First steps to BBQ bliss (at least for me)

So I had to learn a few things……..

First I started with ProtoBoard and added a microcontroller (PIC 16F886) and a two line by 16 character LCD.

I wrote a program that displayed text on the LCD – step one was complete.  I then added an IRF630 MOSFET as a speed controller and learned how to control the speed of a fan –  step two.

The next learning step was a little tougher – I had never worked with any analog to digital sampling before.  Thermocouples work by generating a voltage in relation to the temperature of the junction.  The only problem is that its only 44 microvolts per degree celcius (for the K-type thermocouples I was using). This is a very small voltage to sample, so an OP-Amp is needed to boost the level to something more easily measurable.  In addition, the junction that you measure the voltage at causes issues too – its another junction.

The answer?  The Analog Devices AD595 – a ready-made thermocouple interface that outputs 10 millivolts per degree celcius, compensates for the measurement junction, and even has an alarm output that signals if a thermocouple fails!  Perfect!
Using the AD595’s made my first analog-to-digital attempts easy.

I spent a  couple of weeks writing software to read the thermocouple inside the smoker and control to speed of a 12VDC fan to reulate the temperature.  I also added alarm points and monitoring of the meat temperature through another thermocouple.

Now that I had a way to control it – I needed a fan.  I mounted a computer fan in the side of a Rubbermaid container, put a plate to seal the fan in and mounted a 1 1/4″ hose barb to let the air out.  I unbolted the damper from my smoker, and mounted a stainless steel bowl with a hose barb.  See the below picture for an idea of what I mean.

In the picture, you might notice that the hose barb is plastic – that REALLY didn’t work out so well!  It has since been changed to a metal barb insulated with high temperature rubber tape – works great with plastic hose as the connection between fan and manifold.
I did a bunch of experimenting with the settings of the program, and got everything tuned up so that it keeps the temperature within 5 degrees F or so – more than close enough for my purposes.
Here’s a picture of the test-bed board I ended up with (some parts have been removed and are on the new version):
Image
After that, I decided that I needed it to be a little “classier”.  So I bought a license for one of the entry-level versions of DipTrace and designed a PCB that I had made by OSHPark.  Here’s that board:
pcb
I have since built up the board and it works great!  Next I’ll document the changes I made once I had the couple of mistakes I made on the PCB fixed.  How is it that I spent HOUR AFTER HOUR checking everything on the design, and it still had two mistakes?  They were both easily fixed in a couple of minutes, but still……….
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