Sensors: IR-Detector based Anemometer - Sailing and Windsurfing are common recreational activities

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Sensors: IR-Detector based Anemometer

        Sailing and Windsurfing are common recreational activities but both require a minimum of wind depending on the experience of the user. I wanted to design an anemometer capable of measuring wind speed (and direction) in m/s, possibly with a converter to display speeds in mph or knots. It would also help if it was of sturdy design and portable to allow it to be easily relocated.

        To design my anemometer I first had to investigate the properties of my Sensor, a dual Infrared emitter and Phototransistor. It worked by emitting IR radiation from a diode, and detecting the intensity of reflected IR radiation from the surface it is pointed at. For the purposes of measuring wind I needed to know a speed. The easiest and most accurate way to measure speed in this case was to convert the linear wind speed into rotational speed using a simple “windmill”-type blade, and then to measure the speed of rotation. To do this I would use a cork split into quarters. Alternate quarters would be coloured using matt black paint (a very poor Infrared reflector) and silver foil (an excellent Infrared reflector). This would rotate with the windmill blades

The Phototransistor itself would allow no current to pass unless it detected reflected IR radiation. The greater the intensity of the detected radiation, the greater the created current through the transistor would be. To turn this into a potential figure I could measure, I used the Phototransistor as part of a piece of circuitry similar to a potential. When the Current passing through the phototransistor was lower, the Potential Difference between A and B would be low. When the Current was greater, the Potential Difference would be high. The Phototransistor would need to sit close to the rotating cork to prevent interference and detect the varying quantities of IR radiation being reflected over time, and therefore, how fast it was rotating.

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A Phototransistor works in the same way as a Photodiode, in that light (or IR) gives energy to the atoms freeing electrons to become negative charge carriers in parts of it, causing a current. The main difference is because the effect is used as the base of a transistor, the output is many times greater. The potential difference (A-B) was 0v when the sensor detects no IR, and higher (closer to 6V), when the sensor detected a lot of Radiation. The current is ...

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