251 lines
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10 KiB
Markdown
251 lines
No EOL
10 KiB
Markdown
---
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title: "Smart Yogurt Maker Part 1"
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date: "2022-01-25"
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author: "William Floyd"
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featured_image: "media/IMG_20220125_113949_cleaned.webp"
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categories: [
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"Development",
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"Hardware",
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"Software"
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]
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tags: [
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"ESPHome",
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"Rice Cooker",
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"Home Assistant",
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"Smart Home",
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"Automation",
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"Yogurt"
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]
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series: ["Smart Yogurt Maker"]
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---
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A certain [Rice](/2018/smart-rice-cooker) [Cooker](/2018/dumb-rice-cooker) has been languishing in a box for the last several years - now it is time to revisit it.
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It is a tale of Solder, Software, and Streptococcus thermophilus.
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***
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# "Last time, on Rice Cooker Adventures"
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This story picks up where I left off 3 years ago - the rice cooker has thermistors taped to the bottom of the pot with high temperature fiberglass tape. Specifically, I have 4 of the same thermistor in 2 parallel lines of 2 series resistors, like so.
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![Thermistor Diagram][thermistors]
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This serves to roughly average the resistances, to account for possible hotspots on the pot, while only taking a single ADC pin.
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However, time has not been kind to my memory, and the specific parameters of these thermistors are unknown, so first, it's time to do some testing.
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# Side Quest: Thermistor Calibration
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I have a thermocouple that I can use to verify the temperature of the pot with, so here is the setup: log the resistance through the thermistor bank at close to room temperature, rolling boil, and a middle temperature (during cooldown). These three sample points may then be used to find the parameters of the thermistor exactly.
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From hazy memory (and counting parts in inventory), my suspicion was that I had used a $100k\Omega$ thermistor.
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Brand is unknown, but representative datasheets online cite a $\beta$ value of 3950 - let's see how we compare.
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## Wait, what does $\boldsymbol{\beta}$ mean?
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For a more in depth read, look [here](https://www.qtisensing.com/wp-content/uploads/Beta-vs-Steinhart-Hart.pdf), but in summary early studies proposed that for NTC thermistors, the temperature $T$ could be determined given a measured resistance $R$, coefficient $\beta$ and reference resistance $R_0$ and temperature $T_0$, like so:
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$$
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\frac {1}{T}=\frac{1}{T_{0}}+{\frac {1}{\beta}} \ln{\frac {R}{R_{0}}}
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$$
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While this may work for small temperature ranges (approximately $20^\circ \text{C}$ or so), we require a larger range, and so we are instead going to look at the...
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## Steinhart-Hart Equation
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Briefly summarizing [Wikipedia](https://en.wikipedia.org/wiki/Steinhart%E2%80%93Hart_equation), the equation states:
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$$\frac{1}{T}=A+B\cdot\ln\{\left(R\right)}+C(\ln \left(R\right))^{3}$$
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Which, when using 3 sample points, results in:
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$$
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\begin{bmatrix}
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1 & \ln R_1 & \ln^3 R_1 \\\\
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1 & \ln R_2 & \ln^3 R_2 \\\\
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1 & \ln R_3 & \ln^3 R_3
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\end{bmatrix}\begin{bmatrix}
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A \\
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B \\
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C
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\end{bmatrix} = \begin{bmatrix}
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\frac{1}{T_1} \\\\
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\frac{1}{T_2} \\\\
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\frac{1}{T_3}
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\end{bmatrix}
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$$
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In my case, I found the points of reference using a setup like so:
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Do note that in order to reach a full boil (and $100^\circ \text{C}$ on the thermocouple), the lid of the rice cooker was attached.
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While I could show the calculations, and they wouldn't be difficult, I'll be honest: I used an [online calculator](https://www.thinksrs.com/downloads/programs/therm%20calc/ntccalibrator/ntccalculator.html).
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Using the measurements:
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$$
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\begin{array}{|l|l|}
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\hline \mathbf{R}(\boldsymbol{\Omega}) & \mathbf{T}\left({ }^{\circ} \mathbf{C}\right) \\\\
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\hline 81 \mathrm{k} & 33 \\\\
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\hline 21.3 \mathrm{k} & 66 \\\\
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\hline 6.4 \mathrm{k} & 100 \\\\
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\hline
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\end{array}
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$$
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Yielded
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$$
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\begin{aligned}
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A &= 1.032978070 \cdot 10^{-3}\\\\
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B &= 1.733021623 \cdot 10^{-4}\\\\
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C &= 1.902682261 \cdot 10^{-7}\\\\
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\hline
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R_{(25^\circ \text{C})} &= 117072.68 \Omega \\\\
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\beta_\text{calculated} &= 4202.76 \text{K}
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\end{aligned}
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$$
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As we can see then, the actual $\beta$ value is larger than predicted ($3950\text{K}$ predicted vs. $4203\text{K}$ actual), as is the reference resistance ($100\text{k}\Omega$ predicted vs. $117\text{k}\Omega$ actual).
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In fact, neither of these are even in the commonly stated "$\pm 1 \\%$" range.
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Good thing we're calibrating, hey?
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# "Back to our regularly scheduled programming"
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How to use this data then?
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Being short on time, I opted to take the easy way out, and use the wonderful [ESPHome](https://esphome.io/), as I already use Home Assistant.
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As it turns out, they already have us covered, with a [NTC Sensor component](https://esphome.io/components/sensor/ntc.html) already built in.
