First prototype

Housing

The housing design of the first prototype was based on the design from Tom Onderwater [8]. Below the complete housing including solar module is depicted with two detailed views of the position of the new wind speed and humidity sensors. The wind speed sensor will be mounted on the top of the sensor node. Holes are made on the top of the node to partly embed the sensor inside the housing. Tests will prove if this is enough to make sure the sensor will not detach when the sensor is exposed to high wind speeds. The small off-centre hole in the top is to accommodate the wire from the wind sensor to go to the main compartment. The compartment on the side of the sensor node will hold the humidity sensor. The cap has upwards sloping horizontal holes to keep out rain while still allowing the atmosphere to reach the humidity sensor. The compartment for the humidity sensor has a downwards sloping surface to make sure the water drains through the small holes on both sides of the bottom surface of the compartment. For the model and the details mentioned above, see appendix H.

Interfacing cup anemometer

The wind sensor Froggit WH1080 is a cup anemometer that measures rotations with a reed switch. The reed switch gives a pulses four times per full rotation. According to the datasheet [45] the reed switch gives a pulse one per second if the wind speed is 2.4km/h. From this it can be derived that if the reed switch pulses once per 1.5s the wind speed is 1m/s. See appendix F for the complete calculations. The amount of pulses is measured over a period of 3s. From this amount the wind speed can be calculated according to the following formula: 𝑤𝑖𝑛𝑑 𝑠𝑝𝑒𝑒𝑑 =𝐶

𝑃∗ 2

3 where C=amount of pulses and P=Period in seconds. The anemometer is put in series with a 100nF resistor to get rid of the phenomenon of bouncing contacts. This decreases the chance on false measurements. The resistance of the closed reed switch was measured with a multi-meter and is 6Ω. With 3.3V power supply and a capacitor of 0.1µF this leads to a charging time of 0.1µs. this is so small that it will not interfere with the wind speed measurements at high speeds. See appendix F for complete calculations. Anemometer is connected to a I/O pin on the microcontroller with the capacitor in parallel. The sensor pin provides 3.3v. the ground wire from the anemometer is connected to ground. The amount of pulses from the reed switch can be sensed via the I/O pin.

Interfacing humidity sensor

The humidity sensor is a SHT15 capacitive humidity sensor breakout board form sparkFun [46]. The board is fully calibrated and only needs two communication wires. The humidity sensor itself also includes a temperature sensor which is used in calculating relative humidity. These calculations are done by the board itself. The board’s ground and power supply pin are connected to the 3.3v and ground pin of the microcontroller. The data and clock pin are both connected to one of the I/O pins. The clock pin serves to synchronize the communication between the board and microcontroller. Data is requested and received by the microcontroller trough the data pin.

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Interfacing temperature sensor

The Dallas DS18B20 temperature sensor [47] is programmable digital temperature sensor. The version used for the first prototype is watertight and outfitted with a metal probe for more accurate sensing. The DS18B20 communicates over a 1-Wire bus that requires only one data line and a connection to the ground for communication with a central microprocessor. A 4.7KΩ resistor is connected from the data line to the 3.3v line. This is good practice because the 1-wire bus requires that the control signal be pulled high, so the master device can pull it low to ask for data, and the slave device can pull it low to give the data. This allows you to have multiple 1-wire devices on the same one wire. The data pin is connected to one of the I/O pins. The ‘DallasTemperature.h’ software library was used to retrieve data from the sensor.

Sending data

The data from the sensors inside the sensor node is read from the sensors and compiled into a message. In front of each value is a letter that identifies the value. The message is send using the TTN protocol. This protocol translates the ascii10 _{values in the message to hexadecimals and the}

10 _{Binary representation of common characters. See appendix N for explanation.}

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message is forwarded to the gateway. Here the message is put into a Json package. This package contains extra information about the message such as timestamp, gate way ID and device ID. For now, the data is collected via the TTN dashboard. An example message can be found in appendix G

Deployment

The second prototype was put on top of the ITC hotel next to one of their weather stations. These weather stations are very accurate and will be used for testing the accuracy of the sensors. It was placed there for a bit more than a week. A picture of the placement of the sensor node can be seen in figure 12.