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?
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.
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
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