Solar Powered Weather Station

High level overview

• My pi-yadl project is used to gather and graph the data from the following types of sensors:
  • Argent Data Systems Weather Sensor Assembly contains a wind vane, anemometer, and rain gauge. The wind speed average and gusts over the last 2 minute, 10 minute and 60 minute periods are logged. The amount of rain over the last 1 hour, 6 hours, 24 hours, and since midnight are logged.
  • Supports various types of temperature and humidity sensors: DHT11, DHT22, DS18B20, TMP36 (analog).
  • Supports the BMP180 pressure, altitude and temperature sensor.
  • Battery charge level is read via an analog to digital converter (ADC).
• For the wind vane, wind speed and rain gauge, the pi-yadl program is ran as a daemon in the background so that it can continuously monitor the rain gauge and anemometer via interrupts. The values from these three sensors are written out to a RRD database and JSON file every 30 seconds. The wind speed values are updated internally every second so that it can obtain the correct wind gusts.
• The other sensors are polled every 5 minutes via a systemd timer and written to various RRD databases and JSON files.
• The RRD databases are used to show the historical readings and are recreated at the beginning of each hour.
• The web page uses Javascript and JQuery to download the various JSON files to provide a dashboard showing the current sensor readings. The web page checks for updated JSON files on the server every 10 seconds.
• The nginx webserver runs on the Pi and only needs to serve out static content. Everything runs on the Pi; no third-party services are required.
• Optional ability to publish the sensor readings to Weather Underground. You can view my weather station on Weather Underground.

Hardware Information

Project Box

I used this project box on Amazon. I was initially a little concerned about the quality of the seal on the box but it has held up so after being outside continuously since June 2016.

All of the external sensors are terminated with a RJ45 connector to make it easy to remove the project box without having to bring the entire weather station inside. RJ45 waterproof cable glands are used on the project box to get the connections inside the box. The wire for the solar panel enters the project box using a PG-9 cable gland.

The box is mounted to the top of my fence using hose clamps with some rubber stops that were purchased at a local hardware store.

Solar and Power Setup

This weather station runs on a Raspberry Pi Zero running the latest Raspbian Testing Lite. All of the hardware is powered by a 6600mAH 3.7V lithium ion battery that is charged using a 6V 9W solar panel. The 3.7V is converted to 5V using a PowerBoost 1000. The solar panel is attached to the top of the project box using several large pieces of Velcro. More information about the solar setup can be found on Adafruit's Website. Be sure to connect the PowerBoost 1000 to the battery charge output pins; not to the load terminal. This is because the solar panel can put out 6V however the PowerBoost can only accept a maximum input voltage of 5.5V. See this post on the Adafruit forums for more details. There should not be anything hooked up to the load terminal on the charger. I fried a Pi Zero and a PowerBoost 1000 on a bright, sunny afternoon with the PowerBoost hooked up to the load terminal.

To reduce the power usage of the Raspberry Pi, the LED and display on the Pi was disabled. powertop --auto-tune was used to enable other power saving features. See the files systemd/power-savings.service and bin/power-savings for details. The power requirements could be reduced even further by desoldering the various LEDs on the solar charger and PowerBoost 1000.

I used a USB Charger Doctor to roughly measure the power utilization of the entire weather station at 140 mAH with just the wind / rain collector running in the background and 200 mAH when the main collection processes runs every 5 minutes for just a few seconds. This is with a USB WiFi dongle running the entire time. I would expect to get around 35 hours of usage on a fully charged battery without any kind of backup from the solar panel. See Power Disclaimer section below for more details.

The end of the solar panel is terminated with one of these waterproof cables to make it easy to detach the solar panel from the box. This would also make it easy to run an extension cord outside and plug the unit in to charge without removing or disassembling the project box if there were several days in a row of very cloudy weather.

I mounted this waterproof on / off switch on the project box. Two wires from the switch go to the run pins on the PowerBoost 1000. I initially had it connected to the run pins on the Raspberry Pi Zero but I ran into a situation where the weather station battery got very low and I could not reset the Pi using the power button on the outside, even after the battery was fully charged. Being able to completely power off the Pi itself resolved that issue.

Sensors

A MCP3008 analog to digital converter was soldered onto a solderable breadboard. The ADC is used for the wind vane and obtaining the voltage levels from the battery and PowerBoost 1000. This ADC communicates with the Raspberry Pi using the SPI bus.

The wind vane supports reporting 16 different positions by using a series of reed switches and different size resistors. The wind vane is connected to the ADC and the voltage indicates the direction. For example, according to the data sheet, 0 degrees (N) is 3.84V; 45 degrees (NE) is 2.25V; and 90 degrees (E) is 0.45V. I had an issue with getting accurate readings from the wind vane between 270 and 337.5 degrees that was caused by having the reference ADC voltage in software set to 5V instead of 5.1V when converting the value read from the ADC to millivolts. Adding the argument --adc_millivolts 5100 to the yadl binary fixed the issue.

The anemometer is hooked up to a GPIO pin on the Pi. According to the data sheet, one click of the reed switch over a second corresponds to a wind speed of 1.492 mph (2.4 km/h). A pull down resistor is used and the switch is debounced in software. One complete revolution of the anemometer will cause the pin to go high twice.

The rain gauge is very similar to the anemometer. Each click of the switch over a second corresponds to 0.011 inches (0.2794 mm) of rain according to the data sheet. This also uses a pull down resistor and the switch is debounced in software.

The Stevenson screen (left side of the above picture) contains the temperature, humidity and barometric pressure sensors. A 3D model of the screen was downloaded from https://www.thingiverse.com/thing:158039, 3D printed using HIPS plastic and spray painted using flat white paint.

The temperature and humidity is obtained using a DHT22 sensor. The dew point is calculated using the August-Roche-Magnus approximation. The DHT sensor communicates with the Pi over one of the GPIO pins.

A BMP180 sensor is used to obtain the barometric pressure and communicates with the Pi over the i2c bus. The BMP180 driver came from this project.

The PIN_LAYOUTS.md file contains the pin layouts of the cables that leave the box.

Power Disclaimer

I have received several emails from people asking about using this setup unattended in remote locations. The Pi Zero draws far too much power for the size battery that I used to be used unattended in some far away location. At the time that I wrote this (Nov 2016), Winter is approaching in the Northern Hemisphere, and the solar panel did not completely charge the battery with the shorter days, especially with all of the cloudy days in the area that I live. I currently have an extension cord running from my house to the weather station outside. At some point, I'm planning to get a large 12V battery, a larger solar panel, associated charger, and a 12V to 5V buck buck converter to power the weather station. The other alternative is to go with some low powered microcontroller (such as an Arduino or one of the numerous clones), however none of the code that I have here will run on that platform.

Installation

• Note: This project iniitally started out using Raspian based on Debian Jessie, but the latest version of Raspbian Testing is required for the newer version of rrdtool that supports the --left-axis-format argument.
• Clone this repository and the pi-yadl repository.
• Follow the installation instructions to compile the pi-yadl project.
• Update the paths in this repository's systemd service files.
• Run bin/create-rrds.sh <path to web/ directory> to create the initial empty RRD databases.
• sudo make install
• sudo apt-get install jq nginx
• sudo ln -s /path/to/web/directory /var/www/html/weather-station

Contact

Brian Masney [email protected]