To start up the Raspberry Pi when the external power supply is connected,
a small circuit is necessary, which is pulling down pin 3.
A signal change of pin 3 initiates a wake up of the Pi if is halted.
Additionally the two pads on the back of the UPS must be shorted.
Pin 3 is the I²C clock line.
This means that if the external power is connected,
there is no communication on the I²C interface possible.
Therefor another pin can be used to temporary enable the I²C interface.
This enables temporary communication on the I²C interface.
| Ref | Qty | Part |
|---|---|---|
| R2 | 1 | Resistor 1.8 kΩ |
| T1 | 1 | NPN Transistor S9018 |
| - | 1 | Female Header 4 Pins |
| - | - | Silver Wire |
The UPSLite service is a small python script,
which is regularly checking the power supply status,
as also the battery level.
It will initiate a system shutdown if
- the battery capacity drops below 50%
- the voltage drops below 3.8V
- or the device is running more than 10 min on battery.
Additionally the current readings are written into:
/tmp/UPSLiteStatus.txt
in the format:
<Status (0=Battery, 1=Power supply)>\n<Voltage>\n<Capacity>
- Move
UPSLite.pyto/usr/local/bin/ - Move
UPSLite.serviceto/etc/systemd/system/ - To run, execute
sudo systemctl start UPSLite.service
sudo systemctl enable UPSLite.service