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Does anyone know if mmWave could differentiate between my cat and I?

Possibly at shorter ranges. Those devices are essentially ‘bag of water’ detectors. You can get some data out of most of them that pertains to distance and approximate size of the ‘bag’.

They generally suffer from exact counts of objects. 2 people close to each other will read like one large person. A large cat close to the sensor could read like a large human at a greater distance.

In theory, but probably not in practice. The fancy non-metal-detector airport security scanners use mmWave to detect object through clothing, so the light itself capable of producing high-resolution images. However, cheap home automation sensors are probably not imaging, so you're probably going to be stuck with a single depth measurement.

HomeSeer manufactures a Z-Wave millimeter wave radar sensor that uses imaging techniques to detect motion in specified zones, and they intend to release pet differentiation as a firmware update. I'm seeing enough negative reviews of how it works in practice that I haven't dropped the money on one, but I'm hoping that the technology will improve quickly. It's only $59 too, which isn't much more than PIR Z-Wave sensors. Maybe I will buy one to try out...

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