@coolnewt I looked into the 204CT-4, and it suggests that its a 200Ohm resistance thermistor. Does this mean our readings are way out because we’re only using a 10k voltage dividing resistor?
Victorious.
What a journey, spent until 1am last night with glasses of ice water and a boiling saucepan.
The various online libraries are a strange mix, ie heatermeter has become organically obtuse with Steinhart[3] references in the resistance calc, and I think smartbbq has issues but I haven’t had the time to try their libraries with the coefficients I now know are right. Neither library helped get the clean resistance values that could be validated.
The research and commentary in the forums was crucial, though I hope my output helps the next newbie save time and some grey hair.
My spreadsheet that helped clarify and validate the necessary steps and formulas: http://1drv.ms/1zJZXO2
Pretty close, YMMV. Checkout my spreadsheet below and pop in your resistor (ie the 10 or 20k resistor you added to your circuit) and relevant thermistor coefficients. 150C is 3k for my ET73 with a 19800 resistor.
I might have missed this part (I’ve read the thread at least twice) but what was decided for a power supply? Even if you just have a recommendation that would be helpful.
@cyberhiker, it doesn’t look like they sell the ones I purchased anymore. This one is similar, and would probably work. Anything that is 5.5od/2.1id 12V output and ample current to drive the blower you are using. This one does 3A which is much more than is need for most applications.
@coolnewt Thanks! I’m not planning on doing the blower, just monitoring. So I went with this one from Adafruit (https://www.adafruit.com/products/798) which is 1A. I’m guessing 1A would be enough.
Add a 4 way op-amp to the board. Do you happen to be using the Maverick ET-732 probes?
You may think they’re ET-73, but if they have the slightly longer than normal jack (that effectively shorts the input if you push all the way in) then its ET-732. That little information nugget cost me 5 nights of pulling my hair out because I was using the coefficient table for the ET-73.
Also, my learning curve over the past 2 months has been astronomical, but it always puzzled me why smartbbq bothered with the 1k R on the temp probe input. I got pretty good with excel formulas, but I’m not sure what formula you’d use to combine that with the 10k voltage divider, considering the thermistor is the unknown resistor. Maybe along the lines of R1 / R19 + R1 + ThermR? The smartbbq formula didn’t appear to account for it. Personally I’d use 40k on an op-amp input and leave it with the standard voltage divider calc. YMMV based on your probes as to what R to use in the 20-45k range.
ps, I intend to launch a beta of my novel take on a solution for smoker blower control. Looking to ship the whole unit for less than $75 USD, about $50 USD if you BYO spark.
Cool, I’ll be in touch. The case may be hand made until it’s clear taking the BBQ guru adapter compatibility route is a winner and its economical to use two stacked pcbs to conserve space. What smokers do you use? Do you have BBQ guru adapters or maverick probes?
Primo XL 400 and I don’t have the adaptor or probes yet but I am ready to get them I have almost purchased the cyberQ kit a few times but would much rather go the particle (spark:) route
absolutely perfect. I also have a Primo XL. $2 piece of galvabond and a male 3/4" screw nut is how I attach my blower to the primo.
I actually forked out $80 AUD for the bbq guru ceramic adapter and freight, just to make sure the 1.219 ID measurements online were accurate and I could test it. Its actually a slightly dodgy adapter as it has this extendable clip on the top to flexibly cover up to the top ‘rail’ where the normal vent slider is. Much easier to get a piece of tin, drill the appropriate hole and then screw the nut from the other side. You wouldn’t have the expensive freight, so may prefer to get the adapter.
I’ll be in touch in a fortnight once I’ve got the unit working in the target prototype case (its pretty small, roughly 50mm square, its quite a squeeze!). if you’re still keen it will be easier for you to purchase domestically the ‘Maverick replacement ET-732 pit and food probes’ for about $20.
I’m just looking at developing a controller/probe for my BBQ and this looks ideal!
I wondered how development was going for a new version before I go down the route of trying to roll my own.
Effectively I’m after 2 food probe + 1 type-k thermocouple connections. I don’t suppose this happens to tie up with what’s being worked on?
hey colin,
why do you want to use k-type TC’s? ive used those for building furnaces in the past (im an amateur glassblower). they are generally more expensive, and would require something like this:
So has anyone gotten multiple boards made - willing to sell one? I’ve never gone through getting a board made. Maybe it’s not such a big deal, but I figured if anyone already had some…