Denman Island, British Columbia
Anemometer |
This is a handheld portable anemometer. After almost being blown off several mountain ridges on hiking trips, I wanted to be able to quantify the strength of the wind. The design criteria were that the unit should be light weight, and sturdy enough to be stuffed into a crammed backpack or dropped on rocks.
A standard cup-style anemometer would be too fragile for portable use, so a plastic project box with a fan was used instead. The readout was provided by a standard automotive tachometer. See photo at left.
The unit was equipped with a pushbutton on-off switch to conserve battery life. For protection against the button being held down by other items in my backpack, I also added a slide-type on/off switch, wired in series with the pushbutton.
An earlier incarnation of the anemometer used a small fan with an integral tachometer output, originally intended for cooling computer CPU chips. At a sufficiently high windspeed, the fan would self-generate enough voltage to produce a signal at the tachometer output. A wind speed of about 25 km/h was required to overcome the "cogging" effect of the brushless motor and to generate enough voltage to operate the tach sensor. This made the device suitable for an airspeed indicator in a car, but not for a handheld instrument, which would have to respond to lighter winds.
To overcome the cogging effect, I removed the coil and magnet from a small fan, leaving only the housing, fan rotor, and bearing. On the inside of the fan rotor, I painted two white sectors. I inserted a reflective proximity detector (consisting of an infrared LED and photocell) into the hub of the housing. (See photo at right.) The rotation of the fan would bring the white painted sectors past the detector, producing two pulses per revolution.
The output of the detector did not produce enough of a voltage swing to drive an automotive tachometer. A simple 2-transistor amplifier was added in order to bring the change in voltage up to a suitable level. (See schematic below.)
An automotive tachometer, in 4-cylinder mode, expects to see two pulses per revolution, so it reads the fan RPM directly. The meter's range is 0-8,000 RPM. By switching to 8-cylinder mode, the range can be extended to 0-16,000 RMP.
The instrument was calibrated by strapping it to the windsheild wiper arm of my car and taking it out for a test drive. Since the car was already equipped with a calibrated airspeed indicator, calibrating the anemometer was a simple process.
The dial of the tachometer was replaced with a dial having two scales: 0-60 km/h in 4-cylinder mode, and 0-130 km/h in 8-cylinder mode. (See dial layout at right.)
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Copyright © 2005 Keith Walker
Last modified: 3-Jan-2005