# RPi meassure low level AC sine wave frequency

I have NRG #40C Anemometer and it generate (after manual) low level AC sine wave, frequency linearly proportional to windspeed with range 0 Hz to 125 Hz. Is there way to use this sensor without any additional hardware?

EDIT1:

Specifications
Output voltage at threshold: 80 mV (peak-to-peak) minimum
Output voltage at 60Hz: 12 V (peak-to-peak) typical

• what's the maximum voltage? Aug 6, 2020 at 13:57
• You need to ask the seller,
– joan
Aug 6, 2020 at 19:21
• I found on manufacturer's website info about voltage. Aug 7, 2020 at 6:54

No - there is no way to do this on an RPi without additional hardware.

Neither the question, not the referenced documentation provide the magnitude of this sine wave. Without that information the additional hardware required can not be determined.

## EDIT:

The OP has found some specifications on the amplitude of the sine wave output from the ac generator in the anemometer. Following are some thoughts on that:

### 1. Buy the Manufacturer's Interface Hardware

As it turns out, the anemometer manufacturer also manufactures and sells interfaces to the anemometer. The "Interface 40C to Logic Level, Speed Amp #892E" appears to be the easiest to use with a Raspberry Pi. According to the specifications, this interface could be powered from the RPi 3.3V supply, and connected directly to an appropriate GPIO pin configured as an input. You'll need to write software to count the transitions on the GPIO pin, and convert that to wind speed IAW the manufacturer's specifications; e.g. 100 Hz = 60 mph wind speed - or whatever that conversion is.

### 2. DIY

Knowing the manufacturer's specifications on the amplitude and other characteristics of the anemometer's ac generator enable a "Do-It-Yourself" approach to the interface. Complicating the design of an interface to the Raspberry Pi is the fact that RPi has no analog input. That is exacerbated by the variable amplitude characteristics of the ac generator used in this anemometer, and the fact that at the low end of its measurement range, the generator's output is 80 mV p-p.

That said, the design is not necessarily overly complex either, and probably well within the realm of we hobbyists. There are several/many ways to do this - one method is a "zero crossing detector" which is just a comparator with its REF input tied to GND. The schematic below turned up in a search - see this Application Note for further details.

### Some notes:

• The Raspberry Pi can source the 5V supply needed to bias the comparator.

• Verify the output voltage before connecting it to your RPi GPIO as anything over 5V will likely destroy it.

• Adjust the values of R1 and R2 to keep the maximum output voltage from the anemometer's AC generator to approximately 2.5V

• Your software will need to monitor the GPIO pin you choose, count the transitions, and calculate the windspeed.

Finally, let us know if you decide to take this route, and have questions re the circuit.

• The documentation specifies a supply voltage requirement of 7V to 35V. It's probably safe to assume that the magnitude will not exceed the supply voltage. Aug 7, 2020 at 9:22
• @OleWolf: No - it's not safe - here's one explanation why. The "documentation" for this anemometer is oddly silent on this specification. My experience has been that when a mfr. "hides" specs on an interface, it's because he's got another component he wants to sell! As far as your project is concerned, the "risk" is more likely to be that the "generator" in the anemometer doesn't put out a high enough voltage. It's very easy to deal with too much voltage, but it becomes a bit more challenging when it's too low. Aug 7, 2020 at 20:33
• @OleWolf: Ah! I see you've edited your question to provide a range of ac voltages. I've likewise edited my answer. Let us know if you have questions. Aug 8, 2020 at 3:40
• I'm not the one who asked the question but maybe the OP has added a linke to the documentation. I'd certainly recommend making some few measurements to ascertain that the voltage output is safe. But, one can usually expect that outputs from powered systems are limited by the supply voltage. The most obvious exceptions are AC generators, as you mentioned, voltage doublers, and the like. Aug 10, 2020 at 6:42
• @OleWolf: My apologies - I lost track of the OP's handle. But no - in general, one can not expect outputs from systems to be limited by supply voltages. The device in question shows that. Aug 10, 2020 at 7:12