I would like to build a (snail) mail notifier. My idea is to have an ATTiny85 (unless I need more pins than the ATTiny85 has) inside the mailbox with a vibration sensor and a Tx modules, and another ATTiny85 with an Rx module in the house. ATTiny81-M is the one in the mailbox, and ATTiny85-H is the one inside the house.
The way my mailbox works, the mailman has to push the cover of the slot with the mail to put it inside, so I would glue the vibration sensor there, and the rest of the circuit where it's more convenient. ATTiny85-M is in sleeping mode until the mailman pushes the mail inside, the sensor wakes up ATTiny85-M, which uses the Tx module to tell ATTiny85-H that there's mail, and goes back to sleep. Then ATTiny85-H turns on an LED or something (I haven't decided yet).
I already know how to do all this except for one little detail: Having ATTiny85-M in sleeping mode, and only waking up to notify mail and going back to sleep. The idea is to have a a coin cell battery powering the circuit, and make it last as long as possible. I'm using Arduino IDE, and an Arduino UNO board configured as an ISP to program both ATTiny85's.
Bonus: I'd also like ATTiny85-M to notify ATTiny85-H when the battery is running low. I don't know how to do that. I thought of using a solar cell on top of the mailbox and solve all problems, but that would be too conspicuous and might attract unwanted attention.
If someone can point me in the right direction on how to have the ATTiny85 inside the mailbox to consume as little current as possible and to run off batteries as long as possible, I'll greatly appreciate it. Thanks a lot!