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Flomec-qs100 Open collector output

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We are using multiple of these flowmeters (Flomec-qs 100, please see the link: Now these have only two output wires and they are supposed to be connected as a open collector output. So connecting a pull-up resistor should work between the collector and the Vcc (power supply, 10-20 V). However, we are unable to display the output pulses on a oscilloscope even when we use a pullup resistor (we used 4.7k, 10 k, etc. see the connection diagram below). We have used a PLC before and it was able to count the pulses. But my question is how can I display the pulses in a scope or sample them with a National Instrument DAQ device. Thanks.


CanĀ“t see a open collector output there...

Link is broken, but it also just goes to a marketing site.

Found the manual at
Datasheet available at

or direct links and

Neither give much information, and just say it will work with various irrigation controllers.

I think your problem is just too high a resistance value for the pullup.  Keep in mind these are ultrasonic sensors and are powered by the same two wires that carry the pulses.  Quiescent current when there are no pulses may be quite low, but active current could be a lot higher.

Assuming a 12V supply and off-state quiescent current of 200uA per datasheet, with 4700 ohms you will see a drop of 1V normally, so Off-state V-high is about 11V.  Then for On-state, the current would be 12/4750 = 2.5mA, so On-state V-low should be about 125mV (2.5mA through the internal 50 ohms).  If it is working, those should easily be seen on a scope.  The pulses should go low for 4ms, and repeat at a rate dependent on flow up to 100Hz.

If you are not seeing something, then you should try a smaller value resistor.  I've seen similar sensors that require a minimum of 10's of mA to operate (100's of mA on bigger ones), although many recent models are happy running off 4-20mA current loops.

With a 1k pullup, Off-state V-drop would be just 200mV, so Off-state V-high of 11.8V, and On-state current would be about 11.4mA, so On-state V-low about 570mV.  I'd even try going lower, maybe 470 ohms.

Hi Kean,
Many thanks for your detailed reply. I will note down the voltages for different pullup resistor values and post it soon. Your comments really helps.

I was told by the company tech rep that it is a NPN open collector output. But he did not seem to know much beyond that which is disappointing.


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