Here is the wind vane sensor supplied with the Raspberry Pi Weather Station kit:
A wind vane shows the direction from which the wind is coming, not where it's going (this can be confusing because TV weather maps show the opposite). It works by the wind exerting force on a vertical blade, which rotates to find the position of least resistance; this position is then aligned with the direction of the oncoming wind.
The wind vane is more complex than the rain gauge or anemometer. It does use reed switches and magnets, but it works in a completely different way. Let's look inside the wind vane to see:
- Pull the top off the wind vane; it should come off without much force. On the underside, you'll again find the metal cylinder which is the magnet:
Next, take the screwdriver and remove the three screws in the base, then slide the base panel down the cable a bit to get it out of the way. If you look inside now, you'll see there are eight reed switches arranged like the spokes of a wheel. Remove the three remaining screws, allowing the circuit board to come free. Don't lose these screws.
Examine the green side of the circuit board now. This is the side that faces upward inside the wind vane, and the magnet points down onto it. North is at the top in the picture above, where the two black clips for the anemometer socket are.
Look closely and you'll see there is a ring of metal that goes all the way around the edge. There is also a smaller ring in the centre. Each reed switch connects the outer ring to the inner ring through a resistor. You'll see that SW1 (switch 1) has R1 near it (resistor 1); similarly, SW2 has R2 and so on, up to 8.
So what's going on here? Firstly, we need to understand what a resistor is. These are small components that reduce the flow of electrical current but don't stop it. At the same time, they also reduce the voltage moving through the circuit. Resistors can have different values: a low resistance value would let almost all voltage/current through, but a high resistance value would let very little through.
The wind vane works like a big variable resistor, which you can think of in terms of a volume control. Look at the schematic diagram below (a zigzag line is the symbol for a resistor). The idea is that voltage comes in on the outer ring and can take a path through any of the switches to the inner ring, which is connected directly to ground. As the magnet rotates, different reed switches will open and close, thus switching their corresponding resistor in and out of the circuit.
Each of the eight resistors have different values which you'll see printed in white text next to them; this allows the wind vane to have 16 possible combinations of resistance, since the magnet is able to close two reed switches when halfway between them. You can find more information in the datasheet.
Reassemble the wind vane now. Firstly, locate the letter N on the side of the base, then insert the circuit board with the green side facing away from you so that the anemometer socket aligns with North. Replace the three smaller screws; this step can be tricky and a magnetic screwdriver helps a lot. Next, replace the base, ensuring the knot in the cable remains inside. Finally, replace the three larger screws.
So we now understand that the wind vane is essentially a variable resistor, similar to a volume control but with only 16 positions. Resistance is something that we can't measure directly, because it's a passive property of the wind vane. What we need to do is measure something that changes as a consequence of the resistance: the voltage going through the wind vane. The voltage level passing through it will go up and down as different resistors are switched on and off by the magnet, and this is something we can measure.
This is going to be entirely different to what we have done before. With the rain gauge and the anemometer, we were working with voltage levels changing between 0 volts, meaning LOW, and 3.3 volts, meaning HIGH. Our code could only tell us if a GPIO pin was HIGH or LOW, but not somewhere in between. This is what is known as a digital signal: all or nothing, 1 or 0, HIGH or LOW. For the wind vane we need to accommodate a range between HIGH and LOW, which is known as an analogue signal.
It's important for us to understand the general concept of analogue and digital. Think of a gaming control pad like the one below. The circle is highlighting the thumb joystick and the directional pad. Ask the class which one is analogue and which one is digital.
Answers:
-
Thumb joystick: analogue
The thumb joystick is analogue because it provides a full range of motion between each direction. In a driving game, you have the option to steer gently around a long sweeping corner or hard around a hairpin, for example.
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Directional pad: digital
The directional pad is digital because each direction button has only two states: on and off (just like HIGH and LOW). In a driving game, it would be like trying to steer a car using the indicator stick: you would have full left and full right only. It would be very tricky to control!
Analogue and digital both have their place, and often one works better for a particular task than the other. For a game like a flight simulator, you would want analogue control to aim the plane, whereas for something simple like a jump, run and shoot platform game, digital control is better.
To recap: the wind vane has a voltage going through it, and this will vary according to which resistors are switched in and out by the reed switches and magnet. The challenge we face is being able to observe this analogue signal changing on a computer, which is a digital machine.
To do this we're going to use a clever microchip called an analogue to digital converter, or ADC for short. The weather station board has one of these built-in (as do most games consoles). An ADC chip, like the one above, has a number of input pins. One of them is connected to the voltage going through the wind vane. We don't need to worry about the internal workings of the chip. We just need to understand that it can convert from a continuous analogue voltage to a number (in code) that represents the magnitude of the voltage. More voltage will give a higher number, and less voltage a lower one.
To connect the wind vane to the weather station board, you'll first need to have set up the main weather station box.
- Locate the socket on the weather station board marked WIND and the corresponding grommet.
- Unscrew the grommet from the case and thread the wind vane plug through to the inside of the box:
- Connect the plug to the socket, and tighten up the grommet.
while True:
data = [] # a list to store the measurements
length = 10 # ten seconds
print "Measuring wind direction for", length, "seconds..."
start_time = time.time()
while time.time() - start_time <= length:
adc_value = adc.read(0)
direction = get_direction(adc_value, 10)
if direction != None: # keep only good measurements
data.append(direction)
average = None
if len(data) > 0:
average = sum(data) / len(data)
print "Wind direction:", average