Wind Speed

Anemometer Interface

Voltage-to-Current Converter

Drives distant analog meter, digital meter, computer interface, data logger, etc. Built-in zero offset.

Max Carter



The 1733 Anemometer from Adafruit is a reasonably-priced, well-built wind-speed sensor. It features a built-in transducer that converts wind speed to an analog voltage output - external pulse counter not required. This article describes my 1733 installation and the circuit that converts the voltage from the anemometer to loop current, enabling transmission over a pair of wires to a distant (or local) analog or digital meter, computer interface or logging device.

The advantage of using current to transmit an analog quantity is that the length of the wire pair to the metering device(s) is virtually unlimited. An alternate approach would have been to do an analog-to-digital conversion at the site of the anemometer and transmit the data digitally (and wirelessly) to a microcontroller or computer. But I really wanted an analog meter, and the distance involved wasn't that great (150'/46m), so opted for the current loop. [The distance ended up being ~500'/150m.]

1733 Anemometer

Available from several sources. This one came from Mouser.

1733 Specifications
  • Specified output: 0.4 V to 2.0 volts, 0-32.4 m/s, 0-72.5 MPH
  • Testing Range: 0.5 m/s to 50 m/s, 1.12 MPH to 112 MPH
  • Start wind speed: 0.2 m/s, 0.5 MPH
  • Resolution: 0.1 m/s, 0.224 MPH
  • Accuracy: Worst case 1 m/s, 2.24 MPH
  • Max wind speed: 70 m/s, 156 MPH



Anemometer Installation

Mounting Plate and Pedestal

  • 3.5" electrical junction box cover w/½" knockout hole,
  • ½" close nipple (electrical),
  • ½" locking nuts (3),
  • ½" (F) thread to 1" PVC (sched. 40) reducer,
  • 1" to 1" PVC (sched. 40) coupler.

Mounted on Mast

1¼"x10' (3m) TV antenna mast, secured with #4 self-tapping screws.

Installed on an Outbuilding
Later Relocated
(away from trees and buildings)
18" wall mount (Amazon).

Military surplus tripod,
anchored to short concrete piers.



Voltage-to-Current Converter

Voltage-Controlled Current Regulator

The key take-away from the specs for the 1733 is that, from a baseline voltage of 0.4 volts with no wind, its output voltage increases by 49.4 mV for every 1 m/s increase in wind speed, or 22.1 mV for every 1 MPH increase, and continues to increase (by extrapolation) up to 3.46 V (70 m/s, 156 MPH). The circuit in Figure 1 converts the output voltage from the anemometer to a corresponding current in the remote meter loop. Changing the value of the current sensing resistor (R1) changes the voltage-to-current ratio, making the circuit adaptable to virtually any DC meter. (The value of R1 is calculated using one of the equations below.) The circuit can output currents up to 20 mA.

The Circuit

U1a is the meter loop driver. The opamp monitors the voltage drop across R1 and supplies current to the loop sufficient to exactly offset the voltage from the anemometer. The magnitude of the current flowing in the loop is thus independent of loop resistance. U1b and the associated circuitry provide a regulated current sink for R1 and the meter loop. The circuit maintains the voltage at TP 1 at exactly 0.4 volts. Thus, when the wind is dead calm and the anemometer's output is 0.4 volts, the current through the loop will be zero.

Figure 1

  • 1Resistor R1 sets wind speed-to-current ratio. The value shown (1.10k) provides 1 mA at 50 MPH (full-scale on the 1 mA meter). The resistor can be selected to provide a full-scale reading on any meter at any wind-speed. See Calculating R1.

  • 2Zener diode can be any value from 3 to 9 volts.

  • *The cable from the anemometer to the converter should be as short as possible.

  • The wire pair from the converter to the meter can be any length up to ~10,000 ohms round-trip (wire resistance plus meter resistance) at 1 mA. This works out to something like 50 miles (80km) using AWG 22 (.32 mm2) wire.

  • **Adjust as follows (with current loop connected):

    • Connect multimeter from TP 1 to TP 2
    • Set multimeter to read millivolts DC
    • Adjust pot for 0.0 mV meter reading with dead calm wind (anemometer not turning).

The as-built voltage-to-current converter.


Installed

The gray wire is the cable from the anemometer.
The converter is in the box at lower right. It was later installed in a weather-tight box
and relocated with the anemometer.

