How to make a $30 internet connected door alarm

We run a vacation rental and have always wanted a simple way of knowing when our guests have checked-in. Some owners have a security system or electronic door lock that notifies them. But these often come with a subscription fee and seemed overkill for our purposes.

What I'll describe in this post is how we built a simple device that sends us an email each time the front door is opened or closed. We then use gmail filters to file these emails away most of the time. When we are waiting for a check-in we adjust the filters to bring our attention to it using the LED gmail notifier that I posted about previously.