Oz it would seem that you're suggesting the pump start filling when the upper level goes down a cm...no good for me; more detail given in my update above. Thanks for the response Oz

Your post does not explain why you don't want the pump to switch on when the level falls a few cm ... you want this tank kept full ??? well the switches you linked to will do that with nothing else .... 100's are sold on ebay for just this situation ... they do work as I described , I have used them!!

Do you really think people have all this complication just to keep a tank full up????
You have decided you want it to switch on at a level of your choosing ... unnecessary complication ... All you should want is for the tank to be full within a few cm ... and the float switches in your link will do that. thousands of people use them without problems ... the currents on some of those float switches do seem rather low ..

 

Here is a simple AWLC (Automatic Water Level Controller) that I designed. You can modify it to suit your requirements.
Working video, Circuit (Proteus 8.8 SP1 format, .pdsprj), Circuit (.PDF), Circuit (.PNG) are included in the attachment.

SW1, SW2 and SW3 in circuit are magnetic float switches.

Auto On-Auto Off
Manual On-Manual Off
Auto On-Manual Off
Manual On-Auto Off

features.

Except relay, float switches, C106D (SCR), P2N2222A, float switch and power supply connectors you can use SMD components for all other components and make the PCB very small.

Edit: Oops. I didn't check the section name. It is digital circuit section. If it is not a homework then you can use my circuit for your requirement. My circuit is not digital circuit.

 

Adding an Auto/Manual SPST switch between SW1 (float sensor) and 12V power supply will enhance the circuit such that if mode is auto (new SPST closed) then pump will turn ON when SW1 is closed but pressing BTN2 will turn OFF the pump when pressed but when released the pump starts again. So, the new switch put into manual mode (new switch opened) will make BTN2 to turn OFF pump when SW1 is still closed.

Edit:

Changed the circuit a little. Works fine now (Auto/Manual modes). Please test and reply.

 

Cleaned up final circuit attached.

 

I think this is what TS is looking for ,,, $3 on Ebay ... https://www.ebay.co.uk/itm/AC-DC-12...h=item41f775b675:g:wnAAAOSwUu5cLGrs:rk:2:pf:0

In most situations this would not be needed ,normally only one float switch at the top , to switch on the pump when the water falls a few cm ...

This circuit is designed for something like a hydrophonic tank ... the outlet would have another pump which is switched on periodically to flood the growing trays . It switches off after 15mins and the nutrient solution returns to the tank ... you obviously would not want the input pump activating every time this happened , so a lower sensor is needed which tops up with water only when water reaches the lower level, when the growing trays are flooded

 

@oz93666

My circuit works same. OP (TS) has to use just

Overhead Low Sensor and Overhead High Sensor.
Sump Level Low sensor is optional.

I have not tested my circuit on hardware. I still have to make the PCB after making PCB layout. I will replace all BC547 by BC847 (SMD version) to make the PCB as small as possible.

Here is new version of the circuit with Auto/manual mode selection switch and indicators.

 

Sorry. The was an mistake in resistor naming. Have fixed and tested it. Here it is.

I am now designing the PCB layout.

 

Much simplified circuit attached.

You need Proteus 8.8 SP1 or higher to open the .pdsprj file.