temperature measuremet urgetn help reqd

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sexyabhs

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i amdoing this project on temp measurement. i need to measure two temp's in the body range, i.e. 25-45 deg celsius. and make sure that the difference between these two temp's does not cross 4 degrees. now i need some idea regarding the temp measurement n how to go abt it. preferrably i would like to use thermistor n build my own circuit but i m open to ic's too but my budget is low. also if i could get any schematics? measurement has to be linear in the range of 25 - 45 degrees celsius.is there any other way other than these? the prob is that in the place that i am , the dealers sell the thermistors without knowing their range or sensitivity. so its a blind guess. if u suggest some IC could u please tell me how it will measure the temp,i.e. probe?
thank you
 
The thermister is very non-linear. Check out the LM35 temperature IC the output is a linear 10mV per degree C. National Semiconductor will have application information.
 
the thermistor thing juz isnt working out.....need more details there... also tryin the LM35 approach. now when i connected pin 1 to 5 volts supply,pin2 to ground , and pin 3 gave output.well the output i got was 444 mV,that means that the temperature was around 44.4 deg celsius right but actually the temperature was juz about 32 deg celsius. also when i touched the LM35 with my fingers(temp increase to 36 deg) the output did not change. wat i want to know is whether i should have held one of the pins instead of holding the chip in between my fingers... its a LM35DZ btw (to-92 package)..also are there any other ways that i can check out the temp measure ment without using sensors but rather thermstors or diodes or something...please let me know where i can find the schematics?
thank you
 
Just a comment on sensors. There are many types of temperature sensors. Manufacturers of these sensors provide lots of information on applications, characteristics, etc. While some of these sensors can be expensive, some of the more common types, thermistors in particular, are relatively inexpensive. You might take a look at Newark or similar electronics suppliers for low cost items then go to the manufacturer for detailed information. That manufacturer may have a website or offer applications information that will work for you. Just a thought.
 
Here are some National Semiconductor Application Notes which contain thermometer circuits: AN-211, Figure 22; AN-225, LB-27, LB-24.
 
Try googling for thermocouples.
 
Try a search on Thermopiles. These measure body temp via IR. Same as used in ear thermometers. There similar to PIR modules but at a longer wavelength.

Steve
 
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