555 ic to switch a relay with a catch

Discussion in 'The Projects Forum' started by trevor lloyd, Jun 30, 2015.

  1. trevor lloyd

    Thread Starter New Member

    Jun 30, 2015
    hi guys

    really in need of some help i am completley new to 555 timers and electronics in general actualy

    i am trying to find out how to make a simple project that i was told can be done with a 555 ic so let me start :)

    i am trying to switch a pump on as soon as i open a tap rather simple idea but a bit harder then i thought...

    so what i have got is a flow control device like bellow:

    so now what i want to do is as soon as the flow controller tells me that the tap is open it will switch a relay on thus turning on the pump then as soon as i close the tap it will turn off the pump...

    is this possible and can some one help me do it

    thanks so much everyone
  2. ericgibbs

    Senior Member

    Jan 29, 2010
  3. #12


    Nov 30, 2010
    If the pump is not on until water flows, what makes the water start flowing?
    You must have a way to maintain pressure (a pressure tank of some sort and a check valve) or this won't work at all.
    If you have a pressure tank, you can use a pressure switch to control the pump.
    That's the, "old school" way.

    The, "flow control" device you pointed to doesn't control flow, it measures flow and outputs electrical pulses.
    If you can get the flow meter to output a pulse, you can trigger a 555 timer.
    A retriggerable monostable holds its output high for a fixed amount of time after it has a pulse at its input.
    If the pulses are often enough, the monostable circuit will pretty much stay on until the pulses stop...and then some. One time constant of the monostable timer.

    So, what's your goal? To get a pump to work or to build some electronic stuff?
    I mean, old school is dead simple.
    Still, you're free to try a different way.
  4. trevor lloyd

    Thread Starter New Member

    Jun 30, 2015
    thanks for the info

    sorry i did not give much of the story.

    i have a low presure solar geyser in my house so as a result the water pressure from them is never the best so what i want to do is just put a small pump in line so that when i turn on the shower it will turn on the pump so i get better flow....

    sorry for my stupidity here but can you give me a basic easy words on what or how this "retriggerable 555 monostable" is

    thanks again guys
  5. GopherT

    AAC Fanatic!

    Nov 23, 2012
    And 'astable' 555 circuit just keeps clicking on/off forever at a specific frequency.

    A 'monostable' 555 circuit will trigger one time for a set period of time.
  6. ericgibbs

    Senior Member

    Jan 29, 2010
    hi tevor,
    A retrig monostable output will be set High while the stream of pulses from the flow meter keep coming.
    When the pulses cease, the monostable output will go Low.

    The output of the 555 via a transistor can be used to operate a relay, which will be used to switch the pump On/Off
  7. tracecom

    AAC Fanatic!

    Apr 16, 2010
    From looking at the product in your link, I think what you can do is use the pulses out of the liquid flow meter to retrigger a 555 monostable. A monostable is also called a "one-shot" because when it receives a trigger pulse, it activates for a period of time that is determined by a resistor and capacitor. At the end of that time period, the 555 releases. A retriggerable version allows the activation period to be restarted each time a trigger pulse is received. Thus, you need to be sure that the trigger pulses from the flow meter recur in less time than the 555's activation period. Fortunately, the trigger pulses from the flow meter seem to be "low going," which is what the 555 requires.

    What is the power source you plan to use for your circuit?
  8. MikeML

    AAC Fanatic!

    Oct 2, 2009
    A 555 configured as a retriggerable one shot is illustrated in this forum posting. The timing is likely not what you want for your project, but the concept is. Also, in this example, it is a low-going pulse that resets the timing network.
  9. trevor lloyd

    Thread Starter New Member

    Jun 30, 2015
    WOW guys thats all i can say this is an amazing community !!!! thanks so much for all the info i am going to do a bit of digging into this and il get back to you tomorrow but this does look very promising
  10. AnalogKid

    Distinguished Member

    Aug 1, 2013
    Three questions:

    1. What power is available for the sensor and the control circuit? For example, if the pump runs on 110 VAC and you have no other power supply, then you'll need a small battery charger wall wart that comes with cell phones and cameras to power the sensor and control circuit, and a solid-state relay to control the pump. If it is a DC pump, say 12V or 24V, and you have a power supply for it already, then the control circuit can run directly off of that.

    2. What does the pump need to run? 110 VAC, 24 VDC, etc.? Also, what is the current or power rating for the pump? These will determine what is needed to take the control signal and use it to turn the pump on and off.

    3. What is the water flow rate when the pump is running (gallons per minute)? The sensor is a water wheel, and makes a short pulse with each revolution of the wheel. The faster the water is flowing, the faster the wheel turns and the faster the pulses occur. The retriggerable monostable fills in the gaps between the pulses so the relay controlling the pump sees a continuous signal. A 555 can do this, but so can other circuits. If you have some guess as to the pump flow rate, it can be used to calculate the slowest sensor pulse rate as a starting point for the controller circuit design. For example, one gallon-per-minute (gpm) flow rate is approx. 26 Hz out of the sensor.

    Last edited: Jun 30, 2015