Smart Digital Thermometer - design1

In the previous blog we discussed the requirement of offsetting the voltage we get as output from LM335 temperature sensor.

Now let us design the subtractor ckt we talked earlier.

Here is the circuit diagram (EAGLE CAD)

OK so this is a basic op amp based subtractor ckt. We are using a 2.73V reference (a whole new blog on how to generate temperature independent precise voltage reference) which will get subtracted from the voltage coming from the sensor in the resistor R6.

You may notice that I have left one terminal of the sensor open that is the adjustment terminal which we will use for the calibration purpose again another new blog.

Anyways with this arrangement what we get is 0volt when the temperature of the sensor is more or less 0 deg C.

For the op amp I have used a generic LM358 op amp which has a modest offset voltage. Right now I will use this later if required will upgrade to ultra low offset voltage chopper amplifier.

Now for some simulation

However before we do so let me tell you the above ckt wont work. Reason for a subtractor circuit to work the opamp must be powered from a dual supply how ever above we are using single so output just wont swing. 

Now for simulation let us apply 3.73 volt at the input stage of the amplifier (simulating 100 deg C) we should get 1 volt output right but the opamp just cant deliver this. Let us plot the graph for the above arrangement.

The blue curve represent the input 3.73 volt. The green curve the output which should be 1 volt but here it is merely 100mV.

So we need a dual power supply (talk more during the power supply design stage)

Let us modify the ckt for dual supply +5V and -5V

Now let us simulate again for 100 deg C we should get 1 volt

The green plot the input is at 3.73 volt now the output does swing to 1 volt. 

So the solution is to use dual power supply for our opamp.

Implementing in strip board

Although here I am using two 9volt batteries with a common centre tap to generate positive and negative supply in the actual design we will be using switched mode supplies which will use flying capacitor voltage inverter.

In the next design phase we will be concentrating on the implementation of a temperature independent 2.73 volt precision voltage reference.

to be continued...