How can I make a water level indicator panel for an RV fresh water tank?

I'm rehabbing an RV.   
It has a 100 gallon fresh water tank made out of plastic (polyethylene).
The tank has four built in sensors on the side of it, 0%, 25%, 50%, 75%,
I assume these sensors are just a bolt sticking through the side into the water.

How can I make a simple indicator panel which will light up an LED for 25%, 50%, and 75%?

I doubt I'd just be able to use the electrical conductivity between the 0% sensor and the other three, as the electrical resistance would be too high to send any usable connection through the water, am I correct?

But maybe something based on capacitance?   Anyone know how to build something like that?   My skills are too rusty to think that through.

Maybe I could cannibalize the sensor out of one of those touch sensitive table lamps and use it as the basis for my indicator panel?

I'd prefer not to use an arduino or pi for this, I need something incredibly simple and robust.   I realize those would do the job but I need the quick and dirty solution.

Thanks in advance for your help.

EDIT:   Here is a webpage that talks in depth about the standard tank monitors and how to upgrade it with an Arduino.   Unfortunately, this seems incredibly over engineered for this task and I don't have the liberty of spending the time to do this, as I am working with a tight deadline.  But it does discuss the resistor network which was originally used with the sensors on the tank.   Since the original display panel on mine was too damaged to save, I thought it wouldn't be too hard to rig up something simple to replace it.

sort by: active | newest | oldest
iceng29 days ago

Use the elements of the rain alarm...

Ignor the 555 as it is only an aural indicator..

iceng iceng29 days ago

He got a bad drawing but the rest seems OK

rickharris1 month ago

Most level sensing is done with a float (like your gas tank) or with something like ultrasonics.

Or you could weigh the tank with a load cell.

OF you could put a clear tube up the side open at the top and connected to the tank at the bottom, the liquid level in the visible tube will be the same as the tank.

Water is usually a surprisingly bad conductor, but it might work here. Have you actually tried to measure the resistance ?

You need capacitive sensors for this.
Although you don't need anything labeld "tank sensor".
You can basically take any capacitive sensor that has an adjustment for the sensitivity and can cover a distance at least 2mm longer than what your tank wall is thick - assuming it is plastic.
The sensors should be set so they go from low to high (on the output) when the water level is just covering them.
Most run on 24V but you can get them with a wide input ranges from 10-30V as well.
The output with a LED works by simply using a suitable resistor in series with an LED connected between output and either ground or the supply voltage - depends on the sensor, so please check the datasheet if it gives a positive output or closes to ground.

As a cheap alternative you also use old circuits from the 80's.
There are several, simple, water alarms out there.
Back in the day they were a popular gadget to give you an audible alram when your bathtub is full.
Just two wires going in the tub and when the water bridges them a transistor can switch on to activate the buzzer.
Just replace the buzzer with a LED.
For obvious reasons you want the wires fully insulated and use stainless steel ends as the actual electrodes.

I tried asking Google(r) Images to show me, "four level water level sensor circuit"

and based on what it has shown me, I am guessing the cheap and dirty way to do this is with a single transistor, for each level, and each LED. Basically it is just four copies of that single transistor circuit, running independently.

For all of these, the current to the base of the transistor is connected through the water, when water is touching the electrode corresponding to that level.

I guess you can look at those circuits, and decide for yourself whether or not what is suggested there is impossible.

Well, "impossible" was not the word you used, but you seemed doubtful regarding the existence of electric currents in water.

I doubt I'd just be able to use the electrical conductivity between the
0% sensor and the other three, as the electrical resistance would be too
high to send any usable connection through the water, am I correct?