Implement ADC correction for the soil sensor for a range of input voltages

code/parasite/lib/parasite/parasite/adc.cpp

@@ -23,6 +23,37 @@ constexpr double kBattDividerR1 = 1470;
 constexpr double kBattDividerR2 = 470;
 constexpr double kBattDividerFactor =
     (kBattDividerR1 + kBattDividerR2) / kBattDividerR2;
+
+constexpr double kDiodeDrop = 0.6;
+
+// We need the sensor to behave well as the battery discharge. I plan on
+// using a CR2032 coin cell, whose voltage ranges from roughly 3.0 (charged)
+// to 2.0V (discharged). This is the input range for Vdd, so all of our
+// analog-to-digital (ADC) measurements are somewhat relative to this varying
+// voltage. The soil sensor itself is at heart an RC circuit: it charges to
+// 0.63Vcc within a time constant (RC). More importantly, within a cycle, the
+// final charge is proportional to Vcc. If we sample this with our ADC, and use
+// a sampling mode that is relative to Vcc (AR_VDD4 here), the final voltage
+// should "cancel out", keeping the sampled value constant as we vary Vcc. In
+// other words, in theory, the raw value returned from the ADC should remain
+// constant as I keep the sensor in the same state and vary the input voltage.
+// In practice, I'm seeing a small drift in the raw sampled values. This could
+// be due to some capacitance or lower transistor response times in the fast
+// discharge circuit.
+// I captured some (Vcc, raw adc sample) pairs and did a linear regression to
+// correct for these values when the sensor is out in the air and in the water.
+// This should mitigate the drifting issue enough.
+// In reality, I don't think leaving this correction out would be a big deal. I
+// expect the battery to discharge over a period of a few months, and I'd expect
+// plants to be watered with the period of days/couple of weeks. This drift
+// wouldn't be noticeable in that time scale, but since we can do better, why
+// not?
+// Some assumptions for these hardcoded values:
+// - Sampling resolution of 10 bits!
+// - Input (vdd) is in the range 2.0 - 3.0V
+double GetRawValAir(double vdd) { return vdd * 94.29 + 308.95; }
+double GetRawValWater(double vdd) { return vdd * 36.02 - 57.1; }
+
 }  // namespace
 
 double BatteryMonitor::Read() {
@@ -33,15 +64,22 @@ double BatteryMonitor::Read() {
   return kBattDividerFactor * v_in;
 }
 
-soil_reading_t SoilMonitor::Read() {
+soil_reading_t SoilMonitor::Read(double vdd) {
   analogOversampling(kOversampling);
   // Set up the analog reference to be VDD. This allows us to cancel out
   // the effect of the battery discharge across time, since the RC circuit
   // also depends linearly on VDD.
   analogReference(AR_VDD4);
   int raw = analogRead(pin_);
-  double percentage =
+  double v_out_corr = (vdd * raw) / 1023 + 0.6;
-      static_cast<double>(raw - air_val_) / (water_val_ - air_val_);
+  int raw_corr = v_out_corr / vdd * 1023;
+
+  double air = GetRawValAir(vdd);
+  double water = GetRawValWater(vdd);
+  double percentage = static_cast<double>(raw - air) / (water - air);
+  // Serial.printf(
+  //     "vdd: %f, raw: %d, raw_corr: %d, v_out_corr: %f, percentage: %f\n", vdd,
+  //     raw, raw_corr, v_out_corr, 100 * percentage);
   return {raw, std::max(0.0, std::min(1.0, percentage))};
 }
 

code/parasite/lib/parasite/parasite/adc.h

@@ -19,15 +19,11 @@ struct soil_reading_t {
 
 class SoilMonitor {
  public:
-  explicit SoilMonitor(int air_val, int water_val, int pin)
+  explicit SoilMonitor(int pin) : pin_(pin) {}
-      : air_val_(air_val), water_val_(water_val), pin_(pin) {}
 
-  soil_reading_t Read();
+  soil_reading_t Read(double vdd);
 
  private:
-  // Referencce values for estimating the soil moisture percentage.
-  const int air_val_, water_val_;
-
   // Pin the analog signal is connected to.
   const int pin_;
 };

code/parasite/src/main.cpp

@@ -15,8 +15,6 @@ constexpr int kSoilAnalogPin = 21;       // P0.31, AIN7
 constexpr int kBattAnalogPin = 15;       // P0.05, AIN3
 constexpr int kDischargeEnablePin = 16;  // P0.30;
 constexpr double kPWMFrequency = 500000;
-constexpr int kSoilMonitorAirVal = 680;
-constexpr int kSoilMonitorWaterVal = 60;
 
