How can I read Hertz frequency from a LoPy Pin?

[Lorenzo Comparini]

Hi, I'm using a LoPy 4 mounted on an expansion board.
I need to read the pulses frequency from a water flow sensor, I tried to use PWM library, but I don't really know how that should work.
Here's what I tried:

from time import sleep
import machine

rmt = machine.RMT(channel=3)

rmt.init(gpio="P13", rx_idle_threshold=16000)
lt = 0

while True:
    data = rmt.pulses_get(timeout = 5000)
    print(data)
    print()

This i the output of a single impulse (i think):

((0, 1), (1, 2), (0, 1), (1, 17), (0, 1), (1, 2), (0, 1), (1, 1), (0, 1), (1, 228), (0, 1), (1, 2), (0, 1), (1, 81), (0, 1), (1, 15), (0, 1), (1, 51), (0, 1), (1, 11), (0, 1), (1, 64), (0, 1), (1, 4), (0, 1), (1, 231), (0, 1), (1, 2), (0, 1), (1, 1), (0, 1), (1, 17), (0, 1), (1, 1), (0, 1), (1, 1), (0, 1), (1, 30), (0, 1), (1, 1), (0, 1), (1, 17), (0, 1), (1, 1), (0, 1), (1, 1), (0, 1), (1, 237), (0, 1), (1, 17), (0, 1), (1, 2), (0, 1), (1, 231), (0, 1), (1, 17), (0, 1), (1, 232), (0, 1), (1, 18), (0, 1), (1, 231), (0, 1), (1, 17), (0, 1), (1, 233), (0, 1), (1, 17), (0, 1), (1, 232), (0, 1), (1, 17), (0, 1), (1, 232), (0, 1), (1, 18), (0, 1), (1, 232), (0, 1), (1, 17), (0, 1), (1, 250), (0, 1), (1, 250), (0, 1), (1, 250), (0, 1), (1, 249), (0, 1), (1, 250), (0, 1), (1, 250), (0, 1), (1, 749), (0, 1), (1, 250), (0, 1), (1, 499), (0, 1), (1, 500), (0, 1), (1, 249), (0, 1), (1, 250), (0, 1), (1, 250), (0, 1), (1, 250), (0, 1), (1, 500), (0, 1), (1, 1680), (0, 1), (1, 250), (0, 1))

and if I blow in the sensor, the output list result to be void.
Can someone help me? Even with some example codes, if that's possible.

Thank you

[robert-hh]

@Lorenzo-Comparini I do not know the signal structure of the sensor, but let's assume it's a digital 0/3.3V signal. Please check also the frequency of the sensor. The lowest frequency you can measure with RMT channel 3 is ~300Hz. If you want to get lower frequencies, you have to select for instance channel 7, going down to ~10 Hz. That may still be too high. If I look into your list, there seem to be too many short transitions. That indicates an oscillation at the input. So you will need a proper signal shaping between the sensor and the lopy, consisting of

• a RC lowpass, with about two time the maximum frequency of the sensor and
• a Schmitt-Trigger gate to ensure fast signal transitions at the LoPy input.

Do you have the chance to measure the signal of the sensor, e.g. with a simple logic analyzer or an oscilloscope. The name & model of the sensor may also help in supporting you.

[Lorenzo Comparini]

@robert-hh
Here's the link for the sensor's documentation: http://www.mantech.co.za/Datasheets/Products/YF-S201_SEA.pdf, hope this can help support.

[Lorenzo Comparini]

@robert-hh I tried changing channel to 7, and now the length of the output result changes accordingly to the sensor movement, the maximum level I can reach is 128. How can i obtain the frequency in Hz?

[robert-hh]

@Lorenzo-Comparini So th efrequency is between 16 and 90 Hz, meaning that you have to use channel 7. The frequency should be something like 1000/(2 * pulse_length * 3.125)

[robert-hh]

@Lorenzo-Comparini If the maximum number is 128, then y.ou can as well go for channel 4 or 5, which gives a better resolution. Then replace 3.125 by 1.0

[robert-hh]

@robert-hh Please not, that he sonsor runs at 5V, and the input at the Lopy must not exceed 3.3 V.

[Lorenzo Comparini]

@robert-hh I know, the VCC is joined to an external power source at 5V

[Lorenzo Comparini]

@robert-hh I just tried both, but the maximum legth of the pulse is still 128

[robert-hh]

@Lorenzo-Comparini What you do is guessing. Get yourself an independent method for displaying the signals. Like a 10 € logic analyzer, available e.g. at Amazon: https://www.amazon.de/AZDelivery-⭐⭐⭐⭐⭐-Logic-Analyzer-gratis/dp/B01MUFRHQ2/ref=sr_1_5?__mk_de_DE=ÅMÅŽÕÑ&crid=1LM4V4HE5L3CG&dchild=1&keywords=logic+analyzer&qid=1623852278&sprefix=Logic+%2Caps%2C181&sr=8-5

[Lorenzo Comparini]

@robert-hh thanks, guess I'll try that, can you explain me what is that for and what it does? sorry but I'm very noob at this

[robert-hh]

@Lorenzo-Comparini That is the front end of a software, which display the function of signal level over time. SO you can see the signal. In you case some kind of high/low level changes. And you can measure it's timing properties. It supports 8 input channels. The software you need is either 'pulseview' or the Logic software. Links:
https://sigrok.org/wiki/PulseView
https://www.saleae.com/downloads/

[Lorenzo Comparini]

@robert-hh Thank you very much, hope to solve this soon