LiPoPi Power Up / Power Down version
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This version builds on the original and allows the push button switch to both power up the Raspberry Pi,
as shown already, as well as trigger an orderly shutdown of the Pi when it is pressed once the Pi has booted.
This uses a Python script that is run as a service from *systemd*, the Linux init system that starts up certain processes when the system boots.
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###How It Works
The Low Battery signal path is identical to the LiPoPi original and the Power Up path ony differs by the addition of a 1N4001 diode between
the switch and the Enable pin. This path relies on GPIO 14 being pulled high once the Pi is running. The diode prevents this from pulling GPIO 18 high.
The Power Down path pulls GPIO 18 high when the pushbutton switch is pressed. The Python script reacts to this trigger and initiates a system shutdown.
So what are the two 1N4001 diodes in series doing ? The Battery can have a voltage from around 4.7V when fully charged down to 3.2V when discharged.
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