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How does this water sensor switch work

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icom128

Member
Hi.
I need to build this circuit, but don't fully understand it's operation.
I do understand that the probes when in contact with water, should activate a load i:e (water pump).
Lets say I power the circuit with 12v, will the 5v relay burn out even though the schematic show's 4.5 to 15v input ?.
Or would I use a 12v relay instead.
Also Can the 12v pump share the same 12v psu that powers the circuit, if the 12v+ to pump is placed on the relay common.
Or should the pump be on a separate power supply ?.
I am also not sure of the orientation of the transistors . I have added on the schematic How I think they should be placed on the
Pcb board.

Thanks in advance for any help offered.
 

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WTP Pepper

Active Member
I wouldn't use a 5v relay on a 12v supply. It will eventually get hot. No reason why you cannot power the pump off the same supply as long as the supply can power the pump.
What you have shown is a simply Darlington type switch. Current flows through the water and excites current un the base of Q2. This then does the same for the second transistor and fires the relay. A pull down on the base of Q2 ~100K maybe worth considering to prevent local RF signals like mobile phones triggering the cct.
I would also add a diode reverse biased across the relay coil as the back EMF could kill it. 1n914/1n4148 would suffice.
 

icom128

Member
WTP Pepper Thank you for the quick reply.
Is it an error in the schematic that they show 5v relay with with 4.5 to 15v input ?.
Also with this circuit will the relay flip as soon as the circuit is connected to the power supply, and then flip back when the probes
touch water ?.

Thanks.
 

MikeMl

Well-Known Member
Most Helpful Member
Hi.
I need to build this circuit, ....

It is a typical Internet crap circuit. With > 5Vdc applied to the probes, they will last about two weeks, and they will be totally dissolve due to electrolysis!

To do this right, the circuit must be redesigned to apply only a few hundred mV of AC across the probes, and effectively do an AC resistance measurement...
That is the only way to prevent having to replace the probes frequently.
 

ronsimpson

Well-Known Member
Most Helpful Member
Also with this circuit will the relay flip as soon as the circuit is connected to the power supply, and then flip back when the probes
touch water ?.
No. The probes need to connect to make the relay coil have current. If the probes have no connection the relay coil will have no current.
 

Reloadron

Well-Known Member
Most Helpful Member
If this circuit is actually going to be built and relied upon you really want to rethink things. Heed what MikeML has mentioned. You will be cleaning the probes on a weekly basis and they will be short lived.

Yes, with any extended on time eventually the 5 volt relay coil will cook. I would use a relay coil matched to the supply voltage.

Note the comment of ronsimpson also. When the probes are submerged (bridged by water) the relay will be energized. The moment the water level drops below the probes the relay is de energized.

So if this circuit is to be used in a real application where reliability is important there are much better designs to consider, including simple float switch circuits. Also, you should better define the application. Do you want to pump out a tank when full or pump a tank to full when it is low?

Ron
 

tunedwolf

Well-Known Member
If you are considering redesigning the circuit, you may want to add some hysteresis to prevent the relay chattering 100 times a second at your water limits :)
 

icom128

Member
Thanks to all for your help and suggestions. Would stainless steel probes be effected by electrolysis ?.
I am trying to pump a flood area at the bottom of the garden after heavy rain storm. 4 inch deep
Not worth paying for a drain to be installed, just annoying when going to my garden shed. I pump
it manually at the moment with pump on a switch, but would rather the relay circuit to handle it after a storm.
I didn't know this circuit would be such a problem, to handle the task.
Interesting about the float design that will rise with water level and energize the relay, I will probably have to go with that
due to the hidden limitations of this circuit.
 

icom128

Member
tunedwolf I just built this circuit out of curiosity, your right it does chatter when water level rises. I am not sure what hysteresis is, or how I would add this to the existing circuit. Could you explain what components I need to use and where they would be placed on the schematic.
Thank you.
 

tunedwolf

Well-Known Member
Stainless Steel is just that, Steel that stains less :)

Any metal will be affected, to what degree will depend on many factors. The problem is the dc across the probes, not the probe material itself, although some materials lend themselves to the application better than others, like Stainless Steel, Titanium etc. As Mike said, if you plan on using probes directly in contact with your media, you need to be thinking about using ac here to avoid electroplating and sacrificial corrosion problems :)
Going down the float route, I wouldn't use a single float either. I would use two small floats, one mounted a bit higher than the other, and design the circuit so that the pump comes on when the high limit is reached and doesn't go off again until the lower limit is cleared. This provides some hysteresis :)
 

tunedwolf

Well-Known Member
Control the pump through a re-triggerable delay off timer, say 30 secs to begin with, but you'll need to adjust that to suit your pump's flow rate. That way you allow time for the pump to either clear enough of your media that the probes are no longer sensing that the pump needs to be on. You could add a 555 timer and a couple of resistors/ capacitors to achieve this. There are plenty of example circuits kicking about the web for 555 timer applications, Google is your friend :)
 

Reloadron

Well-Known Member
Most Helpful Member
Given a choice I would think about using a small tilt switch on a small arm to sense water level. Since you are only looking at a few inches of water a standard float switch isn't really practical. A Google of "Tilt Switch" will bring up some good candidates. Years ago they used mercury but the new ones are mercury free. Another merit is using a switch you have your hysteresis. A small miniature version of what is commonly used on basement sump pumps. Switch drives relay and relay drives pump. Personally I wouldn't screw with sensors using probes etc for an application like this. I would leave a manual switch across the tilt switch as a bypass. Doing it this way keeps it very, very simple. Switches like this one are relatively inexpensive and with some RTV can be encapsulated. Anyway, given ways to do this I would likely just use a switch.

This is the data sheet for the switch.

I would just run 12 VDC using a 12 VDC relay.

Another option would be a magnetic reed switch design but I would go with the tilt switch or similar as a water sensor.

Just My Take....
Ron
 

MikeMl

Well-Known Member
Most Helpful Member
Go to a boat store and buy the float switch designed to mount in the bilge to automatically start the bilge pump. I have one in my boat, and it was cheap...
 

Reloadron

Well-Known Member
Most Helpful Member
Go to a boat store and buy the float switch designed to mount in the bilge to automatically start the bilge pump. I have one in my boat, and it was cheap...

Duh, had a 30 foot boat in NC for years and never thought of that. The ideal solution and they work in shallow water by design. Since it is going in a garden I would place it in a sort of chicken wire basket to prevent debris from effecting it.

Ron
 

icom128

Member
Thanks to All. I have scrapped the silly circuit now. And Will go with the boat bilge pump float. Just been having a look at them on Ebay.
They look ideal, I think they work by magnetic reed switch. I will dig a small square about 6" deep ,and cement a thick surrounding wall
so that I can place the pump and float in. That way all flood water will pour into the square and be pumped before it grows into a swimming
pool.
Thanks again to all who helped.
 
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