AVR-Chat

Kathy Quinlan wrote:
> Hi all,
> 
> Well I have to make 100 units (and pot them in Epoxy Resin) of an octopus lure 
> I designed for a proto run.
> 
> I am now thinking SERIOUSLY about building my SMD oven (as it is mainly SMD 
> only 4 through hole solder connections and 2 through hole non solder 
> connections (fishing line to hold the lure in the pot))
> 
> I also have a lot of "other" projects including a run of 200 units for my 
> speed alarm unit for the Au Targa Rallys.
> 
> So I need a circuit for Type K to AVR. I know there was a design note in the 
> Atmel applications Journal, unfortunately the WRONG schematic was placed in 
> there :(
> 
> So if anyone knows of a good easy schematic (with cold junction compensation) 
> I would love to see it. I do not need high accuracy, as solder does not mind 
> if it is 310C or 315C so anything to within +/- 2.5C is fine (I think the over 
> will have problems in coming up quick enough ;) (if so more elements and more 
> insulation is in order ;)
> 
> This will be an OPEN SOURCE Design. FULL PCB artwork (in Gerber and Protel 
> 99SE format), Schematics, Photos, source code, etc. will be on my site.
> 
> I will also be recognising all who contribute to this design.
> 
> I know I can buy chips like the MAX6675 as used in the Elector Jan06 issue, 
> but these are not at hand ATM and I am running out of time.
> 
> I will also use the over for post curing of the epoxy etc. so I would like to 
> build something that can have MULTIPLE presets.
> 
> I will design around the Mega 128 (huge overkill, BUT I have them in stock ;)

I haven't done cold-junction stuff, but only read about it.

I think you could amplify and lowpass filter the thermocouple output with
an opamp, then feed into the avr ADC. Connect an ambient temperature measuring
device to another ADC input, and subtract the effect of ambient temperature
from the thermocouple reading.

http://www.sensorsmag.com/articles/0203/temp/main.shtml