Calculate NTC temperature using Steinhart-Hart equation

src/config.json

@@ -28,19 +28,25 @@
           "name": "PCB_NTC",
           "channel": 0,
           "system": "controller",
-          "control": false
+          "control": false,
+          "resistanceAt25C": 10000,
+          "bValue": 3977
         },
         {
           "name": "SAUNA_NTC",
           "channel": 1,
           "system": "sauna",
-          "control": true
+          "control": true,
+          "resistanceAt25C": 10000,
+          "bValue": 3950
         },
         {
           "name": "Outside",
           "channel": 2,
           "system": null,
-          "control": false
+          "control": false,
+          "resistanceAt25C": 10000,
+          "bValue": 3950
         }
       ],
       "saunaOnWhileDoorOpen": false,

src/platformAccessory.ts

@@ -640,7 +640,7 @@ export class OpenSaunaAccessory {
               }
 
               // Convert the ADC reading to a temperature value
-              const temperatureCelsius = (reading.value * 3.3 - 0.5) * 100;
+              const temperatureCelsius = this.calculateTemperature(reading.value, sensor.resistanceAt25C, sensor.bValue);
               const displayTemperature = this.config.temperatureUnitFahrenheit
                 ? this.convertToFahrenheit(temperatureCelsius)
                 : temperatureCelsius;
@@ -990,6 +990,24 @@ export class OpenSaunaAccessory {
     );
   }
 
+  private calculateTemperature(adcValue: number, resistanceAt25C: number, bValue: number): number {
+    const pullUpResistor = 10000; // 10k ohm pull-up resistor
+
+    // Calculate the resistance of the thermistor based on the ADC value
+    let resistance = (1023 / adcValue) - 1;
+    resistance = pullUpResistor / resistance;
+
+    // Apply the Steinhart-Hart equation
+    let steinhart = resistance / resistanceAt25C; // (R/Ro)
+    steinhart = Math.log(steinhart); // ln(R/Ro)
+    steinhart /= bValue; // 1/B * ln(R/Ro)
+    steinhart += 1.0 / (25 + 273.15); // + (1/To)
+    steinhart = 1.0 / steinhart; // Invert
+    steinhart -= 273.15; // convert to Celsius
+
+    return steinhart;
+  }
+
   // Utility function to convert Celsius to Fahrenheit
   private convertToFahrenheit(celsius: number): number {
     return celsius * 1.8 + 32;

src/settings.ts

@@ -44,4 +44,6 @@ export interface AuxSensorConfig {
   channel: number; // ADC channel number associated with the sensor
   system: 'sauna' | 'steam' | 'controller' | null; // The sensor to system association, or null if not associated
   control: boolean; // Whether the sensor affects control logic (e.g., turns off power if overheating)
+  resistanceAt25C: number; // NTC resistance in Ohms @ 25°C
+  bValue: number; // NTC beta value
 }