Prologue: Some time ago, in an hot summer night, was surfing the web to find a suitable car power adapter for my Raspberry Pi and couldn't find what I was looking for; there were some options, but none fit my design (without substancial hacking or a ton of wiring)... so I designed this board. A programmable power board 12v->5v with some extras.
The problem: I needed a way to power a Raspberry Pi and a 7inch display in my car, a way to sense ignition key and power On/Off the screen and, in turn, cleanly shutdown the Pi and the buck converter.
Being it a Dash/CarPc I needed to keep startup times reasonable and avoid continuosly power on/off the Pi (thinking of a normal car usage, start in the morning, go to work, get back and stop at the mall and so on)...
The solution: I made a tiny board with a 12v->5v buck converter rated 3a@5v, an Attiny85, a separate linear voltage reg for the Attiny, an RTC DS3231, a backup battery (for RTC), and a 25mm fan.
How it works: The Attiny85 senses ignition switch, car battery voltage and Raspberry Pi status (Powered On, Powered Off or Screensaver), then (based on a Python script on your Pi) determines to shut off the buck converter or keep it on.
The board design frees almost all the gpio pins as Attiny and RTC relies on i2c to communicate back and forth with Rasberry, Fan is on the same 5v bus as the Pi, so It's ON whenever the Buck Converter is on.
For quiet operations and non stressing applications I choose a 12v fan working on 5v, for more demanding uses It's best to use a 5volt one at full speed. It could also be tied on a GPIO pin to enable PWM control based on processor temp.
The Attiny85 senses ignition switch, car battery voltage and send data to the Pi, which, in turn sends its status (Powered On, Powered Off or Screensaver).
The Raspberry, based on a Python script on your Pi desktop determines to stay on, on screen saver or to shut down; in case of shut down sends a command to the Attiny to shut off the buck converter to minimize battery consumption. A battery voltage threshold can be used to trigger also a clean shutdown before battery goes flat.
Risks and challenges
I have absorbed most of the project risks by shouldering the financial and engineering uncertainties on my own already, as a proof one PiPowerPack is fully working in my car since 4 weeks, never missed a beat, a second one is on my Kodi Pi, on 24/7 since august.
Other risks could come from the supply chain, but I'm 100% confident on my PCB maker (serving me since 5 years and never had a single problem on quality and timing) and local parts supplier.
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I feel confident that I'll ship (in case of funding) by the given timeframe.