Measuring temperatures with the LM335
This page ©2012 Andrew Wylie all rights reserved
I bought a couple of LM335 temperature sensors on eBay very cheaply. This device looks like a plastic transistor, but behaves like a Zener diode that has a breakdown voltage linearly proportional to the absolute temperature. Yes, that's the Kelvin scale, not a normal domestic scale. The device is specified to operate from -40°C to 100°C, a wide range.
It is trivial to connect the LM335 to the USB25IO, all you need is an external resistor of say 2.2 Kohm, and you connect it as shown to the +5V supply, an ADC input, and ground. You need some software to read it, and convert the voltage to a temperature in whichever scale you prefer, Celsius in my case. Having done this I was somewhat disconcerted to find that the example I had chosen was in error by several degrees, although that only equated to a few millivolts, or ADC counts. I had to add a calibration feature to my software to allow for this. The graph below shows the resulting temperature measured in the shade inside my conservatory over 24 hours. Sunrise is obvious, but the conservatory loses the sunlight gradually over the morning, hence the temperature rise over the day is not great.
I started out with my seismograph in the conservatory, but I was concerned about thermal expansion and contraction. I have now moved it to the garage, which is half underground, and below is the measured daily temperature range: less than 1 C degree change over 24 hours.