Dummy guide & advise on building a simple fire alarm circuit using "LM35" & "AD620" ICs.

Thread Starter

Isaac Po

Joined Nov 27, 2022
26
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).

1669531306401.png


My assignment specifically requires to me use a temperature sensor(LM35 IC) & instrumentation amplifier(AD620 IC) to complete it. 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).


1669532052439.png

1669532158904.png

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

Thanks in advance,
Isaac.
 

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Thread Starter

Isaac Po

Joined Nov 27, 2022
26
If you need a comparator, use a comparator.
https://www.analog.com/media/en/technical-documentation/application-notes/AN-849.pdf
Op amps don't generally make good comparators, instrumentation amplifiers make extremely poor comparators.
Just put in a comparator where you have the AD620 and it will work fine.
I am required to use AD620 for the assignment, the sensor components that available with me are temperature sensor(LM35), load cell(Straight bar weigh sensor), & LDR(light dependent resistor). Any other method to utilize this particular instrumentation amplifier IC(AD620)?
 

WBahn

Joined Mar 31, 2012
27,942
If I were evaluating your assignment and part of the assignment was to use an instrumentation amplifier, I would look far more favorably on a project that at least attempted to use the amp in a reasonable way compared to a project that was a poor match to the components available.
 

ericgibbs

Joined Jan 29, 2010
16,834
hi Isaac,
What is the problem you report when using the AD620 with an LM35 circuit.
Also, what part does the load cell play in the project.?
E
 
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Thread Starter

Isaac Po

Joined Nov 27, 2022
26
hi Isaac,
What is the problem you report when using the AD620 with an LDR circuit.
Also, what part does the load cell play in the project.?
E
Hi Eric,

What I meant when I reply to Ian0 previously is that for this particular assignment of mine, I am required to use one single type of sensor: LM35 or Load Cell(Straight bar weigh sensor) or LDR. The signal captured by the sensor would be required to pass through an active filter(MC3403 IC) and be amplified by an AD620 instrumentation amplifier.

I apologize for the confusion. and also apologize due to the limited components in our laboratory.
 

Thread Starter

Isaac Po

Joined Nov 27, 2022
26
If I were evaluating your assignment and part of the assignment was to use an instrumentation amplifier, I would look far more favorably on a project that at least attempted to use the amp in a reasonable way compared to a project that was a poor match to the components available.
May I ask would load cell sensor be a favorable component to be used with AD620 instrumentation amplifier?
 

Thread Starter

Isaac Po

Joined Nov 27, 2022
26
hi Isaac,
What is the problem you report when using the AD620 with an LM35 circuit.
Also, what part does the load cell play in the project.?
E
The problem I encounter in my circuit is:

1. I would like my LED1 to light up while LED2 is off when my LM35 output is below 500mV(below 50 degrees), and my LED2 to light up while LED1 is off when my LM35 output is above 500mV(50 degrees). But for this to work, my AD620 IC has to act as a comparator, how can this be achieved?

2. If i would like to add another LED to the circuit for 3 different conditions: LED1 for below 30 degrees, LED2 for above 30 degrees, LED3 for above 90 degrees. Is it possible to achieve this without the use of microcontroller?

Thanks in advance,
Isaac
 

ericgibbs

Joined Jan 29, 2010
16,834
Hi, Isaac.
A load cell is usually a full bridge configuration, a IA amp is often used in conjunction with a Wheatstone bridge.
E
 

crutschow

Joined Mar 14, 2008
31,151
The LM35 requires a voltage source to generate an output voltage. Please read its data sheet.
I see none in your schematic.
If i would like to add another LED to the circuit for 3 different conditions: LED1 for below 30 degrees, LED2 for above 30 degrees, LED3 for above 90 degrees. Is it possible to achieve this without the use of microcontroller?
Certainly.
But you would need to add another AD620 and some simple logic gating circuitry.
 
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Ian0

Joined Aug 7, 2020
6,736
The LM35 requires a voltage source and resistor to generate an output voltage..
I see none in your schematic.
Certainly.
But you would need to add another AD620 and some simple logic gating circuitry.
If you wanted 10 LEDs, try an LM3914
 

crutschow

Joined Mar 14, 2008
31,151
If you are using the AD620 as a comparator, you want a high gain, so R5 should be set to a small resistance value (1 ohm or less).
Gain equation below (Rg is R5).

1669577334452.png
 

Thread Starter

Isaac Po

Joined Nov 27, 2022
26
If you are using the AD620 as a comparator, you want a high gain, so R5 should be set to a small resistance value (1 ohm or less).
Gain equation below (Rg is R5).

View attachment 281614

May I know why AD620 turns comparator when Rg pin 1 & pin 8 is shorted(connected together) without a resistor in between?


The output is (-)12.4V when input(+) is lower than input(-):

1669780391954.png



The output is (+)12.4V when input(+) is higher than input(-):

1669780418053.png
 

ericgibbs

Joined Jan 29, 2010
16,834
hi Isaac,
The Gain cannot be infinite, look at page #7 of this PDF, to show the gain limits.

The AD620 or any INA is not suitable for use as a Comparator.

E
 

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ericgibbs

Joined Jan 29, 2010
16,834
hi Carl,
Zipped the LM35 and AD620 and a test AD620_LM35 asc file.
I have checked the zip in another PC, all OK.
Keep them all in the same folder for initial testing, run the sim in Alternate mode.

E
@crutschow Added a zip of the 'selection' *.cir types.
 

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