From 8c87d939ab9e522e79a4bf7e466db7ded28498db Mon Sep 17 00:00:00 2001
From: Mikhail Grushinskiy <mgrouch@users.noreply.github.com>
Date: Sat, 23 Nov 2024 23:51:26 -0500
Subject: [PATCH] Create ResistanceSensor.h

---
 .../ResistanceSensor.h                        | 65 +++++++++++++++++++
 1 file changed, 65 insertions(+)
 create mode 100644 bbn_m5atomS3_lite_resistance_Ohm/ResistanceSensor.h

diff --git a/bbn_m5atomS3_lite_resistance_Ohm/ResistanceSensor.h b/bbn_m5atomS3_lite_resistance_Ohm/ResistanceSensor.h
new file mode 100644
index 0000000..3cd6514
--- /dev/null
+++ b/bbn_m5atomS3_lite_resistance_Ohm/ResistanceSensor.h
@@ -0,0 +1,65 @@
+#ifndef ResistanceSensor_h
+#define ResistanceSensor_h
+
+// readVoltage is used to improve the linearity of the ESP32 ADC
+// see: https://github.com/G6EJD/ESP32-ADC-Accuracy-Improvement-function
+double readVoltage_util(byte pin) {
+  double reading = analogRead(pin); // Reference voltage is 3v3 so maximum reading is 3v3 = 4095 in range 0 to 4095
+  if (reading < 1 || reading > 4095) return 0;
+  // return -0.000000000009824 * pow(reading,3) + 0.000000016557283 * pow(reading,2) + 0.000854596860691 * reading + 0.065440348345433;
+  return (-0.000000000000016 * pow(reading, 4) + 0.000000000118171 * pow(reading, 3) - 0.000000301211691 * pow(reading, 2) + 0.001109019271794 * reading + 0.034143524634089) * 1000;
+} // Added an improved polynomial, use either, comment out as required
+
+double readVoltageAvg(byte pin, int samples) {
+  double sum = 0.0;
+  for (int i = 0; i < samples; i++) {
+    sum += readVoltage_util(pin);
+  }
+  return sum / samples;
+}
+
+enum ResistanceConfiguration {
+  UPSTREAM,
+  DOWNSTREAM,
+};
+
+struct ResistanceSensor {
+  int adc_voltage_pin = G8;
+  ResistanceConfiguration voltage_divider_type = DOWNSTREAM;
+  double reference_voltage = 3.3;     // in volts
+  double reference_resistance = 100;  // in Ohms
+};
+
+void ResistanceSensor_init(ResistanceSensor* rs, int pin, ResistanceConfiguration conf, 
+    float reference_voltage, float reference_resistance) {
+  rs->adc_voltage_pin = pin;
+  rs->voltage_divider_type = conf;
+  rs->reference_voltage = reference_voltage;
+  rs->reference_resistance = reference_resistance;
+  pinMode(rs->adc_voltage_pin, INPUT);
+  analogSetPinAttenuation(rs->adc_voltage_pin, ADC_11db); // for range 0-2.6v
+}
+
+void ResistanceSensor_read(ResistanceSensor* rs) {
+  double rs->volt = readVoltageAvg(rs->adc_voltage_pin, 16) * rs->reference_voltage / 4095;
+  double rs->measured_resistance = 0.0;
+  switch (rs->voltage_divider_type) {
+    case UPSTREAM:
+      if (rs->volt == 0.0f) {
+        rs->measured_resistance = NAN;
+      } else {
+        rs->measured_resistance = (rs->reference_voltage - rs->volt) / rs->volt;
+      }
+      break;
+    case DOWNSTREAM:
+      if (rs->volt == rs->reference_voltage) {
+        rs->measured_resistance = NAN;
+      } else {
+        rs->measured_resistance = rs->volt / (rs->reference_voltage - rs->volt);
+      }
+      break;
+  }
+  rs->measured_resistance *= rs->reference_resistance;
+}
+
+#endif