Flow sensor needs signal pulled up?

Products & Technology Grove
#1

In the wiki page about the flow sensor (seeedstudio.com/wiki/G3/4_Water_Flow_sensor) it says that a 10k pull-up resister is needed on the signal wire. Is this really correct? I’m looking at my flow sensor right now with my scope and with no pullup whatsoever the signal wire is already high and pulls down for pulses. And it works great with my arduino with no pull-up resister and without using the internal pull-up resister in arduino.

Am I correct? Is the wiki wrong?

Also the breadboard picture of how to hook up the flow meter is wrong; if you hooked it up that way you’d short the +5v and the yellow wire of the flow meter sensor itself, connect the ground to the red wire, and leave the black wire with no connection. A simple mistake in rendering of the diagram. On a breadboard the rows are connected, not the columns! But if a newbie attempted to use this diagram exactly as presented, they just wouldn’t get anything!

#2

hey~ I think maybe Arduino has an internal pull-up resister.
of curse, an external pull-up resister will make your system more stable~ :smiley:

#3

Yeah so I said I used a scope on the signal wire and it was normally high, dropping to low with each pulse, without being plugged into the arduino at all. I guess you missed that. To see what I mean just plug the flow sensor into power (ground and 5V) and then stick a scope on the signal wire. It’s possible that adding a 10k pull-up to this signal wire will make sure it’s up to 5v instead of some lower value like 2.5v. I’ll do some more experimentation.

But in the meantime it works fine for me on arduino with no pull-up resistor and I’m not using the internal pull-up resister.

#4

Okay, so since the flow sensor uses a hall effect sensor, the voltage on the signal wire is about 2.5V. And when the blades turn, it will go down to zero every pulse. But if you spun it the other way, it would probably go from 2.5V to 5V. Anyway, it is correct to use a 10k pull-up resister on the signal wire. That ensures that you get a clean 5V to 0V drop for every pulse.

#5

yeah~ so maybe we should add this to the wiki~ how do you think~ :slight_smile:

#6

The wiki already talks about a pull-up resister, though it doesn’t say why a pull-up resister is recommended or needed.

#7

yes,you are right, we can get a clear TTL level after adding a 10K pull-up resistor.

#8

Yesss!! :sunglasses:

#9

The 10 k pull-up resistor is a protective measure used to limit the current to 0.5mA if the sensor becomes short circuited. It is a standard procedure in digital electronics.