Cap Sensor -Redux

[bigkim100]

Just to get everyone up to speed here is the original post-
I have a small tank (about the size of a pop can) that is made out of plastic, and I need to monitor the water level inside. I CANNOT USE PROBES..no matter what, so does anyone have a circuit that can be used that uses capacitance in such a small container. I only need to know when the level gets low, not terribly accuratly, BUT the circuit does need to ignore short bursts of the tank "sloshing" the water around. .Again, the circuit only needs to trigger when there is about 1/2 to 1 inch of water in the container, after being full.
Is the "sloshing" sensor part of this something that only a microcontroller can do, or can a analogue way be designed.

So Im going to use some of the ideas given for the sensor....now its time to ask about the "Slosh Rejection" part of the project

The circuit does need to ignore short bursts of the tank appearing to be empty when the contents are "sloshing" around.
Again, I would prefer a non-microcontroller answer to the question, and tye circuit must be battery powered.
Again....the stats for this job are not very tight, it is not a life-or-death application.

 

[dknguyen]

Filter

Assuming that the sensor outputs a voltage that represents fluid level, take that voltage, and buy an analog filter IC, possibly from Maxim-IC or something, and plug the votlage into that. Then tune the cutoff frequency with a variable capacitor/resistor (or however the IC requires you to tune it's cutoff frequency)until the sensor starts ignoring splashes. You need an IC capable of going to a very low cutoff frequency like 1Hz minimum or something. Also, you probably need a sharper frequency cutoff than most filters because sloshing happens at a frequency that can be quite close to the frequency components involved in changing fluid levels. So, pick an IC that has more "orders" of filtering.

THe alternative is to make a time-delay circuit that only reacts to a fluid level change when that fluid level is maintained or greater than the level in question for X seconds.