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Make a flow switch from a hall-effect flow meter

Discussion in 'Sensors and Actuators' started by Danneskjold, Jun 7, 2019.

  1. Danneskjold

    Danneskjold

    17
    0
    Apr 16, 2010
    Hello,

    I need to have a water flow proving switch in a current project build. A simple flow switch would work, but they are all physically too large. So I was thinking, maybe I could take a hall effect flow meter and find out how to turn the output pulses into a "go-no go" kind of trigger event.

    I have not the faintest idea how to start with this. can someone point me in the right direction?

    Thanks,
    RD
     
  2. Bluejets

    Bluejets

    3,542
    719
    Oct 5, 2014
    If I understand correctly what you want to do i.e. if there are pulses then ok , go ( in your words)

    and if no pulses from the hall effect, then no-go.

    Pretty simple to set up the hall effect as a "re-trigger" on a 555 timer.
    Set the timer to say a few seconds,or whatever, and if there are no pulses to re-trigger the 555, it times out and closes a relay or whatever.
    If in the set time pulses arrive from the hall effect, they re-trigger the 555 so it will not time out.
     
  3. Danneskjold

    Danneskjold

    17
    0
    Apr 16, 2010
    OK, very cool. I kinda figured it would involve a 555. but here's a complication: a flow switch has a definite threshold. how would we work that into this concept? let's say for example that 2 pulses per second is not enough but 4 pulses is.
     
  4. Bluejets

    Bluejets

    3,542
    719
    Oct 5, 2014
    2 pulses per second re-trigger would be plenty if your 555 time-up period is more than 0.5 second.

    You naturally would have to make allowance for any start-up time in the whole design.
     
  5. Danneskjold

    Danneskjold

    17
    0
    Apr 16, 2010
    thinking on this: it seems like this would only turn the output from continuously on to intermittently on. I need it to sense that if the pulse rate is slower than x, then stay off. what am I missing?
     
  6. Danneskjold

    Danneskjold

    17
    0
    Apr 16, 2010
    Here is what I don't understand: if water is flowing, the hall sensor is sending pulses. those pulses keep the 555 from timing out, assuming they arrive more quickly than the set time period. BUT if there is flow and it's not enough flow, pulses still arrive. but now, they arrive after the 555 timer has expired. so in between the timer running out and the arrival of a new pulse, the circuit will be off. and then turn on again with a new pulse.

    How do we make a threshold of say 2 pulses per second, such that any pulses that arrive more slowly will be 'discarded'?

    I'm learning...thanks for your patience!

    RD
     
    Last edited: Jun 10, 2019
  7. Bluejets

    Bluejets

    3,542
    719
    Oct 5, 2014
    Could be done with discrete components but when you start to get into if and then what if scenarios , you might be better off using an Arduino and state machine logic.
    If you used a simple "start-stop" arrangement as many motor starters have, then once the unit dropped out, you would have to re-initiate the run with a button press.
     

    Attached Files:

  8. duke37

    duke37

    5,256
    726
    Jan 9, 2011
    I am old school and would use a charge pump to give a voltage proportional to pulse rate.
    Use a comparator to determine whether there is sufficent flow to switch. Use positive feedback to give a deadband to prevent unneeded switching.
     
    hevans1944 likes this.
  9. Danneskjold

    Danneskjold

    17
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    Apr 16, 2010
    Damn, I need to learn to code.... ;)

    OK, my ignorance is going to speak loudly here, so buckle in....

    What if we used the pulses from the hall sensor to charge a capacitor. said capacitor also connected to a bleed resistor. so the pulsing will raise the voltage faster than the resistor can bleed it off, to a point at which the 2/3 threshold of a 555 is met and the connected relay closes.

    what say you?
     
  10. duke37

    duke37

    5,256
    726
    Jan 9, 2011
    I have already said it.
     
    hevans1944 likes this.
  11. Bluejets

    Bluejets

    3,542
    719
    Oct 5, 2014
    Same here.

    Wonder if the Op even read about "retrigger 555".

    I'm still wondering what exactly is required. The explanation from the first post does not match that from the latter.

    Is this "water flow sensor" somehow connected into the control of the water flow?

    so in between the timer running out and the arrival of a new pulse, the circuit will be off. and then turn on again with a new pulse.

    If it's a flow indicator above a certain flow rate only then this is exactly what you need.
     
