Turning on Android device with motion detector (external device?) Problem

I'm looking for a motion detector that can be set up to wake a rooted Android (4.3) tablet and turn the screen on when someone passes by it in a radius of maybe 2-3m.

Ideas

I'm definitely willing to get an external device for this and invest a (reasonable) amount of money, it would just have to be one that I can somehow connect to the tablet.

Are there maybe any proximity detectors from the home automation field with a decent detection quality (good angle and configurable distance) that can send Android push notifications?
I'm able to write some code from scratch if necessary, but I'm very clueless and unexperienced in the hardware field, so I wouldn't know how to hold a soldering iron, for example. ;)

Background

The idea here is to transform an old tablet into some kind of stationary dashboard screen, so I'm not particularly worried about battery wear (it's end-of-life now and will from now on be used with the power cable plugged in until it dies) or intense usage (unless we're talking about 100% CPU 24/7, which would probably kill it in a few days).

Previous research

I investigated a little bit whether this could be done with the existing sensors in the device and I believe that is not possible.

The problem with using the proximity sensor is that it really only returns whether something (assumed to be your ear while in a call) is right next to it (a few centimeteres away). But I'm looking for something in the 2-3m area.

The other idea would be the front-facing camera, but the problem seems to be that a) the camera in many devices automatically shuts off when the screen is off and b) only foreground apps can access the camera, but I want the tablet to show another app on detection.

I had a quick look regarding waking up the screen in response to push notifications and that doesn't seem too hard using the PowerManager API. Not sure how quickly the device would respond to a push when in deep sleep, but that's a secondary problem.
Just to let you know I have some up with a simple solution it combines an app from the app store called Motion Detector by jastrzab: https://play.google.com/store/apps/details?id=org.motion.detector 
Then install a plugin called Turn Screen: https://play.google.com/store/apps/details?id=org.jastrzab.plugin.screen


Install both apps. 
Open the Motion Detector app and turn on "Send" at the bottom of settings. Click on motion detector (at the top) and then scroll down to "camera" and select "front". 
Open the Turn Screen app. click the only button to make it active. 
Open android settings, Turn off the lock screen or select "none".
Change the sleep settings to time out in the desired time. 
Turn off Daydream.
Open the app (or home screen) you want to have displayed on the screen when the tablet wakes, and you're all set! Wait for the screen to go to sleep and walk by or wave at your camera and it will wake it up. 


Works great during the day or with the lights on. I'm currently working on an external PIR motion sensor that will allow the tablet to wake in darkness. Simple design so far just uses a cheap PIR sensor and a small led light mounted below the camera. When I walk by the PIR triggers the LED light allowing the camera to see, which triggers motion. Hoping to figure out how to eliminate the LED and just run the PIR directly into the tablet using the headphone jack or USB on the tablet. Suggestions welcomed.