Finalize water level meter code

hardware/arduino/water_level/water_level.ino

@@ -1,5 +1,6 @@
 // need 0.1uf cap between vcc and ground for proper operation
 // need 10k pull up resistor on scl and sda for i2c to work correctly
+// need VCC hooked up to VREF pin 5
 
 // Pin Mappings (Per TinyCore)
 // ATTiny Pin  -- Arduino Pin
@@ -18,21 +19,86 @@
 
 #define I2C_ADDRESS 0x5F
 
+// eTape sensor info / values for calculations
+// 5-inch: 400-1000 ±20%
+//         Ref Resistance 1000 ±20%
+//         Active Length  4.9" (213 mm)
+// 8-inch: 400-1500 ±20%
+//         Ref Resistance 1500 ±20%
+//         Active Length  8.4" (213mm)
+// 12-inch: 400-2000 ±20%
+//         Ref Resistance 2000 ±20%
+//         Active Length  12.4" (315mm)
+// 24-inch: 400-4000 ±20%
+//         Ref Resistance 4000 ±20%
+//         Active Length  24.34" (618mm)
+// 32-inch: 400-5000 ±20%
+//         Ref Resistance 5000 ±20%
+//         Active Length  32.4" (823mm)
+
+#define SENSOR_PIN A3 // Pin 2 on physical package
+#define SENSOR_REF_RESISTANCE 1000.0
+#define SENSOR_VALUE_MIN 400.0
+#define SENSOR_VALUE_MAX 1000.0
+#define SENSOR_ACTIVE_LENGTH 4.9
+#define Vin 3.3
+
+// Average/Smoothing
+#define NUM_SAMPLES 15
+int samples[NUM_SAMPLES];
+
 #include <Wire.h>
 
-void setup() {
-  pinMode(6, INPUT);
-  
-  Wire.begin(I2C_ADDRESS);
-  Wire.onRequest(requestEvent);
-}
-
-void loop() {
-  delay(100);
+// Map a value from one range of values to another -- support decimals
+float longmap(float x, float in_min, float in_max, float out_min, float out_max) {
+  return out_min + ((1.0 * (x - in_min) * (out_max - out_min)) / (1.0 * (in_max - in_min)));
 }
 
 // function that executes whenever data is requested by master
 // this function is registered as an event, see setup()
 void requestEvent() {
-  Wire.write((uint8_t) analogRead(6)); // Only get 0 - 1023 back so move this into a 1 byte int
+  // Sensor readings averaged out
+  float average = 0;
+  for (uint8_t i = 0; i < NUM_SAMPLES; i++) {
+    samples[i] = analogRead(SENSOR_PIN);
+    delay(10);
+  }
+  for (uint8_t i = 0; i < NUM_SAMPLES; i++) {
+    average += samples[i];
+  }
+  average /= NUM_SAMPLES;
+
+  // convert the value to resistance
+  float buffer = average * Vin;
+  float Vout = (buffer) / 1024.0;
+  buffer = (Vin / Vout) - 1;
+  float R2 = SENSOR_REF_RESISTANCE * buffer;
+
+  // Map resistance to water level
+  float height = longmap(R2, SENSOR_VALUE_MAX, SENSOR_VALUE_MIN, 0, SENSOR_ACTIVE_LENGTH);
+
+  // Write the value
+  char data[4];
+  dtostrf(height, 1, 2, data);
+  Wire.write(data);
+}
+
+void setup(void) {
+  analogReference(DEFAULT); // DEFAULT == Vcc
+  pinMode(SENSOR_PIN, INPUT);
+
+  // From arduino docs (https://www.arduino.cc/reference/en/language/functions/analog-io/analogreference/)
+  // After changing the analog reference, the first few readings from analogRead() may not be accurate.
+  // Fix this situation during setup prior to sending any values out (this chip may be on/off frequently so this is a real concern over time and needs to happen ahead of i2c sensor reads)
+  delay(100);
+  for (int i = 0; i < 6; i++) {
+    analogRead(SENSOR_PIN);
+  }
+
+  Wire.begin(I2C_ADDRESS);
+  Wire.onRequest(requestEvent);
+}
+
+void loop(void) {
+  delay(100);
 }