Being a active Ebay shopper I often end up compulsively checking my (physical)mailbox. I wanted a system that would tell me if it was worth the journey.
Made this circuit and mounted it in the mailbox:
Once the switch closes momentarily the ESP8266 module is powered. The first thing the program inside the module does is to write GPIO2 low and thus keeping the K2989 (N-MOSFET) closed.
When activated the ESP module connects to my WiFi and sends a message to ifttt.com which in turn send me a message of my choosing.
After the message is sent GPIO2 is written high and power to the module is cut. This way no power is consumed from the battery until the next activation.
I use 3 1.2V rechargable Ikea batteries