Using Smart Lock is an easy way to mix security with convenience — keep your Android locked unless you want it unlocked.
Introduced with Android 5.0 Lollipop, Smart Lock includes the addition of what's called a Trusted Device. A trusted device is anything — from an Android Wear smartwatch to a smart luggage tag to your car's stereo — that can be connected to your phone via Bluetooth, or a programmable NFC tag that's been set up to unlock your phone when the two are close enough to communicate.
While folks who are truly security-focused won't use any of the Smart Lock options (if someone steals your phone they will probably steal your smart watch, too) they are one of those good things that get more people to lock their phone because it's more convenient to unlock it — just like a fingerprint scanner. Anything that causes more people to protect their data and keep things locked away, the better we say.
This post was previously published. It has been updated with more current information and instructions.
Secure your lock screen
The first thing you'll need to do is make sure you have some sort of security on your lock screen in the first place. There's no need to set up a devices to unlock your phone if you don't keep it locked.
You'll find options to set a password, pattern or PIN in your phone's settings. Look for entries under Security or Lock Screen — different phones have different ways of getting there, but every Android has these options. Once that's done, you'll be able to add a trusted device.
Adding a trusted Bluetooth device
In the Smart Lock settings, you'll see the section for trusted devices at the top of the list. Go on and tap it. The first time you use it, you'll be told a bit of information about what types of devices can be used and given the option to add one. Tap that link at the bottom.
On the next screen you have a choice to make — Bluetooth or NFC? We'll look at adding an NFC trusted tag next, but for now choose Bluetooth.
You'll then be presented with a list of Bluetooth devices that your phone is currently paired with. Notice that only devices that are currently connected and in range can be enabled, so make sure you're connected to the device you want to use here. All good? Then just tap its entry in the list. You'll see a dialog box asking if you're sure and letting you know that this trusted device unlocks your phone for everyone, not just you. If you're cool with that, tap yes and add it.
From now on, as long as your phone is connected to that Bluetooth device, the screen will stay unlocked and a tap of the power button will wake it up and you're ready to go. Do note that things like the periodic "refresher" you get to enter your login credentials is still active, but that's for your protection. Stay safe.
Adding a trusted NFC device
This is just as easy as adding a trusted Bluetooth device, and is a little more secure. Your phone doesn't stay unlocked when connected to a device in range — it uses an NFC device to act like a password or a key to unlock your phone.
Head back to the Smart Lock settings and tap the trusted device item. If you already added a Bluetooth device — let's say your car stereo — you'll see it listed as well as an option to add another one, Tap the add trusted device listing, and this time choose NFC.
You'll come to a screen that tells you to tap your NFC device to your phone, so go ahead and do that. Give things a second or two so they can talk to each other, and you're done. From now on anytime you want to unlock your phone all you need to do is tap it to that same NFC device and it will unlock. You will need to tap the power button and turn the screen on first, so keep that in mind.
You can add multiple trusted devices, and they all work at once. That means if your phone is set to be unlocked whenever it's connected to your watch, you won't need to tap it to an NFC tag when it's connected.
Again — remember that your phone stays unlocked when it's connected to a trusted Bluetooth device no matter who is holding it, and anyone with with NFC device can unlock your phone by using it — there is no secondary user authentication at play. Still, using trusted devices is convenient and a much better alternative than not locking your phone at all!