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Oven controller for pizza

Oven switching taken over by Arduino for better control and increased temperature range

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I found the wires to the temp probe in the oven and cut them to read the temp into the Arduino. I soon realized that the temperature sensor was not a thermal coupler as I expected, but a thermistor. I connected a relay to the oven wiring to switch between high and low resistances to turn on and off the oven by spoofing the thermistor. I use a separate digital thermometer along with analog readings of the thermistor to find an equation that would correlate the analog signal to a correct temperature.

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Nick Sayer wrote 04/24/2015 at 15:27 point

Ah. You could use one output to go to the base of an NPN transistor, emitter to output. The collector would go to a 1.2K resistor. In parallel with that would be a 10K to output. 

You may need to finesse that depending on the voltages of the input and output. But that's the basic idea of switching between two resistances - just have the larger be always there and switch a slightly larger than desired one in parallel on demand. 

I've typed all that and now realize that may be exactly what you're already doing. If so, I apologize. 

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Andreas Mergner wrote 04/24/2015 at 17:08 point

I did try to do what you are saying with a MOSFET, but it was not working as expected.  I'm not sure if it was because of the shared ground or a voltage drop or what.  I don't know exactly what the stove circuit is doing other than supplying 5V, but I would think it would be a voltage divider circuit with a voltage ADC.  I switched over to a relay and it just worked.  I think I like the relay because I can make it so that when it is not energized, it is the lower resistance which will turn the oven off.  That way if the oven is on and the Arduino loses power, I won't have the oven staying on until it melts/causes a fire.  It doesn't need rapid switching/PWM.

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Nick Sayer wrote 04/24/2015 at 17:34 point

I've not tried the same thing with a MOSFET. For me, BJTs are a little easier to wrap my brain around. My mental model is two diodes with the emitter being the shared lead. NPNs have a common cathode emitter, PNPs a common anode emitter. The magic is that the base-to-emitter current will cause a larger (hFE) collector-to-emitter current, possibly leading to saturation.

What you wind up needing to figure out is when you want the collector-to-base current to flow, what sort of base-to-emitter current must you make to have that happen?

One of the most interesting puzzles of that sort I had to solve was a J1772 pilot generator driven by TTL logic levels. That is, a +/- 12 volt square wave from TTL. To do it, I wound up with a complimentary pair - PNP for +12 and NPN for -12 with the collectors common for the output. Each had the base weakly pulled to the emitter to keep them turned off normally. But each base was also tied to the collector of a complimentary transistor acting as a TTL switch. A PNP transistor would draw the -12 NPN transistor's base up from -12 towards +5, but only when the input logic signal was low. An NPN transistor would draw the +12 PNP transistor's base down towards ground, but only when the input logic signal was high.

So what you need to do is figure out the voltages in the off state for the transistor and the direction you're going to want the current to flow in the on state. Having done that, you then figure out how to get the appropriate base current when you want it to be turned on.

For the fail-safe, you would just add a pull-up or pull-down resistor to the base, whichever is appropriate.

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Nick Sayer wrote 04/24/2015 at 07:02 point

You might be able to alter the firmware of my Toast-R-Reflow project for this. It's a very similar problem. The controller board is an ATTiny84 or ATMega328p (depending on the model), a 2x16 LCD display, and either an analog or digital thermocouple amplifier (again, depending on the model). It can control two output channel optoisolators (they're intended to connect to my Toast-R-Reflow power board, which has two BTA-20 triacs with optoisolator drivers).

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Andreas Mergner wrote 04/24/2015 at 14:50 point

Thanks Nick!  I took a look at your project and it is very well written.  I am not getting rid of the oven's controller.  I am connecting the Arduino to the thermistor already in the oven to read the temp.  Then I trigger the oven's controller by switching between two different resistances using a relay.  The thermistor is around 1K ohms at 70 degrees F and increases resistance as it gets hotter.  I set my oven's controller at 350 degrees and when I want to turn on the burners, I use a 1K ohm resistor.  When I want to turn off the burners, I use a 10K ohm.  This way, I can use the oven's controller for bake or broil to select the appropriate burner.  I am going to use optoisolators and triacs for another project I have in mind!  

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