Taking a trip to the mailbox in winter or rainy weather can be a hassle, more so when you haven’t received any, making it a wasted trip altogether. On the other hand, imagine waiting on a PCB order for a project that’s time-limited and not knowing they have been sitting in the mailbox for a few days. Electronics engineer Zak Kemble knows those issues all too well and designed a wireless mailbox notification device that lets him know the exact moment a parcel has arrived.
Kemble designed his Remote Mailbox Notifier around Ai-Thinker’s A9G GSM/GPRS module, which also functions as a listening bug and GPS tracker as the module comes equipped with a GPS receiver. On its own, the A9G draws too much power as a standalone remote unit for Kemble's project (even in shutdown mode @ 240uA), so he paired it with an ATtiny402, which handles power management. Additionally, the ATtiny402 detects when the mailbox door opens and closes (via magnet) when the GPS tracking mode button is engaged when battery charging is completed, and making sure there’s enough juice before powering on the A9G.
Powering the Remote Mail Notifier is a rechargeable 10440 (380mAh) Lithium battery, which splits its power between the A9G (3.5V to 4.2V) and ATtiny402 (1.8V to 5V). Kemble added a BME280 environmental sensor as well, which he admits should have been mounted outside of the plastic case, as it only renders an accurate reading at initial startup as the internal temperature rises and humidity falls during normal operation inside the enclosure.
New mail is detected by a reed switch and magnet attached to the mailbox door, and when the connection breaks, it engages the ATtiny402, which in turn, powers up the A9G that sends a Telegram notification via GSM network to the user. It also sends other notifications, such as when battery charging is completed, and status updates. Kemble has uploaded a complete walkthrough of his Remote Mail Notifier on his blog, complete with code and schematics for those looking to recreate his build.