Amazon's Alexa can now sense the presence of people. Here's how to use the uncanny new trick

New Amazon Echo Show smart displays have started shipping nationwide with a host of new features and tricks -- one of which I've been eagerly anticipating: person detection. Although Alexa can't distinguish between individual people the way Google's Face Match or Apple's FaceID do, the new Echo Show 8, Echo Show 5 and Echo Show Kids can at least figure out whether or not someone -- anyone -- is in the room with them. Although that's a pretty nifty feat in its own right, what piqued my interest most was what Alexa can now do with that information.

Watch this: The new Echo Show 8 adds just enough to be a worthwhile upgrade

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Namely, you can now use person detection to fire off a set of commands as part of an Alexa routine. Think about lights that turn on and off depending on whether anyone is in the room. An air conditioner or space heater that kicks on or off, depending on occupancy. Your front door could lock if no one's in the foyer, your bedtime routine could start when you open your bedroom door, your garage door could close, your kitchen could fill with song -- all based on whether or not anyone's there.

Of course, there's usually a bit of a gap between the way a new feature is hawked versus how it works in the real world, and Alexa's person detection is no exception. Don't get me wrong -- it's a fantastic new trick, but it comes with some limitations, which I'll explain in a bit, after I show you how to set it up.

First, however, there are a couple of things you need to know about: a hidden setting you can't change (but still need to account for), plus one other slight (but significant) limitation to consider.

Alexa's person detection has a cool-down period

When I first started testing Alexa's new person detection feature, I was convinced it didn't work. I had set up a brand-new Echo Show 8 on my desk and aimed it at myself while I worked. Beside it, I positioned a desk lamp, which I then plugged into a smart outlet. I created a simple routine that told Alexa to turn on that smart outlet whenever the Echo Show 8 detected a person, and yet -- no matter how much typing, suspicious leering or frantic thrashing of limbs I did -- that daggone light never came on.

I was almost ready to write off the new feature as DOA when an Amazon rep informed me that Alexa abides by a 7-minute cool-down period in between person detection events. To test this, I moved the Echo Show 8 into another room, turned off the smart plug and set a 7-minute timer. When the alarm went off, I moseyed into the other room. Lo and behold, my presence triggered the routine, the desk light lit up -- and my dilemma was solved.

The takeaway here is this -- if you set up a routine with the new person detection trigger, then -- for whatever reason -- turn off the devices used in that routine (either manually or with an app), Alexa won't turn them back on again until you've disappeared for at least seven minutes.

Alexa isn't actually a motion detector

Most motion sensors work using ultrasonic sound waves, meaning they don't depend on lighting conditions to detect movement. Alexa, however, uses a camera and a computer processor to figure out whether someone's in the room or not, and that means the Echo Show has to see you before Alexa knows you're there.

In my testing, Alexa almost never noticed when I entered a room unless it was bright enough to at least read a print magazine. For me, that made person detection triggers essentially useless for turning on nightlights, which is a huge drawback. If that's your goal, you may need to consider dedicated motion sensors.

With all that out of the way, here's how to set up person detection and how to use it with Alexa routines:

To enable Alexa's person detection, check this setting first

I recently discovered a new Alexa setting called Home Monitoring that turns your Echo Show's webcam into a security camera (or something quite like it). Basically, Home Monitoring lets you check your Alexa camera feed in real time without also sharing a selfie video like during a video call. Whether you want to use a feature like that or not, Home Monitoring also needs to be turned on before person detection will work. You can get the full rundown on Home Monitoring here, but here's what you need to do to enable person detection:

1. Starting on the Echo Show device itself, swipe down from the top of the screen and tap Settings, then scroll down and tap Camera.

2. Make sure the toggle for Home Monitoring is switched on and, if not, tap to do so.

Once you've turned on Home Monitoring, in addition to appearing in the places you'd expect, like under Echo & Alexa, your Alexa device will now appear in the Devices submenu of the Alexa app under Cameras (that's also where you'll find the live video feed, FYI).

Start with a super-simple Alexa routine like this

Before attempting to craft any long, complicated automations based on person detection, I wanted to get a sense of how the feature worked using a dead-simple routine. To that end, I plugged a small desk lamp into a smart plug and set it to turn on whenever the Echo Show 8 detected a person. Here's how to do it exactly as I did:

1. Open the Alexa app and tap the More menu in the lower right corner, then tap Routines.

2. Tap the + (plus) sign in the upper right corner, then tap Enter routine name (I called mine "Show 8 Motion On") and tap Next when you're finished.

3. Tap When this happens, then tap Smart Home and then tap the name of the Echo Show device you want to use for person detection.

4. The next screen will present two options: People are detected and People aren't detected. For this routine, choose the first one, then tap Next. (We'll use the People aren't detected option in the next example. 

5. Tap Add action then scroll all the way down and tap Smart Home. Tap All devices and scroll until you find the device you want to control with this routine (mine was "Desk Lamp") and tap it.

6. The next screen sets what that device does when this routine runs -- in this case, the box next to Power should be checked and the toggle should be turned on, then tap Next.

7. Your routine should look something like the adjacent screenshot. Tap Save. The Alexa app will now double check that you've followed all of the steps in the previous section to turn on Home Monitoring -- tap Next and Done.

Once you've mastered those steps, building a more complex routine with even more commands is literally just a matter of repeating steps 5 and 6 as many times as necessary. What this doesn't, however, do is turn the light (or whatever device you turned on) back off again. For that, you'll need another routine (that's just as easy as this one, quite frankly).

What turns on must turn off: An Alexa routine in reverse

Setting up a routine to turn off a device when people aren't detected is almost identical to the previous example turning one on. Rather than copypasta all the previous steps, I'll save us both the trouble and just point out the three steps that are slightly different:

• Step 2: To keep things nice and tidy I named this routine "Show 8 Motion Off."
• Step 4: Instead of People are detected, you'll want to choose People aren't detected.
• Step 6: Leave the Power check box checked, but tap the toggle to turn it to Off.

If this isn't your first Alexa Routines rodeo, you might be curious why I didn't combine these two routines into one. After all, Alexa has a Wait function, right?

While it may very well be possible to create one long routine to handle both on and off functions, considering the trip-up I had with the unpublished 7-minute cool-down period and the discovery that Alexa is effectively blind in the dark, I kept them separate for simplicity's sake. Plus, it's not uncommon to discover that a routine has unintended consequences, and it's a lot easier to isolate and correct for them when you keep things separate like this.

In other words, I'm still testing this feature and will continue to do so for a while. I'll come back and update this story when I really cinch using person detection as a routine trigger and have something new to share.