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I'm in the process of doing a thermostat project and have been looking over Digital Thermometer schematics that use thermistors. I've seen some with elaborate dual op-amp circuits and others that feed strait to a PIC's ADC VIA voltage divider. Is a buffer of some kind recommended? If so why? I want to use this in a farm's out-building when I'm done so a semi harsh environment is likely.
It's down to the Thermistor specs... and MAX PIC ADC impedance...
Or just make it easy on yourself and use a DS1820.Quote:
Originally Posted by Neosec
I considered a DS1820 but was concerned about the 1-wire network immunity to noise... I want to have at least 5 temperature sensors 3 indoor, two of which will be up to 200 feet from the PIC probably running along side Romex 120v AC lines. An outdoor sensor and a sensor in a thermowell in a boiler ~180 Deg.F. Thermistors are industry standard in a boiler so I didn't want to reinvent the wheel. And as for the longer runs I just figured the thermistor lines could be filtered with some caps since they're DC so noise would be less of a problem. I guess adding a buffer will provide a more robust signal. Thanks... I'll look into it a bit more now that I know what to look for.
I have successfully used the Microchip MPC9701. Since it has an analog output, you can run the signal 50' or more. Just use a low-pass filter (2K in series, .1uF to GND) with the A/D input and average 16 samples with a 1mSec delay between samples (so that you average out over one complete cycle of 60Hz). The output is linear with temperature (unlike thermistors), and doesn't need calibration. Cheap, too.
LM34 or LM35 will work too. A cap from signal to ground for longer distances and good to go.
Like the Microchip MPC9701 these are also linear. Your harsh environment is not a problem. Good for around 300 F.
Industry standard? I use the LM34 in thermo wells at the plant where I work. One has an average temp 250 F. One degree resolution, no problems. We must be Non- standard. :)
Lots of options.