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This, coupled with a [resistance](https://esphome.io/components/sensor/resistance.html) and [ADC](https://esphome.io/components/sensor/adc.html) sensor allow using a voltage divider to measure the resistance of the thermistor.
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You know those calculations and equations we did above?
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Unnecessary, all of it.
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ESPHome supports doing Steinhart-Hart calculations itself, and only needs the three sample points.
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In the meantime, a Wemos D1 Mini on a breadboard is ready to go (they're cheap and chearful).
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I wire it up with a $6.8\text{k}\Omega$ resistor (what I had handy), and away I went.
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The circuit looks something like this, where the two trailing wires connect to the thermistor bank:
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To prevent confusion, here is my final working config, then I'll explain.
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```yaml
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sensor:
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- platform: ntc
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sensor: resistance_sensor
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calibration:
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- 81.0kOhm -> 33°C
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- 21.3kOhm -> 66°C
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- 6.4kOhm -> 100°C
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name: Rice Cooker Temperature
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unit_of_measurement: °C
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- platform: resistance
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id: resistance_sensor
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sensor: source_sensor
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configuration: DOWNSTREAM
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resistor: 6.96kOhm
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reference_voltage: 3.306V
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name: Resistance Sensor
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- platform: adc
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id: source_sensor
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pin: A0
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name: Voltage Sensor
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filters:
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- multiply: 3.2
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update_interval: 1s
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```
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# Calibrate all the things
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## Series Resistor
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Easy, measuring mine I found it to be $6.96\text{k}\Omega$, certainly different from the $6.8\text{k}\Omega$ it's meant to be.
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Throw that in the config.
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## Reference Voltage
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This is also easy enough, measure the $3.3\text{V}$ pin off the D1 mini.
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Mine came out to be $3.306\text{V}$, respectably close to rated!
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That goes in the config too.
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## Multiply Filter
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As it turns out, the D1 mini has an internal voltage divider to convert the $3.3\text{V}$ of the input voltage down to the $1\text{V}$ the ADC is rated for.
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This is nice, as it protects the ADC, but it must be accounted for.
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But there's a catch - on my board this wasn't quite right - the voltage read was still slightly wrong.
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I could try manually tuning this, but there is an easier way.
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Instead, I measured the actual voltage ($3.306\text{V}$), and compared it to the voltage being read.
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Say there was no error, then we would say $V_\text{actual}=V_\text{measured}$, and if there is an error of a multiple difference between the two, we can say $V_\text{actual}=m\cdot V_\text{measured}$. Thus, to find this multiplier, we say:
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$$
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m = \frac{V_\text{actual}}{V_\text{measured}}
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$$
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Using this method, I find my multiplier to be $3.2$, not $3.3$.
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With that, the board is finally reading the correct voltage, the correct resistance, and the correct temperature.
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# "You're so controlling!"
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Given that I have Home Assistant, automating it becomes easy.
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I'm currently using a [Tasmota](https://tasmota.github.io/docs/) flashed smart outlet to control the power to the heating element of the rice cooker (for reference, it pulls up to $400\text{W}$ at full load).
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In future, I would like to condense all functionality into a relay controlled directly by the ESP8266, to allow greater safety in case of network issues or even local PID tuning.
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I am also using a [PID controller for Home Assistant](https://github.com/soloam/ha-pid-controller).
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It seems to work fine, I am just bad at PID tuning, so results may vary depending on hardware, amount of mass in the cooker, ambient temperatures, etc.
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That being said, a simple automation to turn the heater on when the PID response goes high and turn off when not high yields acceptable results.
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From fiddling with the settings, I currently am using the following parameters:
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$$
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\begin{aligned}
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P &= 60 \\\\
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I &= 5.6 \\\\
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D &= 6.7 \\\\
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\end{aligned}
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$$
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Further tuning is required, but this held well overnight - mostly...
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I did change the set point from $105^\circ\text{F}$ to $107^\circ\text{F}$ halfway through, as well as tweaked some of the PID tuning.
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The concerning spike at 9:00 is marked by a momentary loss of connection with the ESP board - my automation failed to account for a loss of connection and so allowed the heat to continue rising.
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Thankfully, this was a short-lived event, but it is concerning nonetheless, and will require testing of my automation to prove that power loss tends towards a "safer" outcome.
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Do note that "safer" means no dead yogurt culture - the safety features of the rice cooker remain unchanged and functional.
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# The proof is in the... yogurt?
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So how did it do?
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I am notoriously _not_ picky when it comes to food, but I think it turned out great!
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Filling the rice cooker about halfway with whole milk, I heated it to $180^\circ \text{F}$ for a few minutes (I read I ought to go for longer...), cooled to about $110^\circ \text{F}$, then added a liberal few spoonfuls (think 1/5 volume of milk) of plain Dannon yogurt.
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I let it proof in the rice cooker for 7.5 hours, with the lid sealed.
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I put the whole rice cooker in the fridge to cool, with the lid sealed.
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Trying it later that same evening, I quite enjoyed it.
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I'll start my next batch from this one, and hopefully it'll be even better.
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# "Next time, on Rice Cooker Adventures"
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Now that I have a temperature controllable rice-cooker, and the ability to remotely start, stop, and modify set points, I may try other foods.
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For now, I need to:
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* solder a perfboard version of the circuit
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* add a relay to control the cooker directly
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* ensure the outlet will turn off if the ESP8266 loses connection
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* PID tune the setup for the most common scenario (steady $107^\circ \text{F}$ with the lid shut)
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First though, I'm gonna go eat some fresh yogurt...
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[thermistors]: media/circuit.svg "Thermistor Diagram]"
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{{< mathjax >}} |