# Measure Temp

#### Overclocked2300

Joined Apr 24, 2005
124
How could I measure temp using a thermister or the LM34? I thought of a circuit using a A/D converter and then powering a BCD to Seven segment display, but I dont know how to use A/D converters and what chips are out there that would allow 8 bits (for 2 Digit operation, ie 0 to 99) Can I want to measure in Celcious ©

#### beenthere

Joined Apr 20, 2004
15,819
Hi,

Why not use a digital panel meter? The LM34's output is linear with temperature change.

For that matter, why not use an LM35? It reads in deg.C. In order to use the LM34 for Centigrade, you have to use an op amp with an offset to compensate for the 0 point difference, and then a gain of .455 to scale from farenheit to C.

Thermistors have response curves, and are not very linear over a reasonable range of temperature. An LM35 and a DPM plus a battery will do the trick.

#### Overclocked2300

Joined Apr 24, 2005
124
You mean to use the DPM to measure the LM35's voltage? Wouldnt I have to set up the right scale though? I mean like use resistors so it will read as a while number instead of 0.xx?

#### thingmaker3

Joined May 16, 2005
5,083
The LM35 has an output of 0.01 volts per degree centegrade. Put your meter on the two volt scale, pretend the decimal point does not exist, and read the temperature directly.

For very high or very low temperatures, one can use a Type K thermocouple and a AD594/AD595 - although this will only give an approximation. (Using an EPROM would allow for exact temperature readings.)

#### Overclocked2300

Joined Apr 24, 2005
124
I could always take the DPM apart and just cut the decimal point lead off..

#### thingmaker3

Joined May 16, 2005
5,083
Brilliant! Why do I always miss the obvious solutions like that one? Thanks!

#### Overclocked2300

Joined Apr 24, 2005
124
Yea but hopefully the decimal point doesnt move, it should stay in one place, thus the display would look like this:

xxxx

xx.xx (and hopefully 0's dont show up infront of the decimal point)

So then Really it would be able to measure up to 9999C...which isnt nessary but! Good for science labs.

EDIT: wait a second, dont you mean 200mV scale? Because the LM35 outputs 10mV Per C.

So this meter would be good for the job: http://www.mpja.com/productview.asp?product=6929+ME

#### michael

Joined Oct 5, 2005
2
Hi,

It is best to determine the application of this device that you intend to build. The temperature range of 0 to 90 Celcius seems to be very huge for measurement purpose. What is the accuracy of this device? If the range is small (10 to 30 Celcius) and accuracy of 1 Celcius is tolerable, then a thermistor as its sensor will do. Otherwise a standard linear temperature IC from national semiconductor may be suitable. You may want to look at more details of thermistor specs at the following site.

http://www.electronics-project-design.com/thermistors.html

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#### wael_sal

Joined Sep 16, 2006
2
Can plz list the test ckt,, I had build as on Datasheet ,,, but the LM35 heats quickly !!!!
plz provide any help??