Can someone help me how to connect a Thermistor lm335 to a cr800?
My sensor is based on this diagram: http://web.mit.edu/rec/www/workshop/lm335.html
That is not a thermistor but a semi-conductor diode based sensor. If you follow the instructions for wiring as shown in the document you linked to using the 5V output on the logger as a supply. You can measure the voltage as a single-ended voltage using VoltSE and then scale the reading to temperature as also described.
You might want to increase the resistance you use to reduce self-heating effects, perhaps using a resistor of up to 5K.
* Last updated by: aps on 7/7/2011 @ 8:09 AM *
Dear Andrew,
You are right it is a zener diode!
I made the wiring as shown here: http://diycontrol.com.au/?p=32 and used a 240K resistor.
It works fine on a cr200 with the following prog line:
'Connections
'-----------
'Yellow ---(signal) --- SE3
'Black ---(signal ground)---AG
'Red ---(Excitation)--- 12V
VoltSe (Tair,1,3,0.001,0)
Tair=((Tair*2)*100)-273
What would be the correct line and proper connection on the cr800?
You need to be careful with the resistor you select. The spec for the LM335 says a minimum of 400 microamps is needed to pass through the sensor. IF you take off the voltage drop across the sensor (about 3V) this leaves you with 2V for a 5V supply or 9V for a 12 V supply to induce 400 microamps to flow through the resistor you select.
For 5V that means 5K is the maximum resistance to use to stay within the manufactures spec. For 12V about 22k. If you have used 240k you will be passing far too little current which will make the sensor inaccurate.
(Incidentally there are newer variants of this type of detector that need less current, and therefore also warm up less.)
The program is otherwise OK for a CR800, although you could put a multiplier and offset directly in the VoltSE equation if you want.
What confuses me though is how you got this to work with a CR200. At room temp the sensor should output about 3V (300K) whilst the maximim voltage a CR200 should measure is 2.5 V???