ADC improvement & fix (#1314)

* fix when ADC_DIVIDER is defined
NEW multiple readings option

* publish at least every 1min

* define minimum pub adc interval

main/ZsensorADC.ino

@@ -36,7 +36,7 @@ ADC_MODE(ADC_TOUT);
 
 //Time used to wait for an interval before resending adc value
 unsigned long timeadc = 0;
-
+unsigned long timeadcpub = 0;
 void setupADC() {
   Log.notice(F("ADC_GPIO: %d" CR), ADC_GPIO);
 }
@@ -45,18 +45,29 @@ void MeasureADC() {
   if (millis() > (timeadc + TimeBetweenReadingADC)) { //retrieving value of temperature and humidity of the box from DHT every xUL
 #  if defined(ESP8266)
     yield();
+    analogRead(ADC_GPIO); //the reliability of the readings can be significantly improved by discarding an initial reading and then averaging the results
 #  endif
     timeadc = millis();
     static int persistedadc;
     int val = analogRead(ADC_GPIO);
+    int sum_val = val;
+    if (NumberOfReadingsADC > 1) { // add extra measurement for accuracy
+      for (int i = 1; i < NumberOfReadingsADC; i++) {
+        sum_val += analogRead(ADC_GPIO);
+      }
+      val = sum_val / NumberOfReadingsADC;
+    }
     if (isnan(val)) {
       Log.error(F("Failed to read from ADC !" CR));
     } else {
-      if (val >= persistedadc + ThresholdReadingADC || val <= persistedadc - ThresholdReadingADC) {
+      if (val >= persistedadc + ThresholdReadingADC || val <= persistedadc - ThresholdReadingADC || (MinTimeInSecBetweenPublishingADC > 0 && millis() > (timeadcpub + (MinTimeInSecBetweenPublishingADC * 1000UL)))) {
         Log.trace(F("Creating ADC buffer" CR));
         StaticJsonDocument<JSON_MSG_BUFFER> jsonBuffer;
         JsonObject ADCdata = jsonBuffer.to<JsonObject>();
         ADCdata["adc"] = (int)val;
+        if (NumberOfReadingsADC > 1) {
+          ADCdata["adc_reads"] = NumberOfReadingsADC;
+        }
 #  if defined(ADC_DIVIDER)
         float volt = 0;
 #    if defined(ESP32)
@@ -71,9 +82,14 @@ void MeasureADC() {
 #    endif
         volt *= ADC_DIVIDER;
         // let's give 2 decimal point
-        val = (volt * 100);
-        volt = (float)val / 100.0;
+        int v = (volt * 100);
+        volt = (float)v / 100.0;
         ADCdata["volt"] = (float)volt;
+#  endif
+#  if defined(ADC_CALIBRATED_SCALE_FACTOR) // calibration: scale factor = ref-Vcal-mV / sum-of-cal-readings @see https://skillbank.co.uk/arduino/calibrate.htm
+        float voltage = sum_val * ADC_CALIBRATED_SCALE_FACTOR; // convert to voltage in millivolts
+        ADCdata["adc_sum"] = (int)sum_val;
+        ADCdata["mvolt_scaled"] = (int)voltage; // real scaled/calibrated value in mV
 #  endif
         pub(ADCTOPIC, ADCdata);
         persistedadc = val;

main/config_ADC.h

@@ -40,6 +40,14 @@ extern void ADCtoMQTT();
 #  define ThresholdReadingADC 50 // following the comparison between the previous value and the current one +- the threshold the value will be published or not
 #endif
 
+#if !defined(NumberOfReadingsADC) || (NumberOfReadingsADC < 1)
+#  define NumberOfReadingsADC 1 // number of readings for better accuracy: avg adc = sum of adc / num readings
+#endif
+
+#ifndef MinTimeInSecBetweenPublishingADC
+#  define MinTimeInSecBetweenPublishingADC 0 // pub at least at defined interval - usefull to publish values in case they do not change so much ; 0 = disabled
+#endif
+
 /*-------------------PIN DEFINITIONS----------------------*/
 #if defined(ESP8266) || !defined(ADC_GPIO)
 #  define ADC_GPIO A0 //on nodeMCU this is D3 pin