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File: 1480484437079.png (63.82 KB, 300x226, turn off circuit.jpg)

No.2907

how do I know how big a capacitor to use; what purpose does the Atmel AVRtiny serve?

or where can I go to learn what is happening here? http://www.instructables.com/id/Raspberry-Pi-Shutdown-Button/

  No.2921

>>2907
After taking a quick look it seems the attiny is used to send a signal to the Pi to turn it off safely before removing the power with a relay, it basically removes the human component from switching the Pi off, this is done to avoid data corruption.
The capacitor only seems to remove the noise from the power supply of the attiny according to the circuit.
Capacitors are like a pond in a stream, the bigger the pond, the calmer the water. The same applies to electronics, a big capacitor will smooth all the ripples.
In this application anything avobe 100uF will do fine. Calculation of the capacitor value isn't really important in this case.

  No.2962

File: 1480996011328.png (4.32 KB, 200x150, halfassed power circuit.png)

you could use the pi to control its own relay or a really simple delay circuit or something if you're not dedicated to cramming an arduino in there somewhere

here's one that uses a fet and no arduino. software polls the shutdown pin, prepares for shutdown and then raises the run pin to cut power. requires a dpst switch because I cba to figure out the proper pull ups/downs.