 // parasite::SquareWaveGenerator square_wave_generator(kPWMFrequency, kPWMPin);
 
@@ -24,8 +22,7 @@ const parasite::MACAddr kMACAddr = {0x01, 0x02, 0x03, 0x04, 0x05, 0x06};
 parasite::BLEAdvertiser advertiser(kMACAddr);
 
 parasite::BatteryMonitor batt_monitor(kBattAnalogPin);
-parasite::SoilMonitor soil_monitor(kSoilMonitorAirVal, kSoilMonitorWaterVal,
+parasite::SoilMonitor soil_monitor(kSoilAnalogPin);
-                                   kSoilAnalogPin);
 
 SoftwareTimer timer;
 
@@ -54,25 +51,24 @@ void updateAdvertisingData(parasite::BLEAdvertiser* advertiser,
 void timer_cb(TimerHandle_t timer_handle) {
   parasite::SquareWaveGenerator square_wave_generator(kPWMFrequency, kPWMPin);
 
-  digitalToggle(kLED1Pin);
+  // digitalToggle(kLED1Pin);
-
-  square_wave_generator.Start();
-  digitalWrite(kDischargeEnablePin, HIGH);
-
-  parasite::soil_reading_t soil_reading = soil_monitor.Read();
-  // Serial.printf("Moisture val: %d, %f%%\n", soil_reading.raw,
-  //               100 * soil_reading.parcent);
-
-  square_wave_generator.Stop();
-  digitalWrite(kDischargeEnablePin, LOW);
 
   double battery_voltage = batt_monitor.Read();
-  // Serial.printf("Batt voltage: %f\n", battery_voltage);
+  Serial.printf("Batt voltage: %f\n", battery_voltage);
+
+  digitalWrite(kDischargeEnablePin, HIGH);
+  square_wave_generator.Start();
+  delay(10);
+  parasite::soil_reading_t soil_reading = soil_monitor.Read(battery_voltage);
+  Serial.printf("Moisture val: %d, %f%%\n", soil_reading.raw,
+                100 * soil_reading.parcent);
+  square_wave_generator.Stop();
+  digitalWrite(kDischargeEnablePin, LOW);
 
   updateAdvertisingData(&advertiser, soil_reading, battery_voltage);
 
   // Keep adversiting for 1 second.
-  delay(1000);
+  delay(500);
 
   advertiser.Stop();
 
@@ -81,10 +77,14 @@ void timer_cb(TimerHandle_t timer_handle) {
 
 void setup() {
   Serial.begin(9600);
-  pinMode(kLED1Pin, OUTPUT);
+  // pinMode(kLED1Pin, OUTPUT);
   pinMode(kDischargeEnablePin, OUTPUT);
 
-  timer.begin(2000, timer_cb, /*timerID=*/nullptr, /*repeating=*/true);
+  // digitalWrite(kDischargeEnablePin, HIGH);
+  // parasite::SquareWaveGenerator square_wave_generator(kPWMFrequency,
+  // kPWMPin); square_wave_generator.Start();
+
+  timer.begin(1000, timer_cb, /*timerID=*/nullptr, /*repeating=*/true);
   timer.start();
 
   // Suspend the loop task. Under the hood this is a freeRTOS task set up

resources.md

@@ -140,9 +140,13 @@ Vcc = 2.0 => Moisture Air: 547; Hand: 23
 
 Possible issues:
 * Fast discharging circuit is not fully open with lower voltage?
+* Weird capacitance stored in the transitor messes up the quick switching?
+* Time it takes to overcome the 0.6V diode drop in the output circuit?
 
 Possible fixes:
-* Hack a software interpolation
+* Measure and  correct it via a software interpolation
+
+For now I'm implementing the software fix that seems to work well in practice. The measurements and regression coefficients are in [this spreadsheet](https://docs.google.com/spreadsheets/d/1F6rtkCX7626tzZff0NANZVknawb_U4uS38zjyy6ZPEE/edit#gid=1442651809).
 
 # Battery
 * [CR2032 datasheet](https://data.energizer.com/pdfs/cr2032.pdf)