Sous Vide Cooker

thingiverse

A sous vide cooker made from a small rice cooker. Instructions Equipment Rice cooker Thermistor 3.3 kΩ resistor 10 kΩ resistor Transistor (mine was TIP120) SPDT Relay Diode Extension cord Rice Cooker The rice cooker provides many components needed for sous vide cooking. It has a pot for the water bath and a metal plate used for heat transfer. However, it does not allow for arbitrary temperatures to be set and maintained, so for the purpose of the sous vide cooker, we will remove all the electronics. All that should be left is the heating plate, the pot, and the housing. Thermistor A thermistor works by varying resistance with changes in temperature. The temperature as a function of resistance is T(R) = {A + Bln(R/Rref) + C[ln(R/Rref)]^2 + D*[(ln(R/Rref)]^2}^(-1) where A through D are constants dependent on the material and Rref is the resistance at a reference temperature. Since I wasn't sure of which exact thermistor I had, I took resistance and temperature measurements for a series of water temperatures. The Arduino checks the temperature using the thermistor but cannot measure the resistance directly. To make up for this, I made a simple voltage divider, or adding a resistor in series to the thermistor, such that the voltage across the thermistor will also change with resistance. The thermistor had a resistance range of roughly 1 - 10 kΩ during the earlier test. Using this information, I picked a 3.3 kΩ resistor so that the voltage across the thermistor would vary roughly 2.6 V from min to max. NOTE: If you want to go the easier route, get a LM35/36 temperature sensor. You can skip the voltage divider step, and it's a linear curve!http://www.ladyada.net/learn/sensors/tmp36.html Relay To heat the water, we need a lot more power than what the Arduino can provide. However, we still want the Arduino to act as a power switch. This is where the relay comes in. The relay I used had five pins. Two are connected to the Arduino and form a circuit internally. Inside there is a coil, and when power runs through it, it flips the relay switch. The other three pins are for the actual switching. The switch can flip between two things, but in this case, we are using it as an off-on. My recommendation is that you test the relay with the Arduino first. When you apply enough power, you should hear a click. Transistor My relay can only switch with a minimum 5V@80mA. The Arduino can do 5V but I believe only outputs 40-50mA. I admit, I'm still not entirely sure how transistors work, but they can be used as amplifiers. More information: http://www.youtube.com/watch?v=cEz1i5xzGEE Wall Plug My rice cooker had some extra circuitry for a light and the button to initiate cooking, so I cut that out and away left with a plug. I decided not to use it and instead went with making my own. I bought an extension cord and cut off the female end. I then stripped the wire a bit and then attached one end to the relay and the other to the rice cooker's heating plate. You will complete the circuit using another wire going from the heating plate to the relay. The wall provides a lot of power, so it's important to be safe while doing this step. More information: http://www.sparkfun.com/tutorials/119 Your First Victim To be on the safe side, try your new sous vide cooker on an egg first. More information: http://www.edinformatics.com/math_science/science_of_cooking/eggs_sous_vide.htm Improvements These are things I would like to do in the future. Create a PCB to house the circuit. Estimate error in thermistor curve fit Timer UPDATE 4/19/12 My thermistor appears to be a bit broken, so I am taking the opportunity to switch it out for an LM35/36 temperature sensor. The new code will be added once it comes in. I am also switching to a solid state relay. LCD is now supported! Mine is a simple 16x2 screen.http://arduino.cc/en/Tutorial/LiquidCrystal