yogurt: Add part 1 block post (force push to use lfs to xcf).

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+---
+title: "Smart Yogurt Maker Part 1"
+date: "2022-01-25"
+author: "William Floyd"
+featured_image: "media/IMG_20220125_113949_cleaned.jpg"
+categories: [
+    "Development",
+    "Hardware",
+    "Software"
+]
+tags: [
+    "ESPHome",
+    "Rice Cooker",
+    "Home Assistant",
+    "Smart Home",
+    "Automation",
+    "Yogurt"
+]
+---
+
+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.
+It is a tale of Solder, Software, and Streptococcus thermophilus.
+
+***
+
+# "Last time, on Rice Cooker Adventures"
+
+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.
+
+![Thermistor Diagram][thermistors]
+
+This serves to roughly average the resistances, to account for possible hotspots on the pot, while only taking a single ADC pin.
+
+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.
+
+# Side Quest: Thermistor Calibration
+
+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.
+
+From hazy memory (and counting parts in inventory), my suspicion was that I had used a $100k\Omega$ thermistor.
+Brand is unknown, but representative datasheets online cite a $\beta$ value of 3950 - let's see how we compare.
+
+## Wait, what does $\boldsymbol{\beta}$ mean?
+
+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:
+
+$$
+\frac {1}{T}=\frac{1}{T_{0}}+{\frac {1}{\beta}} \ln{\frac {R}{R_{0}}}
+$$
+
+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...
+
+## Steinhart-Hart Equation
+
+Briefly summarizing [Wikipedia](https://en.wikipedia.org/wiki/Steinhart%E2%80%93Hart_equation), the equation states:
+
+$$\frac{1}{T}=A+B\cdot\ln\{\left(R\right)}+C(\ln \left(R\right))^{3}$$
+
+Which, when using 3 sample points, results in:
+
+$$
+\begin{bmatrix}
+    1 & \ln R_1 & \ln^3 R_1 \\\\
+    1 & \ln R_2 & \ln^3 R_2 \\\\
+    1 & \ln R_3 & \ln^3 R_3
+\end{bmatrix}\begin{bmatrix}
+    A \\
+    B \\
+    C
+\end{bmatrix} = \begin{bmatrix}
+    \frac{1}{T_1} \\\\
+    \frac{1}{T_2} \\\\
+    \frac{1}{T_3}
+\end{bmatrix}
+$$
+
+In my case, I found the points of reference using a setup like so:
+
+![Measuring water temperature and thermistor resistance](media/IMG_20220124_174144.jpg)
+
+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.
+
+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).
+
+![Calculator results](media/calc.png)
+
+Using the measurements:
+
+$$
+\begin{array}{|l|l|}
+\hline \mathbf{R}(\boldsymbol{\Omega}) & \mathbf{T}\left({ }^{\circ} \mathbf{C}\right) \\\\
+\hline 81 \mathrm{k} & 33 \\\\
+\hline 21.3 \mathrm{k} & 66 \\\\
+\hline 6.4 \mathrm{k} & 100 \\\\
+\hline
+\end{array}
+$$
+
+Yielded
+
+$$
+\begin{aligned}
+A &= 1.032978070 \cdot 10^{-3}\\\\
+B &= 1.733021623 \cdot 10^{-4}\\\\
+C &= 1.902682261 \cdot 10^{-7}\\\\
+\hline
+R_{(25^\circ \text{C})} &= 117072.68 \Omega \\\\
+\beta_\text{calculated} &= 4202.76 \text{K}
+\end{aligned}
+$$
+
+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).
+In fact, neither of these are even in the commonly stated "$\pm 1 \\%$" range.
+Good thing we're calibrating, hey?
+
+# "Back to our regularly scheduled programming"
+
+How to use this data then?
+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.
+As it turns out, they already have us covered, with a [NTC Sensor component](https://esphome.io/components/sensor/ntc.html) already built in.
+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.
+
+You know those calculations and equations we did above?
+Unnecessary, all of it. 
