WeatherPi Solar Power System Performance
WeatherPi is a do-it-yourself Raspberry Pi based Solar Powered Weather Station. The solar power system consists of two Voltaic Systems 6V solar panels connected to a SunAirPlus solar power controller/data collector. The weather sensors, databases and displays are done by the Raspberry Pi.
This posting talks about how the shut off and turn on system for the Raspberry Pi running outside in the sun.
Solar Powered System
As you can see in the bock diagram to the right, the solar power panels and battery charging are done by the SunAirPlus board, while the monitoring of all the currents and voltages are done by the Raspberry Pi reading the embedded I2C INA3221 3 channel ADC. SwitchDoc Labs also provides a breakout version of the INA3221.
One of the critical design aspects of the WeatherPi is how to turn the device on and off in the case of not having enough Solar Power to run the device 24/7. This is called brownout and it happens with virtually every stand-alone solar power system. Read more about solar power brownout here.
USB PowerControl
Turning on and off the Raspberry Pi is handled in WeatherPi via the SwitchDoc Labs USB PowerControl 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 and if the line is LOW (< ~3.3V) the USB Port is off. If it is HIGH (above 3.7V-3.8V) the USB Port is turned on and you have 5V of power to the USB plug.
The input to the board was designed to come directly from a LiPo battery so the computer won’t be turned on until the LiPo battery was charged up above 3.8V. We provide 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). This really happens!!!! You kill Raspberry Pi SD Cards this way.
We monitor the battery voltage using SunAirPlus and when the LiPo battery voltage goes below 3.5V we shutdown the Raspberry Pi via software. When SunAirPlus and the solar panels charge the battery above ~3.7V, USB PowerControl turns the Raspberry Pi back on and the Raspberry Pi is back on the air. We also use a Dual Watchdog board to watch out for Raspberry Pi lockups which we will talk about in another article.
RasPiConnect Control Panel Graph
The RasPiConnect control panel shows the wake up and shutdown cycle. It works perfectly! We placed the WeatherPi system in a position that makes sure it doesn’t quite get enough sun to run 24/7 to fully test this system. Next it goes up on the roof where it will get lots of Sun. Yes, the rain gauge has gone crazy. We have had no rain, but lots of wind.
