newbie Rctime question
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Posts: 46,084
Hi all,
Newbie questions here.. first off:
1) Is it possiable to use a Rc Network with a LM334 Sensor to get an
accurate reading of the temp? Or do I have to use an A/D chip?
2) If it is possiable, could someone show me a schematic?
Thanks a bunch..
Sean
Newbie questions here.. first off:
1) Is it possiable to use a Rc Network with a LM334 Sensor to get an
accurate reading of the temp? Or do I have to use an A/D chip?
2) If it is possiable, could someone show me a schematic?
Thanks a bunch..
Sean
Comments
>Newbie questions here.. first off:
>1) Is it possiable to use a Rc Network with a LM334 Sensor to get an
>accurate reading of the temp? Or do I have to use an A/D chip?
>2) If it is possiable, could someone show me a schematic?
>Thanks a bunch..
>Sean
Hi Sean,
Yes, you can use the LM334 with the RCTIME command on the Stamp in
the same way that you would use the AD590 family. Those chips
produce a current proportional to temperature, 1 microamp per Kelvin.
The LM334 needs a 226 ohm resistor to set it for 1 microamp per
Kelvin:
,
, 226 100
Vdd ----| LM334 |---/\/\---o
o--/\/\-- P0 Stamp
| |
' |
'
' |
===== 0.1 film
|
|
Vss
The current does not depend on the voltage across the sensor. The
capacitor charges linearly due to the current and the RCTIME command
measures the time it takes to reach the 1.3 volt stamp threshold:
x var word
loop:
rctime 0,0,x ' raw time
low 0 ' ready for next charging cycle
debug ?x,?65000/x ' 1/time is proportional to temperature.
goto loop
To calibrate, put the sensor in an ice bath, and adjust the constant
(nominally=65000) until you get a reading of 273 Kelvin. For best
results do a two point calibration.
Accuracy is another issue. The RCTIME command is good for about 10
bits, and you also have to take into account the stability of the 0.1
uF capacitor. One Kelvin accuracy is about all you can expect.
There is a lot more information in the Earth Measurements course
material. That includes how to construct probes using the LM334, the
AD592 and the TMP17. For a while, there was a parts shortage from
Analog Devices, so Parallax had to have a lot of options to keep the
course materials in stock.
http://www.owlogic.com/EarthM/em3.pdf
http://www.owlogic.com/EarthM/TK334fab.pdf
There is more information on RCTIME and interface with the current
mode probes here:
http://www.owlogic.com/BS2rct.htm
http://www.owlogic.com/OWL2face.htm#AD590
-- regards,
Tracy Allen
electronically monitored ecosystems
mailto:tracy@e...
http://www.emesystems.com