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I am hobbyist only with some basic idea about electronics..

I am trying to measure rooftop watertank water level and to display it on a small led bargraph. So by googling I thought about to proceed with ULN2003 darlington array.

Here is the circuit I'm trying. enter image description here

But my challenge is it's little far from tank to the bargraph at room. So taking 7 wires from rooftop is not sounds to be a good approach. Is there is any encoders or something like that to convert this 8 output (in my case 6) to a single serial output and to decode it?

I thought about shift registers but it does require some mcu to control it and also some voltage restrictions.

So which is the best way.

I have one more confusion, which is the best metal to use as an underwater probe. Or how can I eliminate the risk of corrosion or oxidation issue after some period.

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The simplest 'no MCU' solution would be analogue.

Have an octal (or hex as you only want 6 electrodes) buffer at the tank, either the ULN2003, or perhaps better still a CMOS type like an HC7404, with a simple resistor to rail at the outputs. As each output goes valid, it draws more current from the rail.

The receiving end uses a current monitor resistor and a bargraph voltage measurement IC (like an LM3914) to monitor the current drawn.

The advantage of CMOS is that you can use very low currents at your tank probes to minimize electrolytic corrosion, perhaps 1 MΩ resistors, or more.

I'd tend to use stainless steel as an electrode material, relatively inert, and cheap to replace when they finally rot.

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the easiest two-wire way Would be to use somerthing like LM3914 which is a chip that converts a voltage level into a bar-graph

on the tank you'd have a bunch of resistors dipping in the water pulling the voltage upwards and have pull down resistor or current sink to set the empty voltage.

kind-of like this:

schematic

simulate this circuit – Schematic created using CircuitLab

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