I am trying to make a device that would act as a switch when it detects
water or any liquid at its 2 electrodes. I thought I could just use 2 wires
as the switch and put them in water but that doesn't work. I have bought a
small 9 volt water detecting unit that sounds a piezo horn when water is
present, but there doesn't seem to be any way to use it as a switch because
there seems to be continutiy all the time at the 3 poles of the horn.

I need this device to time the length of time it takes for paint to dry. I
figure that while the paint is liquid it will allow current to carry and
when it is dry it won't.

Any ideas, kits, anything would be greatly appreciated.
Thanks,
Rudy

For whatever it's worth, another approach would be to measure the
capacitance between two plates half-immersed in the liquid. As the
liquid changes state, two things should happen -- its level should
change, and its permittivity should change, thus changing the
capacitance of the two plate system.
I'm not sure if there is a capacitance-to-voltage chip to buy; I kind
of suspect it's something you'd have to concoct yourself.
Assuming you can transduce the capacitance into a voltage signal, you
can use an op-amp as a comparator to generate the on-off signal for the
horn. You'll have to measure the capacitance cutoff for dried paint,
and that would be your reference voltage, which you can make by
voltage-dividing a battery. Use large resistors.
As to what kind of signal your piezo horn needs -- an oscillating or a
direct signal -- I don't know. If it's like a normal piezo speaker,
you'd need to hook it up to an oscillating source to hear anything.
There might be a VCO IC chip in your kit.
Anyway, I hope this gives you an idea. Good luck!

rudyeb wrote:
I am trying to make a device that would act as a switch when it
detects
water or any liquid at its 2 electrodes. I thought I could just use 2
wires
as the switch and put them in water but that doesn't work. I have
bought a
small 9 volt water detecting unit that sounds a piezo horn when water
is
present, but there doesn't seem to be any way to use it as a switch
because
there seems to be continutiy all the time at the 3 poles of the horn.

I need this device to time the length of time it takes for paint to
dry. I
figure that while the paint is liquid it will allow current to carry
and
when it is dry it won't.

Any ideas, kits, anything would be greatly appreciated.
Thanks,
Rudy

The intent is for the device to be part of a setup that includes a 1 1/2
volt DC battery powered analog clock. The idea being that when there is a
current flow through the liquid paint the clock would run and when the paint
dries the clock would stop. The amount of time the clock ran would tell me
the time it took for the paint to dry.

This is what I need the switching device for.

Thanks,
Rudy

Hmmm,

Are you talking about "paint on a wall" or "paint in a bucket"?
Have you tried taking any resistance reading of wet/dry paint?

My GUESS here, is that the resistance of paint is going to depend on how
thick the paint is, amongst other things.

If you can do this at all, I suspect it will require a microprocessor to
monitor changes in the resistance, and "decide" that the paint in dry
once the delta/time drops to some low level.

- jim

rudyeb wrote:
The intent is for the device to be part of a setup that includes a 1 1/2
volt DC battery powered analog clock. The idea being that when there is a
current flow through the liquid paint the clock would run and when the paint
dries the clock would stop. The amount of time the clock ran would tell me
the time it took for the paint to dry.

This is what I need the switching device for.

Thanks,
Rudy

On Sat, 08 Jan 2005 02:01:16 -0500, rudyeb wrote:

I am trying to make a device that would act as a switch when it detects
water or any liquid at its 2 electrodes. I thought I could just use 2 wires
as the switch and put them in water but that doesn't work. I have bought a
small 9 volt water detecting unit that sounds a piezo horn when water is
present, but there doesn't seem to be any way to use it as a switch because
there seems to be continutiy all the time at the 3 poles of the horn.

I need this device to time the length of time it takes for paint to dry. I
figure that while the paint is liquid it will allow current to carry and
when it is dry it won't.


No, a water detector won't do anything with paint. Paint isn't conductive
either way.

If you're simply setting up some kind of lab test to determine drying time
of paint, then make a test strip that you can actually physically check
each hour or two.

Otherwise, just wait 24 hours - if your paint isn't dry by then, then
you're doing something wrong.

And even if it's "dry", you still need to give it some additional time to
cure before you can put on a second coat anyway.

Or, you could find some exotic coating that dries instantly, but that's a
wnole nother course of study.

Good Luck!
Rich

Hi Rudy:
I believe the paint is latex and it does have water in it. The first
step is to measure the resistance of the paint while it is drying. The
water will evaporate reducing the resistance. Plot the resistance over
time. Now you can set limits with resistace. The next step is to
measure the resistance just using a voltage divider (2 resistors) and a
voltage comparator. LM319 .

If the thickness is like .1", then it will work. However, if the paint
is over a surface then this will not work. The reflectivity will
change while it is drying, Try using a LED and a photo transister.
Allow the photo transistor to measure the reflection of the LED. All
paint is glossy while it is wet and turns dull after drying.
Harold




Good luck
Harold

On Sat, 15 Jan 2005 21:14:14 -0800, hryan wrote:

Hi Rudy:
I believe the paint is latex and it does have water in it. The first
step is to measure the resistance of the paint while it is drying. The
water will evaporate reducing the resistance. Plot the resistance over
time. Now you can set limits with resistace. The next step is to
measure the resistance just using a voltage divider (2 resistors) and a
voltage comparator. LM319 .

And then, does he leave the electrodes painted-in? Or rip them loose and
leave gouges on the wall?

Electrodes to watch paint dry, indeed!

Cheers!
Rich