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 - no external pulse counter needed. 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 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'), so opted for the current loop.
Mounting Plate and Pedestal
Mounted on Mast
1¼"x10' (3m) TV antenna mast, secured with #4 self-tapping screws.
Installed on an Outbuilding
18" wall mount (Amazon).
The 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.
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.
The as-built voltage-to-current converter.
The converter is in the box at lower right.
Chosen for its convenient scale (0-50), the meter was once part of a HP bench power supply.
Calculating R1 (Fig 1)
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).
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:
Read about how to do that here.
Schematics produced with DCCAD.