Build a human firewall with Raspberry Pi
If you're getting tired of answering the front door - only to find unwanted visitors such as energy salespeople, then this project by Leonardo Sarra, Mattia Nicolella, and Andrea Fioraldi will be of great interest.
They have devised a system that uses facial recognition to identify if someone is known to you or not when the doorbell button is pressed - with the results being sent to you via the Telegram app on a smartphone.
The system uses a Raspberry Pi setup with a camera, and various software tools to allow you to train the system to identify people you do want to see - so unwanted visitors can be rejected.
They've put together a quick demonstration in the following video:
To learn more about this awesome system, visit the creators' project page on hackster.io.