Need help & guidance for AD620 Applications for LM35/Load Cell/Light Dependent Resistor

[Isaac Po]

Hi, I'm a college student here. I would like to design a simple fire alarm circuit using commonly found electronic components in the laboratory such as (resistors, LED's, Potentiometer, op-amp ic, & etc).

My assignment specifically requires to me use a temperature sensor(LM35/Load Cell/Light Dependent Resistor) & instrumentation amplifier(AD620 IC) to complete it.

This is the circuit I have built in NI Multisim software, I am currently struggling to make "AD620AN" to act as a comparator for the difference in voltage for LM35 & voltage divider resistor(R6 & R7). For the "Indicator/Display" part of this block diagram, I was thinking of adding 2 LED's; 1st LED for indication of temperature below 50 degrees(indicates: No Fire) & 2nd LED for indication of temperature above 50 degrees(indicates: Fire Occuring).

For the final circuit, I would like to have an active filter(MC3403 IC) included in the circuit somewhere between LM35 IC and AD620 IC.

 

[barry]

First of all, the AD620 is an instrumentation amplifier, not a comparator. You COULD, I suppose, make it work as a comparator, but why?

If the assignment is to use the AD620, then maybe you should use it for it’s intended purpose, like amplifying the output of the LM35. The way you’re using it doesnt take advantage of the whole reason for using an INAMP to begin with (common mode rejection).

Secondly, I don’t think its a good idea to drive an LED directly from the AD920.

Thirdly, why do you think you need a filter?