> Usually this is done with a reed contact/switch in a hollow tube
(stainless
> with closed bottom) and a round float with hole in the center around this
> with a magnet in it when the magnet drops far enough it will activate the
> reed contact. your pump has to stop.

This part is best done electronically. The trick is to provide galvanic
isolation between the probe tips and the input stages to prevent the dc
current from corroding the sensors. I've had excellent results with the
hardware PWM on a PIC continuously injecting a signal in the water (around
1KHz worked fine), and detecting this signal on the sesnor thru' the water
when the level reaches this point.

>
> for this kind of application there would be two stops a bottom one to
> prevent the float from falling of the tube
> a top one to limit the floats top position so it just activates the reed
> contact (and not allows it to go past it )

I agree; this would prevent any hysterisis action. The pump would stop when
the level drops under the lower sensor, but come on only when it rises
*above* the upper sensor. Depending on the size of the tank, and the rate at
which the water gets replenished, these two levels could be as close as a
few inches from each other.

Regards,

Anand Dhuru


>
> Hope this helps
>
> Peter
>
>
> ----- Original Message -----
> From: "John Waters" <john_fm_waters@HOTMAIL.COM>
> To: <PICLIST@MITVMA.MIT.EDU>
> Sent: Tuesday, June 08, 2004 11:35 PM
> Subject: [EE]: water level detection, how?
>
>
> > Hi All,
> >
> > I have a tank with a submersible pump inside, I want to stop the pump
from
> > running dry when the water level is too low, what sort of sensor could I
> > use? I'm thinking of using a float with a magnet attached, the magnetic
> > field will trigger a relay to turn off the power, does anyone get
> experience
> > in building this kind of thing and would like to share the experience
with
> > me?
> >
> > Thanks in advance!
> >
> > John
> >
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