UPSLite

Start on external power

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.

Circuit schematic

Sample implementation

BOM

Ref	Qty	Part
R2	1	Resistor 1.8 kΩ
T1	1	NPN Transistor S9018
-	1	Female Header 4 Pins
-	-	Silver Wire

UPSLite Service

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>

Installation

• Move UPSLite.py to /usr/local/bin/
• Move UPSLite.service to /etc/systemd/system/
• To run, execute
  sudo systemctl start UPSLite.service
sudo systemctl enable UPSLite.service