The output will change state every time it picked up what it thought was a valid pulse, so you will be able to detect any mistriggers immediately on the scope. For a steady flow rate, you should see a nice squarewave, 50/50 mark-space. You will see instantly if a mistrigger is happening because it counts a pulse twice, or whether it is being triggered by other events where you can't see a pulse

10k is about what I'd use, possibly 4.7k. Where does the sensor get its 0V connection from?

Ground is from the board over the sensor cable. So yeah an antenna. I'm waiting for the new board to come back. Completely different layout and connections for a proper RC low pass filter. The sensor will be powered by 24V instead of its minimum 5V. So I will try a few things and update next week. Thanks.

 

I was thinking less of "antenna" than of different ground voltages, if the 0V doesn't come from somewhere near the micro.

 

I was thinking less of "antenna" than of different ground voltages, if the 0V doesn't come from somewhere near the micro.

Well actually you might not be that far off. In the original board design I put a reverse polarity protection diode in the wrong place and ended up making the ground reference about 0.6V. I discovered this and removed the diode. The 300pF cap was already in place so I haven't retested. I'm waiting for the new board. I will start with as few components as needed and go from there.