Meter

Chosen for its convenient scale (0-50), the meter was once part of a HP bench power supply.

  • If full-scale current is not indicated on the meter face, it can be determined with a battery (or DC power supply) and digital multimeter (DMM).
  • Meters may contain one or more internal scaling resistors, shunt-connected or series-connected. Your meter may require the removal of internal resistors to gain access to the basic meter movement. As mentioned, the Voltage-to-Current Converter can handle currents up to 20 mA, which should cover the vast majority of basic meter movements. (The meter above required no modification.)

  • eBay is a good source for meters.


Calculating R1 (Fig 1)

            

Meters per Second:

R = (Vfs x .0494) / Ifs

  • R is the value of R1 in ohms,
  • Vfs is the full-scale wind velocity in meters per second,
  • Ifs is full-scale meter current in amps.
            

Miles per Hour:

R = (Vfs x .0221) / Ifs

  • R is the value of R1 in ohms,
  • Vfs is the full-scale wind velocity in MPH,
  • Ifs is full-scale meter current in amps.
            

Kilometers per Hour:

R = (Vfs x .0137) / Ifs

  • R is the value of R1 in ohms,
  • Vfs is the full-scale wind velocity in km/h,
  • Ifs is full-scale meter current in amps.

Examples

Full-scale wind speed Full scale meter current Calculated R1 Ω Nearest 1%
50 MPH 1 mA 1105 1.10k
100 MPH 2 mA 1105 1.10k
200 MPH 4 mA 1105 1.10k
100 MPH 1 mA 2210 2.21k
200 MPH 2 mA 2210 2.21k
100 MPH 50 µA 44200 44.2k
100 km/h 1 mA 1370 1.37k
100 km/h 200 µA 6850 6.81k
50 m/s 1 mA 2470 2.49k
50 m/s 200 µA 12350 12.4k


4-20 mA

The circuit works in a 4-20 mA system with the following modification:

  • Eliminate all circuitry associated with U1b
  • Set the value of R1 at 100 ohms
  • Ground the left end of R1

Figure 2

4-20 mA Modification

Note: F.S. (20 mA) = 32.4 m/s = 72.5 MPH = 116.6 km/h






Alternate Meter Type - Digital Panel Meter

The circuit in Figure 3 can be used if a digital readout is preferred. The 49.9-ohm shunt resistor across the input terminals of the meter "converts" the current to a voltage. The meter will read full-scale (199.9 mV) at 200 MPH (4 mA).

Figure 3

Digital Panel Meter
  • *Meter inputs must be ground isolated. (The suggested Velleman PMLCDL meets that requirement.)

  • **Shunt resistor value shown produces full scale reading at 4 mA (200 MPH).

DPM

Next to the back door, at eye level, showing about 2 ½ MPH.

DPM Shunt Resistor

Rshunt = Efs / Ifs

  • Rshunt is the value of the shunt resistor in ohms,
  • Efs is the full-scale voltage of the meter,
  • Ifs is full scale wind speed current in amps.



Multiple Devices are Connected in Series

Figure 4

*Meters or other device/s (data logger, chart recorder, etc.), including shunt resistors. Mix or match.



Computer Interface

The anemometer data can also be read and processed by a computer.

Here's a real-time working example:

Interface and Code

Creating the real-time working example involved building a fairly simple loop current-to-computer interface and writing some code: in Basic for the interface; in Perl to acquire and store the data; and in PHP/GD Graphics Library to generate the graph.

Data Logger Suggestions
Building the computer interface and real-time graph was an interesting and virtually zero-cost exercise, but it may not be the most practical approach if you are not into programming. If you'd like to acquire, process and display wind speed data from the 1733 current loop using a computer, check out commercially available data acquisition products. The DLP-IO8-G 8-Channel Data Acquisition Board appears to have most of what you need, hardware and software, at a reasonable cost. Another option, at slightly higher cost, would be the Dateq DI-1100 Data Acquisition USB DAQ and Data Logger System, which also includes software.

- Max


Schematics produced with DCCAD.


Related Pages

Loop Current-to-Computer Interface

Wheatland, Wyoming - Wind Speed Record - Past 24 Hours


External Link

USB-Based 8-Channel Data Acquisition Module







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