Breadmaker - temp sensor question

Discussion in 'Embedded Systems and Microcontrollers' started by roperman, May 15, 2014.

  1. roperman

    Thread Starter New Member

    May 15, 2014
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    Hi all.
    I have an old breadmaker in which the control circuit failed, which is great as I want to make my own anyway.

    I was planning on using a PIC microcontroller, possibly a 16f630 as I have a few, or a 16f886.

    The coding side should be fine, although i've never interfaced with an LCD before (think i'm gonna use a nokia 3310 screen).

    The one thing i'm unsure about is how to 'read' the temperature of the oven. I will need to be able to be able to read the temps around 22-30°C and 210-240°C.

    I think most one wire digital sensors are limited to about 125°C.
    What am I best off doing? Can anyone point me in the right direction, preferably with some sort of example schematic so I can understand what i'm doing.

    Thanks,
    Danny
     
  2. DC_Kid

    Distinguished Member

    Feb 25, 2008
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    use a thermocouple. they come in so many shapes/sizes, etc.
     
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    A thermocouple is an excellent choice albeit no PIC can directly read one. You need it to survive the high temperatures. You can get interface chips where one side plugs into the thermocouple and the other has a digital serial interface for the PIC.

    Your nokia 3310 screen may well require way more resources then a PIC16 can supply. You also have to provide your own symbols, meaning you have to draw any letter or number to that screen. It is possible (I've done it on other devices) but it is a lot of work unless you find a library to reuse.
     
  4. John P

    AAC Fanatic!

    Oct 14, 2008
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    I don't agree. I think a platinum resistance sensor would be much better, because it doesn't require any cold junction compensation, and you might be able to connect it to an A/D pin of your PIC with just a fixed resistor to make a voltage divider out of it. If that isn't precise enough, you'd only need a simple op amp circuit to boost the output.

    http://www.ebay.com/itm/0-2-Class-A-Platinum-Resistance-Thermometers-PT100-3W-/180669771790

    8 bucks on eBay, shipping included. Some people here have said that Chinese vendors are a menace, but I've ordered components from them quite a few times and never had a problem.
     
  5. roperman

    Thread Starter New Member

    May 15, 2014
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    Thanks guys. Hadn't realized how accurate thermocouples were. For some reason I thought they were some sort of thermal switch. Think i'm gonna get hold of a MAX6675 and type K thermocouple.

    The platinum resistance sensor would be good (although it'd rule out my ancient 16f630 because it has no A/D) but I had already ordered a thermocouple. Might have a play with one of those next time.

    Yeah I agree that using a Nokia LCD will be a bit of a challenge but it should be fun. Not really done much asm since I was at uni so will give me something to get my teeth into.

    Danny
     
    Last edited: May 15, 2014
  6. John P

    AAC Fanatic!

    Oct 14, 2008
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    Hah. You can easily build a circuit with the resistance sensor which would make time the variable that you measure--just add a capacitor and drive current into it through the resistor, and count clock ticks until a Schmitt-trigger input changes state. Actually I just checked, and it seems the only suitable input pin on the PIC16F630 would be RA.2, though you could also use the TMR1 gate pin, RA.4, which has the Schmitt-trigger feature when it gates the timer, but it's just regular logic otherwise.

    Seriously, I've measured a variable resistor this way. Much more fun than an external A/D.
     
  7. DC_Kid

    Distinguished Member

    Feb 25, 2008
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    a varistor in a voltage divider will also work. then all that is needed is a table in the pic to map voltage to temp. then call your "whats_the_temp" sub routine every 5-10sec, etc.
     
  8. jjw

    Member

    Dec 24, 2013
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  9. MrChips

    Moderator

    Oct 2, 2009
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    +1

    I have to agree. You don't need an ADC to measure a resistance value. A single I/O port and a capacitor will allow you to measure frequency or period. Even if the result is non-linear, it is easy to convert to temperature. I have done this many times.
     
  10. John P

    AAC Fanatic!

    Oct 14, 2008
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    I think you mean thermistor, and I'm skeptical about whether they're usable at oven temperatures.
     
    ErnieM likes this.
  11. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    If using the Nokia 3310 display it's best to use a PIC with a MSSP hardware module (SPI hardware module).

    That makes it faster and easier to send the SPI data to the display.
     
  12. ErnieM

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    Apr 24, 2011
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    +1

    I would not hazard a guess what he means, but the OP seems savvy enough to use a part rated for 450° F in an area where temperatures reach 450° F.
     
  13. DC_Kid

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  14. MrChips

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    Solder will melt around 180-200°C.
    My guess is you cannot use SMD thermistor in the oven that goes to higher temperatures.
     
  15. DC_Kid

    Distinguished Member

    Feb 25, 2008
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    yeah, that would be bad. for ovens i am seeing AD595 with K tc. thus a simple tc to analog output.
     
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