Water tank sensor circuit

Hi there. Wooo my first proper post.

Right down to buisness.

I have been using the schematics package tina. And I have a problem. This may be simple but I am first year so please bare with me.

Here is the question:

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A circuit is required that will indicate, via a voltage output, the total amount of water contained in four water tanks. Each water tank has a sensor which outputs a certain voltage depending upon how much water is in the tank as follows:

100% Full +20mV
50% Half full 0V
0% Empty -20mV

It is required that the maximum voltage output from the circuit be as close to, but not exceed the supply voltage of +/- 12V.

Using an IDEAL op=amp produce a circuit to fulfil the specifications outlined above.
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So I have a table of values. Now for the diagram I am thinking 4 voltage sources to represent the inputs, but do they then all go straight into the Op amp?

I am not so much looking for the answer more just a help in understanding. I am fairley new to this and there is very little help with the Tina application.
Thanks

The wires in the tank are open circuited and The 180K resistors pulls the switch low hence opening the switch and LEDs are OFF when the water is empty. When the water is filling up the tank, S1 and the + supply is shorted by water, so the switch S1 and turns the LED1 ON. As the water continues to fill the tank, the LEDs2 , 3 and 4 light up gradually.The number of level can be upgraded up to 8, if we use CD4066 ICs. The base of the transistor BC148 is pulled high by the water and this saturates the transistor, when the water is full. It will turn the buzzer ON. To turn OFF the buzzer, The SPST switch has to be opened. while pumping water, Remember to turn the switch ON, otherwise the buzzer will not sound.

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