This all started when we moved to the country and getting the mail became a war of who/when/how far. We run a business and getting checks deposited in a timely manner can mean the difference in making payroll or not. So there's a vigil of constantly walking the 1/4 mile to the mailbox to see "if" the mail had arrived. My wife wore me down, and I finally finished the project. It seems to work really, really well, and I solved many of the challenges the vast majority of all other solutions overlooked. Namely:
- door open/closed monitoring is ineffective. Both my wife and I put outgoing mail in the box, sometimes multiple times a day. this results in false triggers.
- how to discern between outgoing mail and legitimate delivered mail?
- we have a big mailbox. it holds several packages sometimes. how do you detect a small letter in a big box?
- the box is 1/4 mile walk. I'm not keen on loRa long range solutions. They over complicate things.
- Batteries are challenging. Standard Lithium batteries wont work when its below freezing which it is here 6 mos out of the year.
So,those are the PRDs and challenges. I'll address each one.
I decided to use tof (time of flight) laser distance sensors. Ive had really good luck with them measuring pellet levels in both my pellet stoves. Initially I tried to measure from the top down. this proved impossible as the resolution wasnt good enough to detect a post card or single letter. But placed in the bottom so they are covered when there mail and using more than one sensor for a big box proved to be the answer here. These sensors draw some power so I had to figure out a way to put them in standby. Easier said than done.
Luckily our route uses the flag on the box to indicate outgoing mail only, so putting a sensor of the flag solves this issue. Flag upp, theres outgoing mail. Flag down, there is "potentially" incoming mail.
I can barely get a wifi signal on my iphone standing at my mailbox. Choosing an ESP32 with an external ant connector and using a high gain ant proved the right solution. I have 100% connect rate during each wake cycle.
Lithium ion batteries don't like to charge when it's below freezing. LifePO4 batteries do, all the way down to -40F. So I use 2 single 1000ah LifePO4 cells in parallel. After over 2500 cycles they've dropped from 3.4v to 3.29v. So, really quite good. Time will tell on battery life, but I have a few tricks here that really make a huge difference in extending battery life. Im also planning on adding a 5v/5w camera solar charge panel as soon as the LifePO4 charge modules are available again from Adafruit.
I might even add a camera inside so the wife can see all the packages she's gotten from Amazon. Oh wait... That's for me :)
Ill expand on each of these areas in the Discussions sections as they all warrant much further deep dives. https://github.com/jazzmonger/Smart-Mailbox/discussions