Re: MMIR X-10 Macro IR Module

[Marc_F_Hult <MFHult@xxxxxxxxxxxxxxxxxxxxx>]

On Fri, 21 Sep 2007 19:46:49 GMT, "Jeff Volp" <JeffVolp@xxxxxxx> wrote in
message  <JcVIi.119125$ax1.78968@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>:

>"Lewis Gardner" <lgardner@xxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
>news:46f40cc0$0$15935$d94e5ade@xxxxxxxxxxxxxxxxx
>>
>> At around $3 for a LM34DZ I can't imagine why you would want to use
>> thermistors unless you wanted to monitor a LOT of locations.
>
>Thermistors only need a twisted pair.  The National device is 10mV per
>degree, and requires a power source.  An 8-bit A/D has 20mV resolution, so
>the best you can do is 2 degrees F.  You can get better resolution over a
>limited temperature range using a thermistor with proper selection of
>components.  Of course, it takes some math to convert the readings to
>useful  numbers.

And it is useful to have a 'dead' bit on either side of the target value
when creating events (or some other way of averaging) so you can end up
with (for 5vdc ADC) 2F, +/- 2F,  which is a 6F window. For conventional
10vdc FS Input, it would be 12F.

To use the full 8-bits of the analog-to-digital converter (0-5vdv, or
0-10vdc), the LM34D output has to be amplified. If amplified such that the
output at maximum desired  temperature = Full Scale Input of the ADC (eg
128F = 255 count; gain of 40 for 5vdc FS input)) one can achieve 0.5F
resolution and 1.5F window which becomes more useful, but at the expense of
more electronics, more potential drift, and more need for calibration owing
to DC offset.

And if you want to measure temperatures below 0 degrees with the LM34, a
negative supply voltage is needed which adds another needed power supply
and a quick way to fry low-voltage inputs that prefer voltages of the
opposite persuasion ...

For new designs needing below 0 temperatures, the LM94022 is available. It
has an output inversely proportional to temperature, eliminating the need
for negative supply. Alas, in keeping with trends to lower voltages,
although it has a built-in, gain-selectable amp, it can't get to 5vdc FSO
and isn't very accurate.

Like Jeff, I've used thermistors for years. It was the oceanographers that
first showed the long-term stability of thermistors for earth science
research and developed the calibration equations (Steinhart-Hart). But they
could at least could retrieve them later in time for time-trending
recalibration of the data set. My application involved permanently
cementing them into wells that were then backfilled. No going back to
recalibrate once the grout set ;-)

... Marc
Marc_F_Hult
www.ECOntrol.org