Drives distant analog meter, digital meter, computer interface, data logger, etc. Built-in zero offset.
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.]
Available from several sources. This one came from Mouser.
Military surplus tripod, anchored to short concrete piers.
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.
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.
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 (.326 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.
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.
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,
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 pictured above required no modification.)
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).
*Meter inputs must be ground isolated. (The suggested Jameco 108388 meets that requirement.)
**Shunt resistor value shown produces full scale reading at 4 mA (200 MPH).
Next to the back door, at eye level, showing about 2 ½ MPH.
Multiple Devices are Connected in Series
*Meters or other device/s (data logger, chart recorder, etc.), including shunt resistors. Mix or match.
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.