to use a "chirp" signal, but I doubt that is required here.

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