Has anyone succeeded in creating a thermometer using the LM355z temperature sensor? I am keen to use it rather than the popular ds18b20 because of cost and the calibration facilities it provides. I just have not seen any circuit diagrams around. I am new to electronics and the picaxe. Any thoughts/pointers?
The LM335 is a simple zener shunt temp sensor, just a series resistor is all you need, check the datasheet. The voltage is proportional to temperature at 10mV/degK If your PICAXE has an ADC input then you simply read the input voltage from the sensor. Of course there are many ways to then display the temperature - LCD display, LED display, serial upload to a PC etc. There should be plenty of example circuit out there to do any of those things. The LM35 has a 10mV/degC which might be a bit more sensible for you.
Dave :)
does that mean if goes negative if you freeze it ?
-- Bye. Jasen
It does if you have the output tied to a negative rail with a resistor. But i guess one could be clever and float the device up a tad with a resistor on the ground pin, but then one would need to be carful regarding the tempco of the resistor etc.
If you dont mind throwing an op-amp into the equasion, any PN juction will measure temperature. A BC327 or similar jelly bean transistor is good for about 2mV/degC from memory, and with some clean electronics you can get about 0.1degC accuracy without difficulty. Adn IIRC they silicon devices are quite linear, but dont quote me on that, it has been a good 10+ years since i did analog stuff.
Having applied both to the same application, I'd recommend you go with the LM35 unless there's a specified reason such as cost, it's higher output and better specified. I amplify the LM35 output with an opamp, with a noninverting gain circuit, so that the voltage swing across my temperature range of interest is somewhere around the ADC span. If you use an amp that can use a single ended 5V supply the overhead isn't large..
IIRC (also a heck of a long time ago), with a PN junction there was noticeable nonlinearity across a 0 - 70C span.
DEfinately the easiest way.
If i went out to the shed and did some hunting I could probably find some of my old circuits and cal algorithms for PN junctions. Problem is these days I dont have a 5.25" floppy drive :)
Yes I have a Pic based (note not a Pic Axe however the hardware is the same) Data logger with (amongst other options) a front end for an lm335. The interface for the LM335 uses a CA3130 set up as a differential Amplifier running off a supply 0.6V below the Pic chip and up to 6 V - could email you the schematic if you are intereseted and can give a (suitably disguised of course) email address.
Regards Richard Freeman
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