+ESPHome supports doing Steinhart-Hart calculations itself, and only needs the three sample points.
+
+In the meantime, a Wemos D1 Mini on a breadboard is ready to go (they're cheap and chearful).
+I wire it up with a $6.8\text{k}\Omega$ resistor (what I had handy), and away I went.
+The circuit looks something like this, where the two trailing wires connect to the thermistor bank:
+
+![Wired breadboard](media/IMG_20220125_114006_cleaned.jpg)
+
+To prevent confusion, here is my final working config, then I'll explain.
+
+```yaml
+sensor:
+  - platform: ntc
+    sensor: resistance_sensor
+    calibration:
+      - 81.0kOhm -> 33°C
+      - 21.3kOhm -> 66°C
+      - 6.4kOhm -> 100°C
+    name: Rice Cooker Temperature
+    unit_of_measurement: °C
+
+  - platform: resistance
+    id: resistance_sensor
+    sensor: source_sensor
+    configuration: DOWNSTREAM
+    resistor: 6.96kOhm
+    reference_voltage: 3.306V
+    name: Resistance Sensor
+
+  - platform: adc
+    id: source_sensor
+    pin: A0
+    name: Voltage Sensor
+    filters:
+      - multiply: 3.2
+    update_interval: 1s
+```
+
+# Calibrate all the things
+
+## Series Resistor
+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.
+Throw that in the config.
+
+## Reference Voltage
+This is also easy enough, measure the $3.3\text{V}$ pin off the D1 mini.
+Mine came out to be $3.306\text{V}$, respectably close to rated!
+That goes in the config too.
+
+## Multiply Filter
+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.
+This is nice, as it protects the ADC, but it must be accounted for.
+But there's a catch - on my board this wasn't quite right - the voltage read was still slightly wrong.
+I could try manually tuning this, but there is an easier way.
+Instead, I measured the actual voltage ($3.306\text{V}$), and compared it to the voltage being read.
+
+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:
+
+$$
+m = \frac{V_\text{actual}}{V_\text{measured}}
+$$
+
+Using this method, I find my multiplier to be $3.2$, not $3.3$.
+
+With that, the board is finally reading the correct voltage, the correct resistance, and the correct temperature.
+
+# "You're so controlling!"
+
+Given that I have Home Assistant, automating it becomes easy.
+
+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).
+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.
+
+I am also using a [PID controller for Home Assistant](https://github.com/soloam/ha-pid-controller).
+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.
+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.
+
+From fiddling with the settings, I currently am using the following parameters:
+
+$$
+\begin{aligned}
+P &= 60 \\\\
+I &= 5.6 \\\\
+D &= 6.7 \\\\
+\end{aligned}
+$$
+
+Further tuning is required, but this held well overnight - mostly...
+
+![First overnight test](media/log.png)
+
+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.
+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.
+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.
+Do note that "safer" means no dead yogurt culture - the safety features of the rice cooker remain unchanged and functional.
+
+# The proof is in the... yogurt?
+
+So how did it do?
+I am notoriously _not_ picky when it comes to food, but I think it turned out great!
+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.
+I let it proof in the rice cooker for 7.5 hours, with the lid sealed.
+
+![First batch, still warm](media/IMG_20220125_105918.jpg)
+
+I put the whole rice cooker in the fridge to cool, with the lid sealed.
+Trying it later that same evening, I quite enjoyed it.
+I'll start my next batch from this one, and hopefully it'll be even better.
+
+![Spoonful of the good stuff](media/IMG_20220125_180035.jpg)
+
+# "Next time, on Rice Cooker Adventures"
+
+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.
+For now, I need to:
+
+* solder a perfboard version of the circuit
+* add a relay to control the cooker directly
+* ensure the outlet will turn off if the ESP8266 loses connection
+* PID tune the setup for the most common scenario (steady $107^\circ \text{F}$ with the lid shut)
+
+First though, I'm gonna go eat some fresh yogurt...
+
+[thermistors]: media/circuit.svg "Thermistor Diagram]"
+
+{{< mathjax >}}

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