Connecting a Dometic Holding Tank Sensor to Simarine Pico

[Tenedos]

Just purchased a Simarine Pico to monitor our water tanks. We also have a Dometic holding tank sensor. It is one of these:



I don't know the working principle of the Dometic. Does anyone know if there is way to connect this to Pico? Would be ideal to monitor them all from the same unit.

Thanks in advance.

[cal40john]

If I've found the right manuals -

The Dometic has 3 magnetic switches, one for each level.
I would connect the common wire to all the switches to ground. Then put a resistor between each of the switches other lead, and then another resistor to 12 volts from the top switch/resistor. Run your sense line from the top switch.
The Pico has analog inputs and resistive inputs.
Attach the sense line to one of the analog inputs.
As each switch closes the resistance changes, sending a different voltage to the Pico analog input. If you use all the same value resistors you'll get a log scale effect. You have to use different value resistors to make it linear.

The Dometic only has 4 discrete levels because they are switches.

If you don't understand what's really going on here, you should hire an electronics tech and not do it yourself.

https://simarine.net/wp-content/uplo...T107_EN_DE.pdf

https://www.dometic.com/assets/46/80...nual_64680.pdf

[peghall]

I know you'd like to be able to monitor two tank on the same panel, but before buying the Dometic TankWatch system that uses internal senders that WILL require removal to clean off the animal fat buildup on them at least every couple of years, I suggest you take a look at the SCAD tank level monitors that use senders applied to the outside of the tank (any material but metal). Scad Tank Monitors Top rated by Practical Sailor.


--Peggie

[Tenedos]

Originally Posted by peghall

I know you'd like to be able to monitor two tank on the same panel, but before buying the Dometic TankWatch system that uses internal senders that WILL require removal to clean off the animal fat buildup on them at least every couple of years, I suggest you take a look at the SCAD tank level monitors that use senders applied to the outside of the tank (any material but metal). Scad Tank Monitors Top rated by Practical Sailor.


--Peggie

Thank you Peggie. I installed a Scad TM1 on my previous boat for monitoring the holding tank for that reason. Our new boat came with a Dometic tank sensor already and I am installing 240-30 ohm sensors for water tanks. So, I don't plan to change the Dometic sensor for now but I really want a way to monitor everything via a single panel.

@cal40john, thank you for your response too. I am an electronics engineer by training (heck, didn't use it for 20+ years but still...) so I follow what you mean. I wish I could find a way to prevent soldering resistors on a half based breadboard but looks like I will give this a shot. Getting rid of of that ugly display and replacing with a beautiful looking Pico is a super tempting incentive.

[Tenedos]

I think I have an idea. @cal40john, I am thinking of making this a resistive sensor reading for Simarine, instead of a voltage based sensor.

There are 4 cables coming out of the sensor, one being ground (black(1)), one for low (green(2)), one for mid (yellow(3)) and one for full (red(4)).

If I connect 40, 120, 60 and 20 ohm resistors to black, green, yellow and red respectively and then connect their lead terminal together (basically making them all parallel resistors as switches open one by one), reading the resistance from that terminal should give 40, 30, 20 and 10 ohms when the tank is empty, low, mid and full in a linear way. I am assuming Simarine Pico would support a way of configuring this (40 ohms =~ 0%, 30 ohms =~ 33%, 20 ohms =~ 67% and 30 ohms == 100%).

It obviously doesn't have to be 40, 120 etc ohms as long as the multiples are correct. I need to see what kind of resistors I have in my old electronics toolkit

Thoughts?

[cal40john]

The manual seems to indicate that the Pico is expecting a 65k ohm resistance, not that that would be an issue, just pick the right values.

Maybe I'm not placing the resistors as you intend, but I assume the black wire on the Dometic is common to all the switches. In the condition where all the switches are open there would be infinite resistance, which will be a big change from the other values.

[Tenedos]

Ok, I cut the cables and started testing with a multimeter but it turned out to be more complicated than it looks. Switches are not identical. I was getting suspicious from the diagram and that the switches are depicted differently and they are indeed different. The Green switch turns on when it doesn't detect a liquid and it turns off when it detects it. Orange and Red switches are the opposite, meaning they turn on when they detect liquid and they turn off when they don't detect anything. And the gauge has a logic to combine them all, something like this:

• If Green switch is on, turn the Green light on.
• If no switches are on, turn the Yellow light on (at this point, Green switch turns off). At this state, the display basically assumes it is yellow since no switch is on, it doesn't really read this state directly from any switch.
• If Orange switch is on, turn the Orange light on.
• If both Orange and Red Switches are on, turn the Red light on.

I posted an illustration.

Unfortunately, there is no way to turn these states into linear voltages or resistances purely by using a set of simple resistors, it requires something like a microcontroller that can take the combination of switches as an input and spits out a voltage. Quite disappointed...

[cal40john]

These lines in the manual look like you can flip the switches to change them from Normally Open to Normally Closed, or vice versa.

4. To make the #1 float switch activate the “Empty” light, the float must be positioned with the recessed shoulder,
white band, or the letters “NO” at the bottom. Remove the C-clip from the switch body, flip the float 180 degrees,
and replace the C-clip. (See Diagram #1.)
5. The #2 float switch can activate the “Mid” light on black and gray water tanks and activate the “Low” light on
freshwater tanks. The recessed shoulder, white band, or the letters “NO” should be facing up for black and
gray water tanks and down for freshwater tanks. (See Diagram #2.)

[Tenedos]

Ok, finally got to hack this one. I installed an Arduino Nano and connected it to the Dometic sensor and wrote a simple code for it. After calibrating the Pico, I have a working solution!

In case some wants to redo this at a later stage, here is the code:

int greenPin = 2;      // Green line from tank sensor
int orangePin = 5;     // Orange line from tank sensor
int redPin = 8;        // Red line from tank sensor
int outputPin = 11;    // Output to Pico for voltage based reads
int green = LOW;
int orange = LOW;
int red = LOW;
int outputValue = 0;

void setup() {
  pinMode(greenPin, INPUT);
  pinMode(orangePin, INPUT);
  pinMode(redPin, INPUT);
}

void loop() {
  green = digitalRead(greenPin);
  orange = digitalRead(orangePin);
  red = digitalRead(redPin);

  outputValue = 33;
  if (green == HIGH) {
    outputValue = 0;
  }
  else if (red == HIGH) {
    outputValue = 100;
  }
  else if (orange == HIGH) {
    outputValue = 66;
  }
  analogWrite(outputPin,outputValue);
}

[silvercreek]

Outstanding work Tenedos.

For anyone else interested in this, I believe Simarine has built this functionality into the Pico now with a recent firmware update.