toroidalcore

So, now for some solid requirements about my solar-powered Raspberry Pi project. To recap, the idea is to have a Pi hosting a small, static website, while powered by just a solar panel with minimal storage. Enough storage to keep it running if a cloud passes over, and to allow the Pi to gracefully boot up and shut down. This Pi would also act as a 'regular' Linux server, that you could SSH into and do stuff on. In other words, not just an appliance with a read-only file system.

Super capacitors make sense here. One of the goals is to avoid using a battery, and a capacitor should have a long lifespan. The current plan is to charge a cap bank with a solar panel, and have that feed the Pi via a linear regulator. Wasteful, but at the same time simple.

I spent some time looking into the Pi's GPIO, in terms of booting up and shutting down. The idea would be something like this:

• Sun comes up, panel begins charging cap bank.
• Cap bank hits Vmin, which turns on enable pin on linear regulator.
• Cap bank hits designated high voltage, Vmax, somewhere between the panel's max power voltage (Vmp) and open circuit voltage (Voc). This boots the Pi via GPIO.
• The panel presumably puts out more current than the Pi consumes, so this stays running for a while.
• The voltage begins to drop due to overcast conditions or, you know, nighttime.
  
• The cap hits the shutdown voltage (Vsht_dn), which signals the Pi to shut down. This also starts a 1 minute timer.
  • The voltage begins to rise again, because the cloud blew over.
    • After the timer expires, the Pi can be booted again via GPIO, if it hits Vmax.
  • The voltage continues to fall, because it's night.
    
    • The cap is sized so that there's a delay of about 1 minute before the Pi drains it to Vmin, at which point the linear regulator is disabled. This keeps low voltage from damaging the Pi.

This is fairly straightforward, just required some thinking on edge cases. And, some logic stuff I'll have to sort out, but that shouldn't be too bad.

Find me at @toroidalcore@masto.hackers.town