Background

I recently noticed that my mail had been tampered with.

This was around when I was expecting a new credit card, so I was worried. I knew I needed some protection against tampering or theft.

There are lots of options to protect your mail, like having important mail sent to a local drug store for ID pickup or forwarding your mail to a PO box, but having to drive somewhere else every time you want to pick up your mail can be a real hassle. Instead, why not notify yourself whenever your mailbox is open and pick it up soon after? I thought of a quick solution to make sure I get my credit card before the thief does...

There are a lot of mailbox sensors out there, but this one was designed to be discreet: I didn't want to make the mailman/mailwoman feel uncomfortable seeing a sensor staring back at them, and I didn't want the potential thief to see it and take it. The mailbox quietly sends me a notification whenever it is opened and I run over to retrieve the mail before anybody else can.

Summary

I sense the door opening using a hall effect sensor connected to a NodeMCU. The data is sent to AdafruitIO via WiFi and an IFTTT applet responds to each update by sending my phone a notification. This was the easiest and cheapest way I could find to get a connected mailbox up and running.

Details

• Everything is on the underside of the mailbox, out of sight.
• A magnet on the mailbox door moves toward the hall effect sensor when the mailbox when it is opened.
• A battery-powered NodeMCU connected to my wifi detects the hall sensor change and sends it to a "feed" on Adafruit IO. 
• An IFTTT applet connected to the Adafruit IO feed responds to the update by pushing a notification to the IFTTT app on my phone.
• If the battery is low, a separate IFTTT applet sends me a notification using the same pipeline as above.

Results

This is a quick-and-dirty project to get the job done, so I used whatever was lying around my house to make it work.

Construction

I used a metal L-bracket and a small piece of wood to fix the hall effect breakout board to the mailbox post as shown below.

All the electronics were attached to a piece of plywood that I drilled to the bottom of the mailbox. The breadboard and NodeMCU are attached via standard pcb standoffs and the battery with tape. I used more tape to conceal all of the wires below the mailbox, since they were a little long.

You can see above that the magnet approaches the hall sensor when the mailbox door is open. The sensor itself has a large hysteresis band to avoid reporting a bouncy signal. 

Connectivity

Latency

I initially expected the app would respond in seconds, but I quickly found that unrealistic. The delays are approximately (NodeMCU -> AdafruitIO = 2s), (AdafruitIO -> IFTTT = up to an hour), and (IFTTT -> push notification = a few seconds. The largest variance is between AdafruitIO and IFTTT. After a little [online research](https://www.quora.com/Why-is-IFTTT-so-slow-to-react-to-triggers), it seems that IFTTT uses polling, which can take on the order of 15 minutes. The trick is to use non-polling mechanisms, like "webhooks." That will be my next step.

Notes

• IFTTT sometimes missed events from AdafruitIO. I don't know which side the bug was on.
• Power: my first system was a battery hog. I went from a VCC of 6.3 to 5.8 V in 30 min. I could tell that the bulk of the power consumption was from the ESP8266 itself, since the module was warm. I needed a way to poll the sensor at 2 Hz (at least) while burning less power. My solution was the following state machine:

• The hall sensor interrupt required a circuit that would generate a pulse on...