Raspberry Pi Smart Doorbell: Who's at My Door?

This project is intended to replicate some of the smart doorbells that are on the market. This version will detect when someone has touched the doorknob or doorbell and either tweet, text, or email a picture of the person at the door. The photo will be taken with a Raspberry Pi camera through the door viewer (peep hole). I am going to post the hardware instructions and some basic code and then your code will vary depending on whether you want to email, tweet, or text the results to yourself. There are many different ways to do that and I cannot cover them all here.”

Link