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temperature sensor circuit

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mpj111

New Member
I want to build temperture sensing circuit. I want to ditect temperture in two different rooms. Is there any temperature difference, need to be ditected and provide a output to operates a switch to start a fan ( or similar to this.). Do you have any idea about such circuit and sensing device. Your help much appreciated.
 

jack23

New Member
You can use a temperature sensor such as the LM35DT. This particular sensor has a linear output of 10mV / C (centigrade). Being that the output is very small, you might want to amplify that using an Op-amp IC.

Then you can feed the output of the Op-amp into one of the inputs of a comparator circuit. The other input of the comparator can be used as the reference voltage ( which indicates ref temp). So for example, if the output of the op-amp is greater than the reference voltage ( meaning the Temp is higher than a reference Temp), and the output of the op-amp is fed into the positive input, then the output of the comparator will be a High(Logic 1), which then can be used to turn on a switch of some sort.

Or you can implement all of this using a micro-controller.
 

pc88

New Member
As it turns out, I was just investigating this. The LM35 (or LM34 for those who want to use the superior Fahrenheit temperature scale), is an easy way to go. Alternatively, have a look at these articles which discuss some alternatives:

**broken link removed**

**broken link removed**

In your case, since you want a differential temperature reading, you can use a single op-amp to perform the temperature subtraction and scaling.

Another kind of circuit you might be interested in is a "fan control circuit" or a "fan speed control circuit" which will increase the speed of the fan as the temperature increases.
 

blueroomelectronics

Well-Known Member
pc88 said:
As it turns out, I was just investigating this. The LM35 (or LM34 for those who want to use the superior Fahrenheit temperature scale), is an easy way to go. Alternatively, have a look at these articles which discuss some alternatives:

What makes the Fahrenheit temperature scale superior?
 

ericgibbs

Well-Known Member
Most Helpful Member
mpj111 said:
I want to build temperture sensing circuit. I want to ditect temperture in two different rooms. Is there any temperature difference, need to be ditected and provide a output to operates a switch to start a fan ( or similar to this.). Do you have any idea about such circuit and sensing device. Your help much appreciated.

hi,
Use a LM35 centigrade sensor, +4 thru +150 degC.

A programmed device ie: PIC isn't required to control a house fan.

A dual LM393 comparator will accept the inputs directly from the LM35's.

Use a driver transistor, connected to the comparator outputs, to switch a relay that in turns switches the fan,

this will also provide the required mains isolation.

Hope this helps.

Hi Bill,
As a Brit brought up on Fahrenheit, I am well pleased we now use Centrigrade.
Engineering calculations using the old FPS [foot-pound-sec] involving Fdeg were a pain in the butt!
Especially when you then needed to do a costing in Pounds,Shillings and Pence.
 
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TrevorP

New Member
I have a feeling he was joking about F being superior.

It's a stupid scale...Celsius tells you when it's freezing...can be converted to Kalvin quickly...and is good for really simple thermal formulas.
 

Hero999

Banned
Kelvin is really the best scale for doing calculations in as it removes the need for negitive numbers.
 

ericgibbs

Well-Known Member
Most Helpful Member
Hero999 said:
Kelvin is really the best scale for doing calculations in as it removes the need for negitive numbers.

hi hero,
Just in case you dont know, the LM135 [same case style as LM35] works in Kelvin.. +10mV/K from absolute.
 

blueroomelectronics

Well-Known Member
Well the reason I asked is because I'm building a thermostat kit, I've got both LM34DZ & LM35DZ sensors and they work the same just calibrated differently.
I'd like to use the LM34 (even though we're Celcius here in Canada) because it offers a larger temperature range the the LM35 without extra components.
The 20bit A/D in the PIC has a 2.5V + reference so 0.00244140625V per step.
Not too tough to convert the raw A/D result to decimal (ADRES * 125 / 1024), should be easy to convert to C also.
 
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