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Old 02-26-2015, 01:14 PM   #1
JamieGeek
Axis/Vegas Enthusiast
 
Brand: Thor Motor Coach
Model: Axis 24.4
State: Michigan
Posts: 9,837
THOR #1150
DIY Capacitive tank sensor

Long post warning..

I'll start a new thread here to discuss my progress with making my own black tank sensor. The idea first appears on the forums here in the black tank flushers compared thread. Much like my JRV-212 upgrade I'll post updates here as I make any progress. I suspect that this project will take more time than the radio upgrade simply because this will be all custom and not a simple drop-in replacement like that was. In addition some of my activities will be hampered by the winter weather (can't crawl around underneath the RV when there still is 2' of snow on the ground). There is a very good possibility, also, that this may not work--at least what I design. Caveat emptor...

To start with I'll list some of the goals:
  • More accurate black tank level sensing
  • Circuit fits in between tank and existing wiring for probes
  • Circuit's output controls the probe wires so that the existing panel will still work
  • Circuit is removable (e.g. disconnecting circuit and reconnecting the probe wires back to the probes will restore original functionality)
  • Low power draw (ideally circuit only powered when the panel button is pressed)
  • Low cost ($50 or less)(although I'm sure my experimentation will cost more than that!)
At the moment I have a few assumptions about the tank level sensors that I'll need to verify:
  • All probes above the "empty" one are pulled up (e.g. will have voltage on them when the tank level is below the sensor)
  • Bottom probe is ground
  • A given probe is "shorted" to the bottom probe when the level touches the probe (causing the voltage to drop which triggers the indicator on the panel)
  • The probes are only powered when the tank's button is pressed on the panel
I'll need to verify the assumptions by performing some experiments on the coach (perhaps some of you in warmer climes wouldn't mind doing some investigative work?? LOL):
  • With a voltmeter measuring the voltage between the top & bottom probes have someone press the tank button (If my assumption is correct the voltmeter should read 0 until the button is pressed then it should go up..possibly to 12V)
  • (There is a possibility this experiment may cause damage): With someone holding the tank button down (on an empty tank) take a jumper wire and short the bottom two probes together (this would be ground and the 1/3 level probe the expected result is that the 1/3 level indicator lights up. The danger here is if the panel doesn't expect a short for some reason--a resistor could be used here instead of a direct short)
My intent is to use an Arduino Pro Mini microcontroller to measure the capacitance of two "plates" placed on the tank. An empty tank will read 0, as the tank fills up the capacitance will increase (not exactly linearly but it will be close enough). The Arduino will take this reading and translate it to the existing panel LEDs (using transistors to short out the probe wires to the bottom probe wire). Using a small computer means I can add some functionality in like blinking the highest LED according to how much fluid is in the tank. Thus if the tank is less then 1/3 full the 1/3 LED will blink depending on how close the level is to 1/3 (faster blink = higher level).
To save power I'm looking to put a one-shot circuit in so that the Arduino is powered down until some voltage is detected on the probe wires (otherwise I estimate the circuit will draw around 15ma all the time--not good for boondocking). When voltage is detected the Arduino will be powered up for approx 10 seconds (long enough to signal the panel).
In the meantime I'll probably be picking up a storage tote as a substitute for a black tank experimenting with sensors (placement, how big to make the foil patch, how many patches, etc.). Yeah this means I'll have my own "black tank analog" just like the FitRV guy ! LOL
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2022 Thor Axis 24.4
2021 Mach-E
blog - https://spareelectrons.wordpress.com/
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