temperature sensor circuit

[mpj111]

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]

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]

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]

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]

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.

 

[TrevorP]

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]

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

 

[ericgibbs]

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 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.