Have an older phone or tablet that you don’t use any more? You can now turn it into a secondary monitor for your PC using Deskreen. It is a free and open-source desktop app, which lets you use any device with a web browser as a second screen over Wi-Fi. It comes with various versions for Windows, Mac and Linux. Go and get Deskreen here before we get started.
How to convert your mobile or tablet into a second PC monitor?

Launch Deskreen on your computer, you will see a QR code with the connection IP address below it. Connect the other device, tablet or phone, to the same WiFi as your computer. Scan it with the tablet or phone, or manually type the web address in your browser running on the secondary device.
You will see a pop-up in Deskreen on your PC, showing the IP address, session ID, device OS, browser information. Click on Allow button to start your session. Deskreen will now ask you to select what screen source you want to share: the entire screen or sharing an application window.

Once you are selected the screen source, you will have the option to preview it on your computer. If everything is configured properly, hit the Confirm button at the bottom to start sharing your PC screen with your Android phone, iPhone, iPad, Android powered tablet, another PC or Mac, or even smart TV.