Notes

Starting the Pi

Enough about shutting down the computer. What about starting it up?

The Issue

You can’t just let the controller power up the computer. The problem is that the supply voltage will move up and down until there is enough charge in the battery to fully supply the computer. When the computer turns on (connecting a full load), you will pull the battery down hard enough to brown out the computer causing the Raspberry Pi to crash. This constant rebooting cycle can corrupt and ruin your SD card and cause your computer to never boot at all, even when the power is restored. We had this VERY thing happen to us 3500 miles away with Project Curacao. Arduinos are more tolerant of this, but Raspberry Pi’s do not like a ill-behaved power supply. You just can’t be sure of what state the computer will power up at without a good power supply.

This issue can be handled in a number of ways. The first is to use another computer (like an Arduino made to be very reliable by using a WatchDog - see the Reliable Computer series on switchdoc.com - http://www.switchdoc.com/2014/11/reliable-projects-watchdog-timers-raspberry-pi-arduinos/) to disconnect the Raspberry Pi’s power through a latching relay or MOSFET when there isn’t enough power. Project Curacao (http://www.switchdoc.com/project-curacao-introduction-part-1/) used this approach.

We didn't want to add an additional computer to WeatherPi, so we chose a second solution.

Power Your Pi Up and Down with the USB Power Control

A second (and cheaper!) way of handling the brownout and power up problem is to use a dedicated power controller that will shut the power off to the Raspberry Pi and restore the power when the battery voltage is high enough to avoid ratcheting the supply voltage up and down because of the load of the Raspberry Pi. This is called Hysteresis. We have designed a board to do just this (called the USB Power Controller) that will plug between the USB coming out of the SunAir Solar Power Controller and the Raspberry Pi as in the picture to the right.

The USB Power Controller Board

The USB PowerControl board is a USB to USB solid state relay.

Anything you can plug into a USB port can be controlled with USB PowerControl. It's easy to hook up. You connect a control line (a GPIO line or the output of a LiPo battery) to the LIPOBATIN line on the USB Power Control device and if the line is LOW (< ~3.3V) the USB Port is off. If it is HIGH (above 3.8V) the USB Port is turned on and you have 5V of power to the USB plug.

There is a hysteresis circuit so the board won't turn on and then turn immediately off because the power supply is yanked down when the computer turns on (putting a load not the battery).

There is little software for this device. You connect it directly to your LiPo battery for automatic control! The only software used detects the battery voltage and decides when to shut down the computer. The USB Power Control takes care of shutting the power to the Raspberry Pi when the battery voltage gets low enough. Note that a shutdown Raspberry Pi still draws current (according to one quick measurement, about 100ma).









AirBox standby : 92~93mA
AirBox operating : 156~180mA
Sunny day and under the shadow : 20 ~ 40mA (average : 30mA)
Sunny day and under sunshine : 200 ~ 500mA (average : 350mA)
Soloar Panel : 3W
Battery : 5100mAh, 3.7V
Li-Po Rider Pro

C:UsersLawrenceDesktopmosmosquitto>

mosquitto_sub -h 52.193.146.103 -p 80 -t client/200000018/200000018-GIOT-MAKER -I 200000018-generic-service -u 200000018 -P 72498339 M

under lamp max ~ 6-7mAh