As an online shopper, I need to regularly check my mailbox, 5 floors separated from home. It is really annoying to check whether a new delivery arrives.
I came out with this mailbox checker solution that should draw low power (less frequent battery replacement, >6 months) and small (portable). To detect mailbox door activity, I use a reed switch connected to reset pin (RST) of a low-powered microcontroller (I choose Arduino Mini Pro 328 board that runs on 3.3 Volts and has no USB-RS-232 converter).
In regular mode, the controller is in a deep sleep state. Every time a new package delivers, the door will be opened and trigger RST to HIGH. Consequently, the microcontroller wakes from the sleep, sending a signal (a message or missed voice call) to GSM gateway via the attached modem. Since the state is binary (1=new mail), a missed call is enough. Then it goes deep sleep again. In that mode, it draws only tens of microampere. Theoretically, 2xAAA battery should last > 1 year.
I benefit my unused Android phone as the GSM gateway. Sendit MQTT is the application to convert SMS or call signal to MQTT, that is integrated into home-assistant in the smart home network, the “signal” then triggers its notification modules (Twilio, pushbullet)