September 1, 1997

Transistor trio makes vector anemometer

W Stephen Woodward, Chemistry Department, University of North Carolina--Chapel Hill, Woodward@net.chem.unc.edu

A previously published Design Idea (Reference 1) uses a heated transistor as an airspeed-to-frequency converter. Because it uses only one transistor, the circuit cannot determine the direction of airflow. Alternatively, a circuit that uses a trio of hot transistors can digitize both wind speed and direction (Figure 1).

The basic principle of operation is identical to that of the previous circuit. In the zero-airflow case, zero-adjust potentiometers R₁, R₂, and R₃ set quiescent bias currents for self-heated transistors Q₁, Q₂, and Q₃, respectively. If you properly adjust the resistors, collector power dissipation causes a still-air temperature rise in the transistor sensors of approximately 50°C, which in turn reduces the sensors' V_BEs by approximately 2 mV/° to just slightly below Q₄'s V_BE plus the drop across R₄. Then, the voltages at the noninverting inputs of comparators IC_3A, IC_3C, and IC_3D are slightly lower than the voltages at their inverting inputs. These comparators' outputs therefore go low, holding C₁ discharged and multivibrator IC_3B reset with its output high. This action causes a zero-frequency output on IC_2C and holds IC_2A and IC_2B off.

If a nonzero airflow impinges on the sensor array, the resulting increase in cooling rate tends to raise the airflow sensor's V_BE relative to Q₄'s V_BE. This change reverses the voltage relation at one or more comparator input pairs and releases the reset on C₁. C₁ then charges through R₆ until the voltage at IC_3B's inverting input is greater than the voltage at its noninverting input, causing IC_3B's output to snap low, beginning the discharge of C₁ through R₆ and turning on IC₂. IC₁ now directs an approximately 700-µsec pulse through the span-adjust-potentiometer array to whichever sensor transistor triggered the cycle. The resultant pulse of collector current deposits a quantum of heat tending to warm that sensor's temperature enough to restore the original zero-flow voltage balance with Q₄. Until that temperature is restored, IC_3B continues to oscillate and cycle on IC₂. This feedback loop acts to maintain a constant temperature differential among sensors Q₁ to Q₃ and Q₄. The frequency at IC_3B's output is therefore proportional to the extra power required to heat the array and thus directly related to airspeed.

Meanwhile, feedback through R₅ latches the address of the sensor transistor whose cooling triggered the cycle, so only that transistor receives the heating pulse. This binary (0, 1, or 2) sensor address is available at the output. Thermal coupling between the sensors depends on airflow direction. In Figure 1, for example, Q₃ is the most upwind sensor and is therefore the most strongly cooled. This effect is the basis for the computation of wind angle (Click here to download the Basic program from DI-SIG, #2073).

The anemometer's output is suitable for direct connection to the parallel port of a desk or laptop PC. You can then run the accompanying Basic program to report wind speed and direction once per second.

The maximum output frequency for the circuit as shown is 1 kHz. Appropriate adjustment of the span-adjust potentiometers establishes almost any desired full-scale flow rate from less than 1 to greater than 10 m/sec (which is equivalent to less than 2 to greater than 20 knots.) Response time is less than 2 sec because of the constant-temperature operation of the sensors. Tracking among Q₁, Q₂, Q₃, and Q₄ compensates for changes in ambient temperature. The circuit operates from one 5V power source. Power consumption depends on airflow rates but is typically less than 2W. (DI #2073)

Reference

1. Woodward, W Stephen, "Self-heated transistor digitizes airflow," EDN, March 14, 1996, pg 86).

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