# arduino yogurt maker

Posted on Feb 6, 2014

The crock pot is attached to a relay that can switch it on or off. The relay is toggled by a microcontroller based on readings from a temperature sensor ( thermistor ) placed inside the crock pot. Heat the milk to 185 °F (85 °C). This kills off microbes to make way for our yogurt cultures, and denatures enzymes in the milk that may interfere with

yogurt culture growth. Doing things the old-fashioned way, I`d be using a stovetop and candy thermometer for steps 2 and 3, then a warm oven or a radiator for step 4. That takes a lot of attention, and uses more containers than I care to wash later. Many commercially-sold yogurt makers still require you to perform step 2 yourself. With the Arduino setup, I use a few canning jars in a water bath inside the crock pot to ensure even heating. I submerge the temperature sensor in the water. A voltage divider circuit is used to indirectly measure the resistance of the thermistor. In the code, I make use of the Steinhart-Hart Thermistor Equation to translate the thermistor`s resistance into temperature. This gives a pretty good idea of the temperature inside the crock pot. In addition to the thermistor`s resistance at a given time, the Steinhart-Hart equation needs to be fed three coefficients which can be calculated from information on the manufacturer`s data sheet based on predetermined resistances at different temperatures. Since we`ll be measuring a range between 100 °F (38 °C) and 185 °F (85 °C), I used resistance values measured at 86 °F (30 °C), 140 °F (60 °C) and 194 °F (90 °C) to calculate my coefficients. Here`s a simple calculator to calculate your coefficients from a given temp/resistance range, and here`s a more in-depth explanation of the math involved. Since the temperature readings will be taken in the water bath,...

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