Temp. sensor

[Michael36]

Does anyone know how i would go about using a pic to convert the voltage given from a thermister to give an output that could be used to make a display (probably using seven segment displays) showing the temperature in °c?

[Michael 2727]

For a couple of bucks extra the DS18B20 leaves
the thermistor for dead.
The picaxe will display the temp -55 ºC to +125 ºC.

Get the file here called, tempdisplay.bas

<A href='http://porepunkahps.vic.edu.au/home/jef01/files/display/' Target=_Blank>External Web Link</a>

[xstamp]

Only reason to use a thermistor over a IC sensor is if you want a rapid responce to temperature change. Thermistors and thermocouples can be much quicker that an IC sensor chip.

[helipilotpete]

And the conversion time for a DS18B20 is nearly 1second.

regards Peter

[Jeremy Leach]

Plus (I think) thermistors offer the possibility of greater accuracy if you don't mind the hard slog of building the circuitry (my recent interest in the LM331 voltage to frequency converter!).

The DS18B20 makes it all nice and easy - so unless you REALLY want to go there I'd agree with what's been said already.

Edited by - Jeremy Leach on 1/31/2006 4:08:51 PM

[BeanieBots]

As you can see, there are many options. Perhaps if you could indicate the temperature range, how accurate you need it and how often you need to take a reading, we could offer more concise advise.
Also, consider using something like the AXE033 serial LCD for your display. Driving 7-segment displays from a PICAXE can be a problem if you want the PICAXE to do something else as well. In particular, if you use the DS18B20 (one of my favourites) you will not be able to drive the 7-segment displays directly because as mentioned, it takes 750mS to read. That would mean you would need to use some sort of display driver which would make the AXE033 a more tempting solution. It would also give you more scope to do things such as display max and min temps or even (with the optional clock upgrade) the time and when the max/min happened, if that is something you would like to do maybe later on.

[xstamp]

I’ve just remembered a device that offers a good compromise between ease of use and good response time (if needed). Take a look at the Analog Devices AD590. This is a small two pin semiconductor sensor that has a linear output of 1uA/C over the range –50 to + 150C. All you need is a single 10K resistor to convert this to a voltage at the ADC input of the picaxe. I have found it possible to run many metres of unscreened cable to a remote AD590 and still get good reading in an automotive environment (can’t do that with SPI or I2C devices!)

Only two potential problem areas:

The AD590 has an offset of about 2.7V at 0C. Therefore the single resistor approach losses about half the ADC input range, but 10-bit readings still give a resolution to match the sensor accuracy.

You will need to power the ‘top end’ of the AD590 from about 9V but because it’s a current output device this can be an unregulated battery supply.

Basically, this device would be useful for applications that require remote temperature measurements with the minimum of conditioning circuitry and current drain.