Didn't realize that Onsemi had taken them over!

Where have you been? Complete rearrangement. What used to be Motorola is part of the same company that used to be NatSemi. That could never have happened!

 

https://www.onsemi.com/pub/collateral/an-3006.pdf
http://ww1.microchip.com/downloads/en/appnotes/00958a.pdf
but don’t use a pot with a metal shaft!

Here’s the 555 and comparator circuit. The slow triangle wave on the capacitor is buffered and used to vary the comparator threshold slightly over a period of a second or so.View attachment 300152

This is a very good approach. But I need to use this for a smart water heater and this for sure need to implement a control system.

 

This is a very good approach. But I need to use this for a smart water heater and this for sure need to implement a control system.

If you need to control it digitally then swap the pot for a digital pot.

 

If you need to control it digitally then swap the pot for a digital pot.

Instead of the POT I will introduce the high and low of the Arduino ?

 

Water heater temperature is normally set only twice. Once, when the plumber installs it, to the max legal temp of 125° F. Then by the homeowner to 140 so he can take a shower longer than 30 seconds.

 

Instead of the POT I will introduce the high and low of the Arduino ?

That will give you two settings, I thought you might want more. That's why I suggested a digital pot,
https://www.microchip.com/en-us/products/data-converters/digital-potentiometers