Updates ambient brightness estimation code

Also renames the `lux` field from `photo_read_t` to the more generic `brightness`.

code/b-parasite/README.md

@@ -39,7 +39,7 @@ Sensor data is encoded in unsigned 16 bits (2 bytes), and whenever multiple
 | 6-7        | Relative air humidity, scaled from 0 (0%) to 0xffff (100%)     |
 | 8-9        | Soil moisture, scaled from from 0 (0%) to 0xffff (100%)        |
 | 10-15      | b-parasite's own MAC address, big-endian format                |
-| 16-17      | Ambient light, scaled from from 0 (dark) to 0xffff (bright)    |
+| 16-17      | Ambient brightness level from 0 (dark) to 0xffff (bright)      |
 
 # Supported Modules
 

code/b-parasite/src/main.c

@@ -69,7 +69,7 @@ static void rtc_callback() {
 
   prst_ble_update_adv_data(batt_read.millivolts, temp_humi.temp_millicelcius,
                            temp_humi.humidity, soil_read.relative,
-                           photo_read.lux, run_counter);
+                           photo_read.brightness, run_counter);
   prst_adv_start();
   nrf_delay_ms(PRST_BLE_ADV_TIME_IN_MS);
   prst_adv_stop();

code/b-parasite/src/prst/adc.c

@@ -146,13 +146,21 @@ prst_adc_photo_sensor_t prst_adc_photo_read(double battery_voltage) {
       1e4 * (battery_voltage - ret.voltage) / ret.voltage;
 
   // TODO: Now that we have the resistor value of the photo resistor, we need to
-  // estimate the value in lux. This needs to be calibrated with a real board in
-  // complete dark and in a super bright environment.
-  // This current value is just a placeholder.
-  ret.lux = (uint16_t)UINT16_MAX * (photo_resistance / 1e4);
+  // estimate the brightness level. This needs to be calibrated with a real
+  // board in complete dark and in a super bright environment. This current
+  // value is just a placeholder.
+  // Dark resistance: 1 MOhm.
+  const double kDarkResistance = 1e6;
+  // Light resistance: 10 kOhm.
+  const double kLightResistance = 1e4;
+  // A value in 0x0 (dark) - 0xffff (light).
+  // A little better, but still not great.
+  ret.brightness = (uint16_t)UINT16_MAX * (kDarkResistance - photo_resistance) /
+                   (kDarkResistance - kLightResistance);
 
 #if PRST_ADC_PHOTO_DEBUG
-  NRF_LOG_INFO("[adc] Read lux level: %d (raw); %d (lux)", ret.raw, ret.lux);
+  NRF_LOG_INFO("[adc] Read brightness level: %d (raw); %d (brightness)",
+               ret.raw, ret.brightness);
 #endif
   return ret;
 }

code/b-parasite/src/prst/adc.h

@@ -21,7 +21,7 @@ typedef struct prst_adc_photo_sensor {
   // A value from 0x0000 (no light) 0xffff (direct sun).
   // Might need calibration.
   double voltage;
-  uint16_t lux;
+  uint16_t brightness;
 } prst_adc_photo_sensor_t;
 
 void prst_adc_init();

code/b-parasite/src/prst/ble.c

@@ -33,7 +33,7 @@
 | 6-7        | Relative air humidity, scaled from 0 (0%) to 0xffff (100%)     |
 | 8-9        | Soil moisture, scaled from from 0 (0%) to 0xffff (100%)        |
 | 10-15      | b-parasite's own MAC address                                   |
-| 16-17      | Lux level from the photoresistor                               |
+| 16-17      | Ambient brightness level from 0 (dark) to 0xffff (bright)      |
 */
 #define SERVICE_DATA_LEN 18
 static uint8_t service_data[SERVICE_DATA_LEN];
@@ -136,7 +136,7 @@ void prst_ble_init() {
 
 void prst_ble_update_adv_data(uint16_t batt_millivolts,
                               uint16_t temp_millicelcius, uint16_t humidity,
-                              uint16_t soil_moisture, uint16_t lux,
+                              uint16_t soil_moisture, uint16_t brightness,
                               uint8_t run_counter) {
   // 4 bits for a small wrap-around counter for deduplicating messages on the
   // receiver.
@@ -154,8 +154,8 @@ void prst_ble_update_adv_data(uint16_t batt_millivolts,
   service_data[8] = soil_moisture >> 8;
   service_data[9] = soil_moisture & 0xff;
 
-  service_data[16] = lux >> 8;
-  service_data[17] = lux & 0xff;
+  service_data[16] = brightness >> 8;
+  service_data[17] = brightness & 0xff;
 
   // Encodes adv_data_ into .gap_adv_data_.
   uint32_t err_code = ble_advdata_encode(

code/b-parasite/src/prst/ble.h

@@ -13,7 +13,7 @@ void prst_adv_stop();
 
 void prst_ble_update_adv_data(uint16_t batt_millivolts,
                               uint16_t temp_millicelcius, uint16_t humidity,
-                              uint16_t soil_moisture, uint16_t lux,
+                              uint16_t soil_moisture, uint16_t brightness,
                               uint8_t run_counter);
 
 #endif  // _PRST_BLE_H_