Pseudonymous maker "Psychological_Cat_20" has put together a wire-free device, which makes use of an infrared (IR) distance sensor to detect when something's been delivered to a mailbox — ideal, the maker says, "for lazy people."
"I had this IR distance sensor laying around for several months waiting for a quick and dirty project (you know those [projects which] are never quick and always dirty)," Cat writes. "Being very lazy I started to think about a smart postbox notification system which tells me when new paper mail has arrived."
The heart of the system is a Wemos D1 Mini development board, based on an Espressif ESP8266 and offering Wi-Fi connectivity to Cat's home network. An 18650 lithium-ion battery powers the device, which wakes every 10 minutes from its slumber to check the distance between the sensor and the far wall of the mailbox.
Each wake-up sends MQTT message with the measured distance to a Node-RED server running on a Raspberry Pi. "Node-RED logic checks if [the] postbox is full and triggers notification via Telegram and Alexa," Cat explains. "In order to avoid notifications each 10 minutes Node-RED sets a timer and suppresses subsequent notifications for two hours."
More details are available on the project's Reddit thread, with Cat promising to release "Arduino code and [an] STL of the case" if there's sufficient interest.