I have build a differential amplifier comparator and it works fine except that instead of switching voltage between zero and 5v, it toggles it between 3.5v and 5v.
So, I would like to know if there's a way to make it switch between 0-5v.
The schematic is as follows

 

I would like to know if there's a way to make it switch between 0-5v.

Add a PNP transistor with the emitter connect to +5V, the base connected to the top of the 12kΩ resistor (disconnect the resistor from +5V), and the collector connected to a resistor (e.g. 1kΩ) to ground.
You should get a voltage swing of 0 to 5V (minus the collector-emitter saturation voltage) at the transistor collector.

 

Add a PNP transistor with the emitter connect to +5V, the base connected to the top of the 12kΩ resistor (disconnect the resistor from +5V), and the collector connected to a resistor (e.g. 1kΩ) to ground.
You should get a voltage swing of 0 to 5V (minus the collector-emitter saturation voltage) at the transistor collector.

I don't quite understand the way you are telling me to do the connection can you post a schematic.
Also I don't have PNP transistors right now ..is it possible to achieve 0 to 5v swing with NPN transistor

 

I don't quite understand the way you are telling me to do the connection can you post a schematic.

See below

Also I don't have PNP transistors right now ..is it possible to achieve 0 to 5v swing with NPN transistor

Possibly, but I don't offhand how to simply do that.

 

I have build a differential amplifier comparator and it works fine except that instead of switching voltage between zero and 5v, it toggles it between 3.5v and 5v.
So, I would like to know if there's a way to make it switch between 0-5v.
The schematic is as followsView attachment 253580

Replace the right-hand-side transistor with a PNP type, emitter to the positive supply.
The two resistors connected to that transistor are unnecessary.
That makes a conventional differential amplifier.
[Edit] The way you have connected the bottom of V to ground is particularly confusing.

 

 

All right, I guess I understand now..
.. your help is really really is appreciated...thanks

 

I forgot to mention: R7 adds a little bit of hysteresis, and makes the changeover a bit sharper.