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added level sensor resistance picture

edited Jun 14, 2019 at 13:55

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Question

Does anyone have any ideas on how I can use RPI with ADC to measure a tank level sensor which has a range of only 20 to 200 ohm?

Answer

Some months ago I played with a couple of moisture sensor / level detectors. Below is an example.

Water Level Detection Discussion - Rpi StackExchnge

However the above sensor does not answer the OP's question, which asks to detect resistance using ADC, not analog or digital voltage levels. So I think I need to start from a variable resistance and check out how to use ADC to measure the resistance, within a range of 20 to 200 ohms.

I very surprisingly found that all the five samples from different manufacturers have the same resistance of 0.5 ohms for each of the two resistive "legs".

Long Answer

/ long answer to prune later, ...

Introduction and Summary

/ to continue, ...

Question Analysis

I think the preliminary functional spec of the project is very good, because it is concise and precise.

There are many reasons that the question is so short, but I will not list my wild guesses for now. Perhaps later.

The spec includes the following key words/phrases:

"Rpi, ADC, tank level sensor, 20Ω ~ 200Ω.

The word "tank", is crucial. She says "tank", not "container", perhaps she wants to indicate that the "tank" is deep , say, not a 5 inches deep fish "bowl" or "pond".

The phrase "20Ω ~ 200Ω" is also crucial. There are basically two types of liquid level sensors: active and inactive. Active sensors are usually 5V or 12V powered, and the sensor output is a DC voltage level. Inactive sensors don't have any power. Usually the sensor is just a reed switch and floating magnet, or a long "resistor" whose resistance varies as the water level. Usually the resistor is far, perhaps 10ft, and up to 1,000 ft from the measuring instrument.