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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

    14
    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

    3,365
    673
    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

    14
    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

    3,365
    673
    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

    14
    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

    14
    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

    3,365
    673
    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

    5,189
    704
    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.
     
  9. Danneskjold

    Danneskjold

    14
    0
    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

    5,189
    704
    Jan 9, 2011
    I have already said it.
     
  11. Bluejets

    Bluejets

    3,365
    673
    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 at 12:31 AM
  12. Danneskjold

    Danneskjold

    14
    0
    Apr 16, 2010
    Did I get this right?
    upload_2019-6-11_9-31-43.png
     
  13. Bluejets

    Bluejets

    3,365
    673
    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.
     
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