I think I have uncovered the secret of having a second monitor in Windows 10. At least, it works for me, but I’m no expert. There is just one little problem: My solution might pertain strictly and specifically to MY computer and TV set. In searching the Internet for a solution to making that HDMI connection, the “solutions” I found had no direct resemblance to what I saw on MY screen. Therefore, my solution here, while it works just fine for me, is a GENERAL solution for anyone else with the same goal, because it may or may not conform to anyone else’s PC and TV setup. Still, the GENERAL PRINCIPLES of the setup might apply. I could not have made my solution work without the general help offered on the Internet. (Incidentally, I used a 35 foot HDMI cable to make the connection.)
Here’s my setup: I have a custom built desktop PC, with the Windows 10 operating system, an Intel i7 CPU chip, and 16 GB of RAM. This PC also has an NVIDIA Geforce GTX 670 4GB video card, which has a DVI port (used by my PC monitor) and an unused HDMI port next to the DVI port. I have a 28 inch Upstar 4K monitor made in China. (Cheap, but solid.) I also have an old Sony Bravia 32 inch flat screen TV with 3 HDMI ports and built-in speakers. This TV set is located in the same room as the PC. One of these 3 HDMI ports is used by an Amazon TV Firestick dongle, and another is used by a Toshiba DVD Recorder/Player. So there was one unused HDMI port on my TV set, which was later designated as HDMI port #3
Step #1: With both the PC and the TV turned off, connect the HDMI cable from the back of your PC to an unused HDMI port on your TV. On the back of the PC, make certain that the HDMI cable is next to or close to the cable attachment for the PC monitor, to ensure that you are using the same video card for both monitors.
Step #2: Turn on the TV, and then the PC. If you are lucky, the new configuration will still allow you to see your normal screen on the PC monitor. (I mention this because the first time I tried to do this, my PC monitor went dark.) The TV screen will hopefully display HDMI #3 (or whichever port your cable is connected to) with the “No Signal” error message.
Step #3: This is the “key”. You must now get into the Windows 10 “Display” screen, as follows: Right click on the Start button, select “Settings”, and from the “Settings” screen, select “System”. On the “System” screen, select “Display”. Now, with hopefully ALL the Display options visible, the fun begins…as well as the moment of truth.
Step #4: If all is well, you should see a box, within which are smaller boxes containing the numbers 1 and 2. In my case, the 2 box was much smaller than the 1 box. The 1 box reflects your PC monitor, and the 2 box reflects your TV monitor. Just below that larger box are two options, namely, “Identify” and “Detect”. Click on “Detect”, and see if there is another option box below,
Step #5: If there is no box below, see if clicking on “Identify” will produce the additional option box. You may have to click both (Identify and Detect) boxes. This box below should have the following options:
1. (1) Duplicate these displays (i.e., display the image from monitor 1 on monitor 2)
2. (2) Extend these displays (I have no idea what that means)
3. (3) Show only on 1 (Display should be seen only on monitor 1)
4. (4) Show only on 2. (Display should be seen only monitor 2)
Step #6: Select option 1 from the list above. If all went well, you should see the mirror image on your TV set. If not, try again, but experiment and be creative. Be advised that when I successfully created the second monitor on my nearby TV set, there were still some unexpected problems. For example, I had lost the sound on the speakers for my PC monitor, although the speakers in the TV set were working fine. Also, the resolution on my PC monitor changed from its native 4K of 3840 x 2160 to 1920 x 1080, same as on the TV. Also the desktop icons were much larger; there were less of them, and they were randomly placed. There was an option for screen resolution, so I changed it back to 3840 x 2160, and that thankfully worked for both monitors. Also, when I later changed the option to “Show only on 1”, all was as it was before….i.e., normal. Now I can change to option 1 at will (Duplicate these displays), and use the TV as a second monitor whenever I like.
Good luck!