for the Raspberry PI
The idea: have a "smart" SPDT switch that turns the Raspberry PI off by telling it to shutdown, then wait and kill the power, or turn it on by turning on the power.
Also: must be open source.
To start with the first prototype I went with breakout boards for the chips, it's a bit botched, but fits. It was a bit tricky to find pin one on the TPS22917, but with the help of Alex I was able to decide on the right pin *hopefully*. On the left side is an HT7333 - I was thinking about using the AP2112K first, but the lower quiescent current of the HT7333 and general current consumption needs of the Attiny brought me to the holtek chip instead.
At first I thought I would slap an n-channel MOSFET to an attiny85, switch low-side (as in cut off the ground of PI) and call it a day. Then @Alex said high-side switching makes more sense, but complicates things. For a start you need a p-channel MOSFET instead AND to switch it off you need an extra transistor when your Attiny runs on 3.3V to achieve the voltage level that you're switching.
I learned a lot yesterday just by talking over whatsapp with Alex, while we were kind of distracted by the live stream of the "unexpected maker". In the end I found a chip called TPS22917, that Alex waved through as a "possible/passable candidate". This way I don't need a mosfet and a transistor etc.
davedarko
Jon Davies "Woody"
James Thomas
Randy Walker