Onsen Tamago Machine
- some eggs and plain water
- a small, cheap rice cooker: it should be under $20 US. It shouldn't have any screen or other features besides a switch to turn it on and off. I use this one., which you can likely find at any drug store or housewares store.
- a PowerSwitch Tail from Adafruit, SparkFun or their own online store There are two types of PowerSwitches: the standard one with a mechanical relay sold by Adafruit and Sparkfun, and then one with a solid state relay sold only on their website. The mechanical relay version makes a noticeable click sound every time it cycles the power on and off -- dozens or even hundreds of times during an egg cook cycle -- but can safely handle a larger rice cooker with more wattage. The solid state one advises a maximum of 300 watts but is virtually silent. Check the wattage of your rice cooker. The rice cooker I recommend above is small -- can fit about six eggs and pulls exactly 300 watts, so I use the solid state PowerSwitch. I found the mechanical relay sound a bit too loud for the serene act of making an onsen egg.
- A microcontroller. To precisely follow these instructions, I suggest the Arduino Pro Micro -- you can choose either the 5V or 3.3V version. But you can also use most any Arduino code-compatible variant if you already have one. I like the Pro Micro because it is small and cheap. The only real architecture dependent code on top of I2C and OneWire is a 15ms timer.
- This RTC breakout from Adafruit and the recomended coin cell battery. This is the thing that tells the eggs what time to heat up at. If the breakout is not in stock, it's a very simple circuit with parts you can easily acquire yourself. It looks like you can also subsitute the DS3231 breakout, which seems smaller and higher accuracy, but I haven't tried it.
- a DS18B20 temperature sensor on a waterproof cable from either Adafruit or Sparkfun. You also need a single 4.7K resistor, which Adafruit (but not Sparkfun) ships with the sensor.
- This LCD shield from Adafruit. At $25, it's the most expensive single part, and it can be omitted if you're cutting costs. I really like seeing the status of the egg as it cooks, and the buttons are helpful to start or stop a cook in progress.
- Some solder, a soldering iron, hookup wires, jumpers, etc. A breadboard or through hole drilled PCB would be helpful if you want to permanently attach the components.