water level sensor

Discussion in 'Electronic Design' started by nick4cpp, Jan 19, 2008.

1. nick4cppGuest

I'm an electronic engineer but I have always worked ad a software
engineer. So I know the theory but I have no experience on electrical
circuit design.
I need help designing a simple circuit for a water level sensing
system. I have found different sensors that act as a current
generator.
I need to know where I can find information on the best way to read
the output. I was thinking about using an opam.
A simple circuit with a resistor between the negative input and the
output and the Vcc applied at the positive input.
In this case the tension will go from Vcc to somthing less than Vcc.

It looks enough for me but just in theory. Do you know where I can
find info on how to implement it in a real application?

Thanks

2. linnixGuest

Do you mean the sensor output to the positive input?
Can you post a link to the sensor datasheet?

3. Bob EldGuest

What do you mean " I have found different sensors that act as a current
generator"? Are these commercial sensors and if so what are the
specifications?

Or, are you proposing to sense the resistance of the water by making contact
with it. This does work but you have to be careful in selecting the
electrodes that they are not subject to corrosion or that any corrosion or
ion exchange does not contaminate the water or alter the sensing.
Furthermore water's conductivity varys greatly depending on the impurities
in it. Pure water is a "good" insulator.

I designed a water sensor that works on capacitance change. Water has a high
dielectric constant, about 80 and therfore its presence is easily detected
when made the dielectric of a capacitor in an electronic circuit.

In my device there is no contact with the water. The sensor is a few sq cm
of copper foil inside of a thin sealed PVC pipe. When water is present on
the outside of the pipe, the capacitance of the foil is greater than when
water is not present because the dielectric constant to ground is much
greater wet than not wet.

The detector also sealed in the pipe is a CMOS circuit consisting of an
astable multivibrator (oscillator) and a monostable multivibrator (one
shot). The one shot is triggered on each positive edge of the oscillator.
The time out period of the one shot is determinned by the sensor capacitor
(foil) connected to it.

The oscillator also triggers a D flip-flop on each cycle. The output of the
one shot is connected to the D input of the flip-flop.

If the period of the one shot is long enough so that it is high when the
flip-flop triggers, the output of the flip-flop will be and will remain
high. This is the high capacitance, water detected state.

If the period is short, the D input is low when triggerd and the flip-flop
output stays low. This is the low capacitance water absent state.

The internal circuit works on 5 volts at about one MHz, but is conntected in
a two wire 20mA current loop for detection of water in a tank a couple of
hundred feet away from the pump circuitry that uses the information.

Please e-mail me at montassocatyahoocom if you have other questions.

4. Don LancasterGuest

Sprague chips to do this have existed for decades and cost pennies.

--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552

Please visit my GURU's LAIR web site at http://www.tinaja.com

5. Spehro PefhanyGuest

Unfortunately, Sprague Technologies has not existed for decades.

What's left of it is called Allegro and owned by Sanken of Japan, so
maybe they'd prefer yen over pennies. Anyway, from a quick look I
don't see anything much along those lines.

Best regards,
Spehro Pefhany

6. RobboGuest

There used to be the National Semi LM1830, AC sense signal etc all in a
DIP14. AFAIK this is long gone.

7. Spehro PefhanyGuest

Yes, that hoary bipolar chip was specifically designed for this kind
of application. It doesn't even show up on their website search
engine-- but Google finds it in their obsolescence list with last-time
buy date of 12/08/98. Personally, I'd consider using a
microcontroller. ;-) Most practical solution in a lot of situations
is probably a reed (or Hall) switch activated by a magnet float. Made
by the millions for dishwashers and the like.

Best regards,
Spehro Pefhany