Pi Zero 2W

I want it to be small. As small as reasonably possible. As such, have decided on a Raspberry Pi Zero 2 W. You could easily argue that this project is better suited to a micro controller like an Arduino Nano or Pi Pico, but I want to learn about Single Board Computers (SBC). I'm guessing it'll be more accessible to start with.

DS3231 RTC

Of course the system needs to track when the power is on and when the power is off. I considered various ways to do it with battery backup, detection of active power, etc. Ultimately I decided to try and keep it as simple as possible.

Current idea is to save a heartbeat to disk (e.g. every minute save a timestamp to a file). When the system is on, the heartbeat will happen. When the power fails, it won't. Loss of power detected!

The problem is that the Pi won't know the time when the power comes back. Ordinarily computers use Network Time Protocol (NTP) to sync time on boot. If I do that, then the user needs to configure Wi-Fi somehow. I think for simplicity (#2) this is a non-starter.

The solution is to use a board with a battery that can very accurately save the time and update the Pi when it boots. These batteries should run for 3+ years, even further simplifying the problem for now.

PAM8302A

The system needs to alert you if there was a power issue. An audio alert is more effective than a blinking light. From what I can see, the Pi Zero doesn't have the ability to play audio without a break out board or some additional electronics, and this appears to be a mature easy to use product.

Future me discovered that this doesn't work for my intended purposes. This is documented in Hello Sound.

8Ω 1W Speaker

There are too many options around these little speakers that go with the audio board. The PAM8302A connects to the pi, and the speaker connects to the PAM8302A. I'm not clear on the differences between these options yet. I'm just going to order one or two and see what happens.

Push Button

The system needs a button to disable alerts after a power outage. Two buttons are better than three; one is better than two. Ideally, there would be none, but a single button keeps things simple.

OLED Display

Thinking about usefulness (#1), and scope (#3), we can get away with just the audio board (PAM8302A). To meet minimum requirements we could use a Text To Speech (TTS) library on the Pi and provide a statement such as "Severe power outage detected on January 10th lasting 4 hours." or something like that.

As a stretch goal I want to add a display to provide useful information that would be cumbersome with just TTS. The push button can serve a dual purpose to minimize additional requirements. The display could have different "screens" of information. A single press on the button would change screens, and a long press would activate the option on the screen. One of the screens would provide the ability to clear the alert, or maybe the first button press clears it.

I looked at several different screens, such as Freenove I2C IIC LCD 1602 Module (16 chars x 2 lines), as well as some small OLED displays that offer more flexibility. I think I'll defer the decision. Maybe order a few options and play with them once I begin to make progress on the core requirements.

Heatsink

From what I can tell it's advised that you apply a heatsink to the Pi.

My hope is that we can create a smaller overall package without the Heatsink. If this is true, to minimize heat we can try underclocking the CPU and reducing cores. This should be feasible since the Pi Zero 2 W is overpowered for this project.