    Last edited: Jun 11, 2019
  12. Danneskjold

    Danneskjold

    17
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    Apr 16, 2010
    Did I get this right?
    upload_2019-6-11_9-31-43.png
     
  13. Bluejets

    Bluejets

    3,542
    719
    Oct 5, 2014
    Driving a relay in that manner, the relay will be off during the timeout, then you will need to allow for this with the contacts.
    Sometimes it's done like this to allow for "fail-safe" but I don't think this would apply in your case.

    Re-trigger I had more in mind was starting with a oush button on the trigger and continually discharging the cap with the open collector of the hall effect.
     
  14. Danneskjold

    Danneskjold

    17
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    Apr 16, 2010
    like this?
    upload_2019-6-20_9-45-30.png
     
  15. Bluejets

    Bluejets

    3,542
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    Oct 5, 2014
    No.....the output of opto isolators is an open collector as shown however the opto itself is not some magical power generator.

    The collector, (terminal 4) would normally go to terminal 2 on the relay with the emitter (3) to ground as you have connected BUT terminal 9 of the relay would then go to the positive rail.

    Also if ANY LED is used in a circuit as the opto input is ( an LED) then use a current limit resistor.
    The size of the resistor is dependant on the supply voltage, and the maximum required current for the opto.
    All available in the spec sheet for the part.

    As an added note you could get the same result from a standard NPN small signal transistor. In that instance the current limit would apply to the base leg of the transistor.

    Also the output pin of the 555 (3) can support up to around 200mA current so depending on the relay used, it may be able to be directly driven from the pin.
    Get into the habit of fitting a freewheeling diode on the relay coil, it'll save you some head scratching grief further down the track one day.

    Have a look at how circuit diagrams are drawn as one final note, and redo your circuit.
    As drawn it is unnecessarily complex to follow.

    A hint as to your control is, "you want to stop the timer from timing out", so what is the one external component that controls this time period which can easily be shunted to ground to stop this from happening..??

    Take a look at the circuit extract from Talkingelectronics that I posted a few days ago as an example.
    Note that this is a servo driver NOT a control as you require.
    https://www.electronicspoint.com/fo...ervo-w-o-microcontroller.291338/#post-1787415
     
    Last edited: Jun 21, 2019
  16. Danneskjold

    Danneskjold

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    Apr 16, 2010
    Thank you so much for your well-thought reply, Bluejets. I really do appreciate it! Let me absorb this and I'll reply in a few days.
     
  17. PAFMElb

    PAFMElb

    8
    1
    Mar 20, 2017
    I made such a thing a few months ago to bypass a failed flow sensor embedded (in epoxy) inside a tank-water / fresh-water arbitration pump. The overall system checks if there is tank water available, and if there is, fires up a pump to deliver same. When flow ceases (ie tap is turned off, the pump is turned off after a few seconds. If there is no tank water the pump stays off and water is delivered via the townwater mains. In my system the flow sensor failed hence the need for a build.

    An Ebay 3 terminal Hall effect flow switch (around $7.00 from memory) puts out pulses at a rate according to the flow. Internally it is just a magnet on an impeller that passes a Hall device. Super reliable.
    Sadly the thing I bought could come to rest with the output either high or low. The O/P appears to go active high and active low, in other words, it is not an open collector output which would have been ideal. Because the output at zero flow can be in either state it was necessary to AC couple the Hall device output into a charge pump (ie an integrator circuit) which feeds a threshold detector. This is needed to ensure a single pulse as might be caused by mechanical jitter or a tiny flow of water does not trigger the device. It is good to introduce hysteresis into the integrator so if the flow rate happens to be around the switching point you don't get the switched device (pump in my case) turning on and off or hunting. So the device would switch on at about 1 litre per minute and switch off at around half that. My implementation was using 2 transistors, done that way for ease of construction, but if I were to do it all again I would opt for a CMOS Schmitt gate implementation which simplifies the design at the expense of a more complicated layout.Using a Schmitt device means you don't have to supply any hysteresis.

    I am fairly new here so I have not figured out how to create and attach a circuit diagram, or even if the site has a schematic design module. If your need is still current, and if my basic hints are not enough let me know and I will attempt a more expansive answer.

    Regards PAFMelb
     
  18. Danneskjold

    Danneskjold

    17
    0
    Apr 16, 2010
    [QUOTE="I am fairly new here so I have not figured out how to create and attach a circuit diagram, or even if the site has a schematic design module. If your need is still current, and if my basic hints are not enough let me know and I will attempt a more expansive answer.

    Regards PAFMelb[/QUOTE]

    I would love to see your circuit--it sounds exactly like what I need. you can just paste a screenshot in line with your reply, or send it directly to my email [email protected] if that's easier. then I'll put it up here.

    Thanks!

    RD
     
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