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Temperature sensor?

G

Greg T

Jan 1, 1970
0
Gidday,

After losing faith in computer motherboard sensors, I'm trying to design
an accurate (+/- .2 degC) electronic temperature sensor that will plug
into a computer. I'm mostly interested in measuring the ambient
temperature in my server room.

Sensors like the LM75 look ideal but I haven't come across any decent
schematics that can interface with them. I've yet to decide whether to
use USB, RS-232 or IEE-1284 as the external interface.

Can anyone point me in the right direction?

Thanks,
Greg
 
A

Anthony Fremont

Jan 1, 1970
0
Greg T said:
Gidday,

After losing faith in computer motherboard sensors, I'm trying to design
an accurate (+/- .2 degC) electronic temperature sensor that will plug
into a computer. I'm mostly interested in measuring the ambient
temperature in my server room.

Sensors like the LM75 look ideal but I haven't come across any decent
schematics that can interface with them. I've yet to decide whether to
use USB, RS-232 or IEE-1284 as the external interface.

Can anyone point me in the right direction?

The LM75 is only accurate to 2C with a resolution of .5C, so I don't
think you can achieve your goal with that part. You might want to look
at the LM35 since it will at least make more resolution available. You
should be able to easily achieve .1C resolution and excellent accuracy
with some calibration.

BTW, Dallas makes some 1-wire devices that have digital readout with up
to 1/16C resolution and very good accuracy (without calibration).
 
G

Greg T

Jan 1, 1970
0
Anthony said:
The LM75 is only accurate to 2C with a resolution of .5C, so I don't
think you can achieve your goal with that part. You might want to look
at the LM35 since it will at least make more resolution available. You
should be able to easily achieve .1C resolution and excellent accuracy
with some calibration.

BTW, Dallas makes some 1-wire devices that have digital readout with up
to 1/16C resolution and very good accuracy (without calibration).

Ah yes, my mistake. It was the LM35 I was thinking of.

Does anyone know of a site that provides schematics for such projects?

thanks,
Greg
 
A

Anthony Fremont

Jan 1, 1970
0
Greg T said:
Anthony Fremont wrote:

Ah yes, my mistake. It was the LM35 I was thinking of.

Does anyone know of a site that provides schematics for such projects?

The datasheet is a good place to start as it contains some reference
circuits. Since the LM35 outputs a voltage that is proportionate to the
temperature, you will need an external ADC (12 bits would be a good
choice) and associated reference voltage (4.096V makes things easy) to
measure it. You will also need some kind of I2C or SPI capable device
to talk to the ADC. I would use a microcontroller, but there are other
ways.

I have easily achieved .1F resolution using an LM34 and a PIC with only
a 10 bit ADC. I was able to accomplish that by using a Vref of only
1.024V. Currently that means that I can only measure temps up to
102.3F. But since my house should never be any hotter than that, it
works for me. If I needed a higher max temp capability, I could use an
op-amp to offset the output of the LM34 and slide the 102.3F range up or
down as required. For example, by subtracting 500mV from the output of
the sensor, I can increase the max temp limit to 152.3F but still
maintain .1F resolution. Of course the lower limit would now be 50F
instead of 0F. A 12 bit ADC would be a better choice since I could
maintain .1F resolution AND utilize the full temperature range of the
sensor.
 
R

Robert Baer

Jan 1, 1970
0
Anthony said:
The LM75 is only accurate to 2C with a resolution of .5C, so I don't
think you can achieve your goal with that part. You might want to look
at the LM35 since it will at least make more resolution available. You
should be able to easily achieve .1C resolution and excellent accuracy
with some calibration.

BTW, Dallas makes some 1-wire devices that have digital readout with up
to 1/16C resolution and very good accuracy (without calibration).
However, Dallas is another name for Maxim.
And Maxim has a bad habit of advertising vaporware.
Many distributors and companies have blacklisted them as a source due
to the problem of unavailable parts.
 
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