How to use an Android device and NFC tag to turn on a Windows PC
First, open up your Windows Network and Sharing Center. Click Change adapter settings. Right click your active internet connection and select Properties. Click the Configure button to the right. Go to the Advanced tab, look for the option "Wake on Magic Packet", and set its Value to Enabled.
Next, you'll need to get your hands on your computer's IP and MAC addresses. No, you won't be hacking your own PC or anything. You'll just have to run a few simple commands. Hold down the Windows key and press the R key on your keyboard to launch the Run dialog box. In it, type "ipconfig /all" without the quotes. You'll get a command prompt window with a bunch of network data. If the window disappears too quickly, press the Windows + R keys again, type "cmd", hit Enter, then type "ipconfig /all" in the command prompt. What you need to write down are the Physical Address, IPv4 Address, and Subnet Mask values. Keep them near, you'll need them later. Also, your computer has to be on a Wi-Fi connection.
Now, install the following apps on your Android device:
Tasker - download from Google Play. This will be used to automate the processes. The app usually costs $2.99, but you can get a .apk install of a 7-day trial version from here. Install it on your device like any other apk.
Trigger - download from Google Play. This app is used for reading and writing to your NFC tag. It's a free download.
WoL Wake On Lan Wan - download from Google Play. This app sends the command that powers your PC on. It's a free download.
All set? Let's move! Open WoL on your smartphone, and tap Add New button. Type "Home" in the Name field, then type your MAC and IP addresses in the designated fields. Tick the box next to "Send as Broadcast", and enter your Subnet Mask address. You're done! Tap Save and move to Tasker.
In Tasker, tap the three-dot menu button and hit Preferences. Go to the Misc tab and tick the Allow External Access box. Then, go to the Task tab, and tap the + button. Name the task Wake and press the checkmark icon. Tap the "+" button at the bottom, select Plugin, and choose WoL Wake On Lan Wan. Hit the pencil-like icon at the top, and select your Home WoL profile. You can now exit Tasker.
Finally, here comes the NFC tag. We'll be using Trigger to write the sequence in it. Open the app, and tap the + button, then tap the next + button. Choose NFC, hit Next, and tap Done. Now, hit Next, tap the + button, select Tasker Task, and tap Next. On the small pop-up window, tap the magnifying glass-like icon, and select the Wake task we created a few paragraphs ago. Tap Add to task. Hit Next, then Done, then get your NFC Tag. You will be writing the task on it, so place your device on the tag, and tap Done when the writing ends.
Wheee-whew! Not that you've gone through all this, you should be able to power on your PC by placing your Android device on the designated NFC Tag, with the display switched on. Find a clever spot for it and enjoy the little piece of the future. If you have any problems, first, check out whether "Wake On Lan" is enabled in your PC's BIOS. Hopefully, this will solve it.