This is a low-cost anemometer (wind sensor) made for use with electronic projects. It was originally inspired by a student project that required a breath sensor. We used a piezo type vibra-tab for that project but the sensor was glitchy, picked up sound and vibration and was hard to calibrate. I didn’t get this designed in time for the original project but I think it will solve certain problems in sensing wind speeds and human breath.
The Wind Sensor is a thermal anemometer based on a traditional technique for measuring wind speed. The technique is called the “hot-wire” technique, and involves heating an element to a constant temperature and then measuring the electrical power that is required to maintain the heated element at temperature as the wind changes. This measured electrical input is then directly proportional to the square of the wind speed. Hot wire anemometers are available in hand-held packages resembling multimeters, and tend to cost about $300 but this is the first small sensor suitable for electronic experiments. Honeywell also makes larger sensors based on the same principles.
The underlying principle that makes the sensor function is the same as the traditional hot-wire technique. This technique excels at low to medium wind speed, and is the preferred technique for sensing indoor air movement, where the spinning cup anemometers typically seen on weather stations are ineffective.
As an experimenters tool, the sensor is exquisitely sensitive, with a small puff of air being sensed at a distance of 18-24″. Possible applications include human breath detection, room occupancy detection, hvac system monitoring, weather stations and many more.
We have a new version of the sensor out — dubbed Rev C (yes we switched from numbers to letters). It has five pins with three signal pins plus power and ground. Below is a little chart of the pins functions.
The sensor runs fine between five and ten volts but the Arduino calibration sketch (link below) is premised on a regulated five volt supply, so the sketch won’t work at higher voltages.
- Dimensions: .68″ × 1.590″ × .25″
- Supply Voltage: 4 – 10 volts
- Supply current: 20 – 40 mA (depending on wind speed)
- Output signal: analog, 0 to VCC
The sensor can be switched off to save power, but at start-up needs to be allowed to warm up about 10 seconds to thermally stabilize for best accuracy. This time interval allows the sensing thermistor to heat up to operating temperature. It’s probably also not a good idea to hang the sensor power supply off a microcontroller pin, even an Atmel pin, if best accuracy is a concern, as the sensor draws a bit of current (~25 mA) and the current will also fluctuate with wind speed.
The Wind Sensor includes a small trimpot that is used to calibrate the sensor for zero wind. Calibration is simple. Simply put a glass over the sensor to block any breeze and adjust the pot for the desired zero level. We calibrate the sensors roughly for .5V of output at zero wind with a six volt supply, but you are free to calibrate as desired. A lower calibration point, say .2 V will result in a little more sensing capability at the high end. Using a higher voltage supply will require recalibration. Do not use supplies higher than about 10 volts. A regulated supply is highly recommended.
|+V||Power – 5 to 10 volts – this should be a regulated voltage for best results|
|Out||Output voltage. This is the voltage at RV multiplied by about three and shifted by the setting on the potentiometer. This output voltage will saturate (hit Ground or +V) if the ambient temperature changes much from the temperature when the sensor was calibrated. The sensitivity of this output can be changed by by swapping out R11. Higher sensitivity will make the sensor saturate more easily with ambient temperature change.|
|RV||This is the raw loop voltage – and it will not saturate. It won’t go below about 1.8 volts at room temperature either. This voltage is not affected by the calibration potentiometer. The output of the sensor is logarithmic, which means the sensor can capture very slight air movements at the low end, but also not saturate at full output until the air-flow reaches about 60 miles per hour. The Arduino sketch for use with the wind sensor is constructed to use this pin as output.|
|TMP||A temperature output. This is just a voltage divider resistor and thermistor. It will read about 2.8 volts at room temperature and go down with higher temperatures and up with lower temperatures. It will never saturate. An algorithm to translate voltage into temperature is included in the example sketch below.|