Raspberry Pi Weather Station - part 3

Next I wanted to talk about the hardware for the sensors my weather station will employ and how each sensor is wired to the Pi's GPIO pins.

BME280
As I previously said the BME280 can be access via either SPI or I2C.  I decided to use I2C.  In order to wire to the Pi you will need to connect 4 wires.  The first is the power wire which connects to the 3.3V pins on the Pi.  The second is ground which connects to one of the ground wires on the Pi.  And lastly are the two I2C data lines which get connected to the I2C pins on the Pi.  Wiring this sensor up is not hard, it just requires a little bit of soldering and time.

The rain, wind speed, and wind direction sensors were a single package and clearly not designed specifically for the Pi.  The output wires have RJ11 connectors (telephone connectors) on the end of them.  I believe they were originally designed to plug into a specific weather module that reads the sensors.  Since I am using the Raspberry Pi as my homemade weather module, I will need to modify each sensor.

Rain Gauge
The rain gauge is an interesting sensor.  It literally has two buckets side by side.  When one side fills with water it gets too heavy and tips over.  The other side then fills up and eventually tips back.  Each time this bucket tips over a magnet passes by a reed switch.  This causes a momentary close in the switch which can be detected by the Raspberry Pi.  This only requires 2 wires, power in and the signal detect line coming out.  For power in I used the same 3.3V pin as the BME280 sensor, and for the signal wire I picked any available GPIO pin.

Wind Speed
For wind speed there are 3 cups that are free to spin as the wind hits them.  Each time it spins another magnet passes by a read switch.  As with the rain, the Pi can detect these events and use that to calculate the wind speed.  Again, only 2 wires are necessary, one for power (the same 3.3V pin) and a signal detect line which is any available GPIO pin.

Wind Direction
The wind direction sensor works using magnetic reed switches, but unlike the previous sensors it contains 8 reed switches.  Each reed switch corresponds to a direction; N, NE, E, SE, S, SW, W, and NW.  As the magnet moves around according to wind direction it closes different switches which allows you to determine direction.  To complicate matters, it is possible for the magnet to close two neighboring switches at the same time.  This gives you a total of 16 possible bearings.

Reading the wind direction was by far the hardest.  The problem is the wind direction sensor is designed to be read using analog and only has 2 wires, power in and signal out.  So if you apply 5V in then the output voltage will be one of 16 different values between 0 and 5V.  The Raspberry Pi however cannot read analog values, it only reads digital signals.  One possible solution is to use an analog to digital converter which requires additional hardware.  But there is a cheaper way, hard wire the 8 reed switches directly to the Pi.  To do this I had to open the wind direction sensor and remove the circuit board.  On this circuit board are 8 small surface mount resistors which had to be desolder.  In their place I soldered 8 wires, one for each switch.  Lately I soldered a power wire to the outside ring which ties all reed switches together.  This allows me to provide power in (again, 3.3V) and read each sensor one at a time using the 8 signal wires I attached to available GPIO pins.  This required a lot soldering on a small circuit board, but the end result is a digital wind direction sensor that was originally designed to be analog.