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assembly.md

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Assembly of Thermistor Board

Board Recommendation

We need 6 ADC channels from ADC unit 1 exposed. Some boards use I2C pins overlaid on ADC unit 1 channels.

There is also difference in sampling rates between ESP32 models. ESP32-S3 has a maximum sampling rate of 83kHz where ESP32 has 2MHz.

I recommend using Sparkfun Thing Plus or Thing Plus C as they have a 2MHz max sampling rate.

When setting up the Wheatstone bridge the supply voltage is about 3.3V. If the thermistor is NTC type, resistance will decrease with temperature. The minimum resistance is at 45C and will be 4k Ohm and the maximum at freezing about 35k.Therefore one Voltage will be constant at about 1.6V and the other between 0.9 .. 2.5V.

Therefore ADC setting for ESP32 should be chosen as following:

Choice Description Voltage Range
ADC_ATTEN_DB_0, No attenuation 0.1 to 0.95 V
ADC_ATTEN_DB_2_5 2.5 dB 0.1 to 1.25 V
ADC_ATTEN_DB_6 6 dB 0.15 to 1.175 V
ADC_ATTEN_DB_11 11 dB 0.15 to 2.45 V

The bridge resistors should be at a ratio of 2 to 1 so that R2 and R3 resistor are 20k and R1 is 10k. This would allow more accurate measurement of temperature in physiological range.

Soldering

Attach color coded wires to the IO pads. E.g. red for power, black or green for ground and blue or white for digital input/output and yellow for analog wires.

You can insert the wires into the through holes (perpendicular) or or you can attach a short piece of the wires on top of the pad (perpendicular).

Suggested connections for the Sparkfun Thing Plus (USB-C) and the Adafruit Feather ESP32-S3 are given below.

Connections

Analog input needs to be on same ADC unit in order to use fast continuous ADC on ESP32. On Adafruit Feather ESP32-S3 we have only 5 pins available for ADC1 and two are also used for I2C. In addition ESP32-S3 has maximum sample rate of 83kHz where as Thing Plus ESP32 has 2MHz

PAD Function Thing Plus Feather
A5 analog CH1A A5 ADC1-CH7 D9 ADC1-CH8
A4 analog CH1B A4 ADC1-CH0 D10 ADC1-CH9
A3 analog CH2A A3 ADC1-CH3 D11 ADC1-CH4
A2 analog CH2B A2 ADC1-CH6 D12 ADC1-CH5
A1 analog CH3A 06/32 ADC1-CH4 not exposed
A0 analog CH3B 10/33 ADC1-CH5 not exposed
GND Ground GND GND
3.3V Power 3V3 3V3

Require connections are in bold.

Button

The button is configured to pull a pin high. One button pin is connected to 3.3V and the other to the input pin on the microcontroller. In software that pin is pulled low.

PAD Function Thing Plus Feather
3.3V Power 3V3 3V3
Button Button 9 or A0 12

Pinouts

Calibration

The resistors are configured the following way:

drawing

made with https://www.circuit-diagram.org/editor

The Wheatstone bridge resistors should be measured for greater accuracy.

  • TP1 - TP3: R1
  • TP1 - TP2: R2
  • TP2 - TP3: R3

The thermistor resistance is

$R_{Thermistor} = \frac{R_3 (V_{in} R_2 - V_{diff} (R_1+R_2))}{Vin R1 + Vdiff (R_1+R_2)}$

Where $V_{diff} = V_1 - V_2$

$V_1$ and $V_2$ are measured with a microcontroller's ADC converter and they correspond to A0-A1, A2-A3 and A4-A5. ESP ADC is of low quality and linarization and averaging in software is needed.

After measuring the three resistors of each of the channels we need to close all jumpers in the circuit. There are 11 jumpers.

Thermistor

Amphenol 10kOhm MA100 https://www.mouser.com/datasheet/2/18/Amphenol_04022020_AAS_920_321E-1826352.pdf

Temperature Resistance R_3/R_T R_1/R_2 R_3/R_T R_1/R_2
10k/R_T 10k/10k 20k/R_T 20k/10k
V_1 V_2 V_1 V_2
32C 32739 Ohm 2.70 1.76 2.19 1.18
41C 25441 Ohm 2.53 1.76 1.97 1.18
50C 19925 Ohm 2.35 1.76 1.76 1.18

Illustrates that ADC_ATTEN_DB_11 and 2/1 ratio is needed between the resistors so that the measurements are in acceptable linear range of the ADC.