Can a Floor Pad Detect Liquid with Just One Drop?

[sapia]

Hey all,

For a school project, I am trying to design a pad (placed on the floor) which can detect liquid and make a loud buzzing sound immediately upon the detection of the liquids.

Any ideas?

 

[ChrisJA]

Hey all,

For a school project, I am trying to design a pad (placed on the floor) which can detect liquid and make a loud buzzing sound immediately upon the detection of the liquids.

Any ideas?

If this a school project then what technologies are allowed? Is this a middle school project or a first year EE?

In middle school use a float switch. 1st year EE, start with a comparator that is balanced by a resistor network (high value resistors) but conductivity of the water sensor pulls one leg up and flips the conspirator. You only need a cheap part like the lm311 and a resistor network. I think the 311 can even directly drive a small LED

The sensor is just a pair of bare wires.

 

[Danger]

conductivity of the water sensor pulls one leg up

You're making one assumption though; Sapia never stated what the fluid is. It might not be conductive.
If, on the other hand, it's for the same reason that my neighbour needs such a thing, he can get by with a simple video surveillance system to detect when his dog pulls one leg up.
I'm thinking that the conductivity issue can be avoided entirely by using some sort of absorbent gel or fabric that will expand when wet, and thus compress some sort of pressure sensor such as a grid of load cells.

 

[dlgoff]

I actually installed something like this snake

around the perimeter, under a raised floor like this

It worked. I got a call-in from the maintenance guy informing me the alarm was sounding. He said, "be sure to bring your fishing pole as there are some Smallies under the floor.

 

[Danger]

So... what is that and how does it work?

for the fish joke.

 

[dlgoff]

So... what is that and how does it work?

Conductivity. Once "pure" water penetrates or contacts the concrete floor, it's not "pure" any more and will conduct nicely.

 

[Danger]

Fair enough, but what if the liquid that he wants to detect is alcohol or canola oil?

 

[Bobbywhy]

The unit dlgoff pictured is here:
http://rletech.com/product/ld300/

 

[Danger]

Ah, yes. And the first line under "Features" specifies "conductive fluid sensing cable". I repeat myself by saying, "What if the fluid that he wants to detect doesn't conduct?"

 

[Jobrag]

Build in a low wattage constant power heater and a temperature sensor, when the mat gets wet the temperature will fall.

 

[sapia]

For my class project, the liquid that needs to be detected is water. I am hoping to do this as inexpensively as possible (my dad is going to help me pay for the parts so I can build one for presentation). I am hoping to hook the pad up to a buzzer so it makes a loud noise as soon as you spill water on it.

 

[Danger]

the liquid that needs to be detected is water.

I stand corrected.
A couple of bare wires and a doorbell will do it.

 

[sapia]

Thanks Danger! Say I buy a cheap buzzer off Amazon (with the two wires exposed, see picture below).

Would I need to add a battery to power it and create the buzz?

 

[Danger]

Thanks Danger! Say I buy a cheap buzzer off Amazon
Would I need to add a battery to power it and create the buzz?

Yes. Pure water doesn't conduct electricity, but it almost always picks up conductive minerals when let loose. Given the items shown in your pictures, just attach the two red wires to each other, then tape or glue the two black ones very close together (maybe 0.5mm).
Also, smear some sort of grease around the contacts of the battery case just in case the water is conductive enough to short it out.

edit: And if you have a chance, sneak some salt into the water supply.

 

[dlgoff]

Hey sapia,

Take a look at this technical reference from Omega.

www.omega.com/techref/pdf/aboutconductivity.pdf

All aqueous solutions conduct electricity to some degree. The
measure of a solution’s ability to conduct electricity is called
“conductance” and is the reciprocal of resistivity (resistance).
Adding electrolytes such as salts, acids or bases to pure water
increases conductance (and decreases resistivity).

The contacting-type sensor usually consists of two electrodes,
insulated from one another. The electrodes, typically 316 stainlesssteel,
titanium-palladium alloy or graphite, are specifically sized
and spaced to provide a known “cell constant.” Theoretically, a cell
constant of 1.0 describes two electrodes, each being one square
centimeter in area and spaced one centimeter apart (Fig. 3).

 

[berkeman]

Homework/project thread moved from the EE forum the HH/Engineering forum.

C'mon guys, please do not do students' homework for them. That's against the PF rules. Please report misplaced posts like this so the Mentors can move the thread to HH and remind the OP that he has to actually show some work and effort on his schoolwork projects.

sapia, check your PMs.

 

[Danger]

Sorry. I got carried away.

(Does it help that I was wrong?)

 

[CWatters]

Ah, yes. And the first line under "Features" specifies "conductive fluid sensing cable". I repeat myself by saying, "What if the fluid that he wants to detect doesn't conduct?"

Perhaps capacitance change would work?

PS Using DC can lead to corrosion/plating problems so using AC might be a better option even if using conductivity.

 

[Danger]

using AC might be a better option

Possibly, but I suspect that the single "AA" battery case that s/he showed might have difficulty providing that.

 

[ChrisJA]

Ah, yes. And the first line under "Features" specifies "conductive fluid sensing cable". I repeat myself by saying, "What if the fluid that he wants to detect doesn't conduct?"

If you are expecting a flood of cooking oil then use a float switch. Only trouble is that you need quite a lot of liquid to turn it on.
http://www.westmarine.com/webapp/wcs/stores/servlet/ProductDisplay?productId=34205&catalogId=10001&langId=-1&storeId=11151&storeNum=50632&subdeptNum=50633&classNum=50635#.UaGvTJU7Kx4

 

[Danger]

If you are expecting a flood of cooking oil then use a float switch.

I first started dealing with float switches in 1959. They are not suitable for this application.

 

[ChrisJA]

I first started dealing with float switches in 1959. They are not suitable for this application.

The requirement was did not give a detection level threshold.

OK then. If you must detect a non-conductive liquid and you can't wait until the level is two inches above the floor, one common why is a capasitive switch.

It looks like the conductive sensor in the it has two electrodes separated by a small distance. But in this case the electrodes form a capacitor. The dielectric constant of air is different from oil.

The trouble is how to clean the oil out of the sensor, the mechanical design that let's the liquid in and out will be hard if you are dealing with a thick, viscous liquid that was only enough depth to make the floor wet.

These sensors are used for airplane fuel sensors. They make the capacitor with two concentric metal tubes. But the tank has some depth, If you need to detect only 0.1 mm of liquid maybe design a pattern of copper on a printed circuit board and lay the PCB copper side down and track capacitance.

OK, one last idea. This could detect even one drop. Use a video camera aimed at the floor.

 

[Danger]

OK, one last idea. This could detect even one drop. Use a video camera aimed at the floor.

That's the first thing that I suggested, back in post #3.