Dankdryer improvements

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* The LM35 is operating in a nasty thermal environment, and on about ten centimeters of poorly-shielded AWG22. Let's give it some bigger boxing gloves. Put a 0.01µF bypass capacitor across its power and ground leads. Put a 75Ω resistor and a 0.22µF capacitor in series between the signal and the ground. These recommendations come directly from the [https://www.ti.com/lit/ds/symlink/lm35.pdf datasheet]:

[[File:LM35-circuit.png]]

* We should use the AVDD analog power input of the HX711 at 5V.

===PCB considerations===

* We're ditching the Sparkfun NAU7802 breakout board, so we'll need add 2.2kΩ pullups on both I2C lines, 0.1µF and 1µF capacitors on VDDA, and 47Ω resistors on VNN1N and VNN1P with a 0.1µF bypass capacitor across them. See the [https://cdn.sparkfun.com/assets/6/a/5/9/d/Qwiic_Scale.pdf schematic]:

[[File:SparkFun-NAU7802.png]]

* The first version took in 120VAC, 12VDC, and 5VDC (i.e. there was a discrete buck converter from 12 → 5VDC). Let's build that circuit into the PCB, so we're only working with 120VAC and 12VDC (the native output of the AC adapter) inputs. At the same time, let's move the ESP32 over to 3.3V input (at least when we're not powering it with external USB), so we don't exercise its low-efficiency voltage regulator. We'll still want 5V to the LM35 and the HX711's AVDD. Since these are both precision analog devices requiring very small currents, we'll use a voltage regulator from 12->5; this ought be less noisy than a switching transformer.

==Thoughts==

Dankdryer improvements: Difference between revisions

Dank (talk | contribs)

@@ Line 18: / Line 18: @@
 * The LM35 is operating in a nasty thermal environment, and on about ten centimeters of poorly-shielded AWG22. Let's give it some bigger boxing gloves. Put a 0.01µF bypass capacitor across its power and ground leads. Put a 75Ω resistor and a 0.22µF capacitor in series between the signal and the ground. These recommendations come directly from the [https://www.ti.com/lit/ds/symlink/lm35.pdf datasheet]:
 [[File:LM35-circuit.png]]
+* We should use the AVDD analog power input of the HX711 at 5V.
 ===PCB considerations===
 * We're ditching the Sparkfun NAU7802 breakout board, so we'll need add 2.2kΩ pullups on both I2C lines, 0.1µF and 1µF capacitors on VDDA, and 47Ω resistors on VNN1N and VNN1P with a 0.1µF bypass capacitor across them. See the [https://cdn.sparkfun.com/assets/6/a/5/9/d/Qwiic_Scale.pdf schematic]:
 [[File:SparkFun-NAU7802.png]]
+* The first version took in 120VAC, 12VDC, and 5VDC (i.e. there was a discrete buck converter from 12 → 5VDC). Let's build that circuit into the PCB, so we're only working with 120VAC and 12VDC (the native output of the AC adapter) inputs. At the same time, let's move the ESP32 over to 3.3V input (at least when we're not powering it with external USB), so we don't exercise its low-efficiency voltage regulator. We'll still want 5V to the LM35 and the HX711's AVDD. Since these are both precision analog devices requiring very small currents, we'll use a voltage regulator from 12->5; this ought be less noisy than a switching transformer.
 ==Thoughts==