The Smoke-O-Tron

Who doesn't love smoked meats? Yes, I'm ignoring vegetarians and vegans. Who doesn't love them more when you don't require any skill to maintain temperature for the 6+ hours of cooking?

This guy.

I picked up a Brinkman Gourmet Electric Smoker on an impulse one day. It's a steel can with a 1500W heater, a water pan, and some racks for food. For under $100, it's a reasonable device for getting really fantastic food. However, it only has one setting: about 250F degrees, and that depends a lot on the outside temperature. You see, the heater gets HOT, and when that heat rises, it hits the water pan and the water eventually boils. That barely at temperature steam rises up and mingles with a bit of the 400F degree air and you wind up with about a 250F degree cooking temp. 250F is great for pork, beef, and most other smokeables... but not fish, and not if you want to get a really thick bark on your meat.

Enter the Smoke-O-Tron; a PID controller that can handle the 1500W heater to within about 3 degrees F.

Step 1 Hardware selection

OK, so what do we need to do? We need to control temperature in a large air mass using a 1500W heating element.

What does this entail?
1] Measuring temperature in the space
2] Switching the heater on and off at a given speed
3] Having the temperature be easily adjustable: Digital.

Since I wanted a digital read out, I was going to need a Micro Processor on this one. I haven't used one since I was an EE, and even then I spent most of my time keeping large industrial  robots from running me over, which really limited my exposure to the actual processors. I went with the PICAXE 18 line for this one. It seemed to be what I needed, but the development language it uses really limits what you can do with the micro (e.g., couldn't set interrupts). This was kind of an annoyance, but for the precision needed here, it wasn't a deal breaker.

The thermometer was selected to be a K-Type Thermocouple. They tend to be rugged and are good to well beyond the range of temperatures in the smoker. To interface, I went with a Max 6675 Thermocouple Interface IC. This interfaced with SPI to the micro. The PICAXE doesn't do hardware SPI, so I bit-banged it.

Next, the display. The usual 20x2 display seemed enough for this task. The chip almost lacked enough pins to talk parallel to the LCD, so I got a serial one from SparkFun, just to make sure. Again, probably something I'd do differently, but there ya go.

For a relay, I figured that I would leave this thing running for about 8 hours at a time with a switching time of 6 seconds. That's about 2880 on/off cycles for a relay, and with anything reasonable being rated at 20,000, I figured I should look at solid state relays. I picked up a 250V, 20A model from Digikey (about $40), but since then I pick them up on eBay for about $10 with heat sink.

Finally, I went with a rotary encoder for user interface. I was always a fan of "knob with a button" interfaces, and this one was a good way to go.

The power section is your basic AC supply. There's a MOV in there for surge suppression, but you can leave it out. I have no idea why I left it there.

Comments

[samj2]

what does this exactly do? can u please mention the controller required?

[DIY-Guy]

This is a great way to bring some flavor to otherwise bland TOFU!
You haven't ignored vegetarians, you just didn't know how many would appreciate this for things like Tofu and BBQ roasted vegetables. Great invention for foodies, keep it up.

[abefetterman]

Wow! I love this! You did an awesome job here. I've been working on an AVR based PID controller for sous vide cooking, and I'd be very happy if mine looked half as sexy as yours!

One question I have is what on-board power supply you used? It looks like you just combined a transformer and rectifier, and left it at that. Is that enough? Can you suggest which parts you ended up using? Thanks so much, I'd love to give this a shot!

[dafonso]

Nothing fancy for the power supply. It's a 120v->9v transformer, bridge rectifier, 100uF cap, and LM7805 regulator on the board. The power wiring has a circuit breaker and a MOV for protection as well as the power switch.

If done today, I'd go with a Supertex SR037 to get the voltage. It's cheaper to put it and associated parts than deal with the transformer, and the board space would be much smaller.

Sorry I don't have the part numbers... I seem to have lost the BOM for the build.

[gouthro]

Nice job on the smoker. I have been muling over the smoker idea for awhile myself. One think I really would like to have is the capacity for cold smoking. I notice the Bradley that someone mentionned does that. Most commerically bought smokers I have seen have done hot smoking.

But, the price of the Bradley is $500. I am wondering whether it is really worth it, or whether you would be better just throwing together some kind of box with an entry for smoke in it? I'd appreciate any comments or suggestions
gouthro

[dafonso]

Cold smoking is pretty straight forward... I even think there's a Good Eats episode that talks about it. A hot plate with a covered cast iron pan of saw dust will billow smoke. Put that in a box with some aluminum dryer ducting and a fan and you will get a cool smoke out. You can pretty much smoke anything in a plastic bag with some holes poked in it at that point.

[gouthro]

I pretty much understand that part of it. But, where I am stuck is on the stove part. I don"t want to buy a stove just for this. I am not sure I ge the 'covered' cast iron pan idea. How does this work?
thanks Gouthro

[dafonso]

Oh, well, there's no stove, just a hot plate. I picked up my last hot plate at the good will for $6. You can get them new for about $20 at a big box store.

My old smoker generator that got used in this rig (pre-controller) was a cast iron pan filled with wood that I covered in a disposable aluminum pie plate (poke some holes in it). The pie plate was on there to keep the wood from getting enough oxygen to catch fire, so it would just smolder. So you take those two, and put them in a cardboard/metal/ceramic box with a fan to blow out the smoke through some long-ish tubing to cool the smoke, and inject that into your smoke chamber.

The Good Eats episode is available on You Tube, and the second part shows the process around 1:50.

[gouthro]

I finally saw the link to the video in your post. Had a look. I am sure i will get something going
thanks G

[gouthro]

Thanks for the reply. I looked at Good Eats video but the one I saw had him constructing a hot smoke smoker. I looked around though, and found some other things. One possibly useful one for me was someone using a Weber kettle bbq as a firebox. A flexible tube goes from that to whatever the smoke box is. That may be fairly simple for me to set up. Otherwise, as you say, I imagine I could use pie plates.

Joe

[swighton]

The derivative is a measure of the rate that the temperature is changing not just how much. The method that you describe to calculate the derivative will be fine as long as you sample at a constant rate (essentially your delta time will be incorporated into your derivative gain), though it will fail/behave funky if your sample rate is changing. Dividing the temperature change by the time between samples would help to prevent potential problems.

[dafonso]

Very true. And if I could actually tell how much time had passed, I might have been able to factor that in. The Integral component had the same set of issues, but the 1-2% variation in loop calculation time never really affected anything; the time constant is over 2 minutes.

The time between samples is roughly constant and the errors did average out after a while. The derivative gain was not a huge portion of the gain on this system so it got the shaft in terms of care taken to make sure it performed adequately. If I was tuning PID loops for Galvos, it would be a different story.

[aeray]

Excellent work. I've been mulling over smoker designs and products for a while now. You may be interested in these smokers as well, for inspiration for a future upgrade.

[dafonso]

Those look great... I'm looking to make a more portable rig as my condo is tiny and there's just no real point in filling the smoker over here. Carting around a smokey steel can filled with lava rocks is also a royal pain. I liked the portable rig they had there with the Mylar/insulation soft box combo. I might want to take a stab at it.

Thanks for the link!

[aeray]

I'm in a similar situation. The "load-it-turn-it-on-and walk-away" factor is what really intrigued me, plus the low-temp smoking capabilities. Hams, bacons, sausages, etc, etc. I'm willing to buy the wood-chip-cakes, if I can DIY a feeder/burner mechanism. The enclosure will be no problem, I'm thinking junk fridge, or cardboard box.