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New Teensy controller pcb
I've posted my fluid bed roaster design on this forum before, at Not a whole lot new to report there, except... last week I finally swapped out the mess of breadboarded electronics with a custom PCB, and thought I would share the design with the community. See photo below.

I've listed the full set of features on my github page. Here are some of the highlights:
  1. Uses a Teensy 3.5 development board. The 32-bit chip on this board is really fast (120 MHz), and has plenty of storage (512K flash, 192K RAM) which leaves plenty of room for future development.
  2. Includes two thermocouple inputs (BT and ET) inputs, a heater SSR output, and a DC fan output. The MOSFET on the board can drive a lot of current, and has no problem driving the 22A, 260W fan in my roaster.
  3. Features an internal, tuneable PID controller.
  4. Stores an onboard, adjustable roast temperature and fan profile. (I will soon be modifying the code to store multiple profiles, but as of this date it just has the one.)
  5. Roast data saved to an onboard microSD card for later analysis.
  6. 20x4 LCD and button-driven interface. (See github site for a 3D printed panel)
  7. (Optionally) intefaces with Artisan for roast logging and/or control.

The code is fairly mature and bug-free at this point. I've been roasting with it for most of the past year, and have been pleased with the results. If any one does happen to build one of these, I am certainly open to feature requests.

I have no intention of selling this board, but I have made the Eagle and gerber files available if anyone wants to build one for their personal use. I did buy ten PCBs but am only using the one so would be willing to send out the other boards to interested folks for $4 (at cost) + shipping. The components would need to be procured separately, but really aren't that expensive. The Teensy is $25, and the other parts can all be purchased from Mouser or Digikey for less than $20. It does require a little surface mounted soldering, but it really isn't that difficult with some flux and a steady hand.
elkayem attached the following image:

Edited by elkayem on 03/10/2019 11:02 AM
Congrats, I really like how well you documented your project, including a listing of the parts you used. Very much helpful!

I also like the clean and simple design of your board. That's something that does not make any difference when roasting, but being able to visually track what's on your board when something misbehave is very much appreciated.
Thank you, marcov! I left the type of notes I wish I had easily found in when researching this project. Of course everything can be found on this forum if one is willing to read through the historical posts.
That's why I value your project and documentation that much.

I am curious about the TCs frontend: is it something specific to the MAX31855, or can be used for any ADC?
Are you asking whether the board would be compatible with a different converter other than the MAX31855? No, it was designed for the MAX.
I meant the thermocouple frontend, i.e. the components between the thermocouple and the MAX31855. Any reason why did you put the ferrites and the 0.01uF cap? Are those components specific to the maxim chip?
Noise filtering, generic for any thermocouple amplifier input.
Oh, now your question makes more sense to me marcov! Renatoa is correct, the ferrites and capacitors are for noise suppression, and should be generic to any TC amplifier. Keep in mind that K-type thermocouples have a ~40 microvolt per deg C scale factor, and those long probe wires act as antennas. Plus, many of us have powerful fans with brushed motors nearby, which radiate all kinds of EMI. I do notice that the TC4 lacks any filtering, so it appears some are able to get by without it.
Last TC4 version, V6, features RC filters on TC inputs.
Previous versions used a trick instead filtering, forced input low common mode impedance, with 10k resistors to ground, so ensuring a path for noise to discharge.
So this explain why I recently had a lot of troubles with TC measurement done with a TC4+.

Is the trick done by configuring the ADC in common mode?
Common mode is not a feature that can be switched on-off, wiki explains better than me the meaning in this context:

The resistors to ground are the key elements that make the noise a common-mode voltage signal. Without them noise will be mixed with TC signal.
Still have any boards?


mk1 wrote:

Still have any boards?

Yes I do! Sorry for the tardy response. Looks like I forgot to subscribe to my own thread.
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