AVR-Chat

Russell Shaw wrote:

> 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.

I am probably wrong, but I thought cold junction was needed to make the 
circuit accurate ??

-- 
Regards,

Kat.
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