The MagPi - July 2018

raspberrypi.org/magpi May 2018 75

BUILD A WEATHER STATION Feature

raspberrypi.org/magpi July 2018 75

CONNECTION

Anemometers normally have two wires.

Connect one to a GND pin, the other to

GPIO 05. If using the RJ11 connectors,

the anemometer uses the middle two

wires of the cable: normally pins 3 and 4

on RJ11 breakout boards.

TEST WITH CODE

Open IDLE, create a new Python file,

and save it as wind_speed.py in the

/home/pi/weather-station directory.

Add the code from the wind_speed.py

listing. Save and run your code. Test

it by manually turning the arms of

the anemometer.

STEP 3

WIND SPEED

The anemometer produces two signals

per spin, so you can count the number

of full rotations of the sensor by

halving the number of detected inputs.

This can then be used to calculate the

wind speed:

speed = distance / time

To calculate speed, you need to know

the distance travelled in a certain

amount of time. Measuring time is

fairly straightforward, and you can

count the number of signals over

the course of a fixed time period,

for example 5 seconds. The distance

travelled by one of the cups will be equal

to the number of rotations multiplied

by the distance around the edge of the

circle (circumference):

speed = (rotations × circumference)

/ time

The circumference can be calculated as

long as you know either the radius of

the circle. You can discover the radius

made by the anemometer by measuring

the distance from the centre to the edge

of one of the cups. Once you know the

radius, you can find the circumference

with the formula 2 × π × radius. Don’t

forget that a whole rotation generates

two signals, so you’ll need to halve the

number of signals detected:

speed = ( (signals/2) × (2 × π × radius) )

/ time

The radius for the recommended

anemometers used by the original

Oracle Weather Station is 9.0 cm, and

that is the figure which is used in the

code example. Don’t forget to change

this value if your anemometer has

different dimensions!

To implement this formula in Python,

you can use the math library. For

example, if you measured 17 signals

from your anemometer in 5 seconds,

your wind speed could be calculated like

the code in wind_speed.py.

STEP 4

WIND DIRECTION

If you look inside the recommended

wind vane, you’ll see eight reed switches

arranged like the spokes of a wheel.

These can be used to measure the wind

direction. In order to do so, you’ll need

to be able to measure the resistance

produced by the sensor and convert that

into an angle value. There are several

steps in this process, and you need an

analogue-to-digital converter (ADC).

A popular and versatile ADC is the

MCP3008 (magpi.cc/BlHlKm)

Measuring the wind direction is a

bonus project and you can read how to

set up the rotating magnets and ADC

here: magpi.cc/nZNLga.

STEP 1

STEP 2

from gpiozero import Button

import time

import math

wind_count = 0 # Counts how

many half-rotations

radius_cm = 9.0 # Radius of your anemometer

wind_interval = 5 # How often (secs) to report speed

# Every half-rotation, add 1 to count

def spin():

global wind_count

wind_count = wind_count + 1

# print("spin" + str(wind_count))

# Calculate the wind speed

def calculate_speed(time_sec):

global wind_count

circumference_cm = ( 2 * math.pi) * radius_cm

rotations = wind_count / 2.0

# Calculate distance travelled by a cup in cm

dist_cm = circumference_cm * rotations

speed = dist_cm / time_sec

return speed

wind_speed_sensor = Button( 5 )

wind_speed_sensor.when_pressed = spin

# Loop to measure wind speed and report at 5-second intervals

while True:

wind_count = 0

time.sleep(wind_interval)

print( calculate_speed(wind_interval), "cm/h")

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wind_speed.py CODE

Language: Python

Name: wind_speed.py

magpi.cc/ABckLC