Looking for a simple zigbee device that can tell if a car is in the garage or not..
Will mmWave sensor work?
I think it would work to tell you when it arrives or leaves by going active, but it would probably time out to inactive after the car sits motionless for a bit.
I know the Linptech/MOES ES1 community driver provides a distance attribute. You could probably make an automation with distance to determine if it is arriving or leaving. Distance can also tell you if the car is there or not. When motion is activated, have the automation wait for a few seconds, then look at the distance attribute.
When the car is parked, it will show the distance from sensor when it stopped. That attribute will not change again until motion is seen again, so you could just say if distance is is less than some value, it is parked, and ignore the sensor going inactive.
When leaving, the last thing it will see is the car moving at a greater distance for last motion, and then that distance will stay again until you pull the car back in, so with the greater distance change, the car is gone.
The issue I see is when someone walks into the empty garage, it is going to trigger the sensor and change the last distance, so it would think you are the car.
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This was what I was thinking would be important to work around. You'd need some sort of timer or filter of some type on any distance reporting (or other measure of presence) to avoid a person or some other object left in the car's normal area in the garage fooling the sensor.
No.
mmWave sensing depends on the object reflecting millimeter range waves at 24 GHz (or higher). Detection of humans is dependent on attenuation of the reflection by water - and we're mostly water. Due to movement, we also scatter the reflected waves at odd angles.
Your car will not be picked up by mmWave sensors that are calibrated to pick up living objects.
If I were you, I would use an ultrasonic sensor, or something based on a photoelectric cell to pick up your car.
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I just had a thought. A PIR motion sensor on the outside of the garage, that goes active as the car pulls into the driveway. Only react to the presence sensor distance value on active to decide if the car is parked, IF the outside sensor is also active.
Assuming you have a side door or something on the garage, and people will not enter from the main garage door from the driveway.
Why don't you do this?
Use something like this sensor:
And hook up the NO and COMMON terminals to a zigbee binary sensor (like a contact or leak sensor)
You can position the photo electric beam such that the reflected beam is blocked if the car is parked there.
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Thanks for sharing, wasn't aware of the connection w/water, very interesting.
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My quick Google of it says mmWave can detect cars, and many non water objects. The list of things it can detect is pretty large, actually. It detects the micro movements in the ripples of moving water, but does not detect water itself, is what I am finding.
This is also a good idea, but a bit more expensive. People could still break the beam, but a "says closed" for x minutes rule could be used to avoid it picking up people just walking through it.
Not the ones calibrated to detect humans and larger pets. That detection RELIES on attenuation of the reflected signal by water.
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I will be happy to prove you correct tomorrow, when I take my sensor out to the driveway with a power bank and confirm that it will not detect my car pulling into the driveway.
thanks for so much feedback..... now thinking about trying the ultrasonic
Strap a water bottle or two to the front of the car? 
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Be sure to leave the car parked in the garage for a while to see whether the sensor detects the car as “present.” Since that was the OP’s use case.
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Yup.
Overkill for the op, but for reference:
That will detect any car and avoid detecting humans.
If you want to detect only your car a beacon in or on the car is the easiest. Or security camera. Beam sensors and pad sensors are options if you just need to get close.
I'm thinking something plugged into the power outlet for the opener on the ceiling, aimed down. For multi-bay garages. Could an inexpensive zigbee ultrasonic tank level sensor be adapted?
How about hanging a tilt sensor from a string. When the car pulls in the tilt sensor moves to horizontal on the roof of the car. I don't think scratching paint would be a factor. Maybe I'll try it with one of my surplus Zooz tilt sensors. They didn't work mounted on the door, maybe they'll work hanging in the air.
Of course you could run into it with say, your head, but again, light and shouldn't scratch the finish. 
edit: People hang tennis balls on string to gauge where to stop the car. Maybe incorporate a tilt sensor, ie, put it inside a sliced tennis ball.
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Open=Car
Closed=Empty
Perhaps the vibration feature could be used.
If only the sensors continue to work, lol.
edit: I have no need for this.
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Not Zigbee but easy and works very well.
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