Improvements to photo resistor-based lux estimation

- Code now compiles
- Estimation of the photo resistor resistance works
- Lux estimation from the photo resistor resistance needs more work and calibration
- Lux is now encoded in the BLE advertisement payload

code/b-parasite/config/prst_config.h

@@ -11,13 +11,14 @@
 #define PRST_PHOTO_OUT NRF_GPIO_PIN_MAP(0, 2)
 
 // Deep sleep.
-#define PRST_DEEP_SLEEP_IN_SECONDS 300
+#define PRST_DEEP_SLEEP_IN_SECONDS 2
 
 // Analog to digital converter (ADC).
 // Prints out ADC debug info, such as the values read for battery and soil
 // moisture.
 #define PRST_ADC_BATT_DEBUG 0
 #define PRST_ADC_SOIL_DEBUG 0
+#define PRST_ADC_PHOTO_DEBUG 1
 
 // BLE.
 // Prints out BLE debug info, such as the final encoded advertisement packet.
@@ -27,7 +28,7 @@
 // 1. Two most significant bits are set to 1;
 // 2. The remaining bits should not _all_ be set to 0;
 // 2. The remaining bits should not _all_ be set to 1;
-#define PRST_BLE_MAC_ADDR "f0:ca:f0:ca:00:01"
+#define PRST_BLE_MAC_ADDR "f0:ca:f0:ca:01:01"
 #define PRST_BLE_ADV_NAME "prst"
 // Total time spend advertising.
 #define PRST_BLE_ADV_TIME_IN_MS 1000

code/b-parasite/src/main.c

@@ -63,12 +63,13 @@ static void rtc_callback() {
   prst_pwm_stop();
   nrf_gpio_pin_clear(PRST_FAST_DISCH_PIN);
 
-  nrf_gpio_pin_set(PRST_PHOTO_V);                                               // set GPIO for photoresistor HIGH
+  nrf_gpio_pin_set(PRST_PHOTO_V);
-  prst_adc_photo_sensor_t photo_read = prst_adc_photo_read(batt_read.voltage);  // read PHOTO SENSOR value
+  prst_adc_photo_sensor_t photo_read = prst_adc_photo_read(batt_read.voltage);
-  nrf_gpio_pin_clear(PRST_PHOTO_V);                                             // clear GPIO for photoresistor
+  nrf_gpio_pin_clear(PRST_PHOTO_V);
 
   prst_ble_update_adv_data(batt_read.millivolts, temp_humi.temp_millicelcius,
-                           temp_humi.humidity, soil_read.relative, photo_read.lux, run_counter);
+                           temp_humi.humidity, soil_read.relative,
+                           photo_read.lux, run_counter);
   prst_adv_start();
   nrf_delay_ms(PRST_BLE_ADV_TIME_IN_MS);
   prst_adv_stop();

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

@@ -71,8 +71,8 @@ void prst_adc_init() {
   nrf_saadc_channel_config_t photo_channel_config =
       NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(PRST_ADC_PHOTO_INPUT);
 
-  APP_ERROR_CHECK(
+  APP_ERROR_CHECK(nrf_drv_saadc_channel_init(PRST_ADC_PHOTO_CHANNEL,
-      nrf_drv_saadc_channel_init(PRST_ADC_PHOTO_CHANNEL, &photo_channel_config));
+                                             &photo_channel_config));
 }
 
 prst_adc_batt_read_t prst_adc_batt_read() {
@@ -131,14 +131,28 @@ prst_adc_soil_moisture_t prst_adc_soil_read(double battery_voltage) {
 }
 
 prst_adc_photo_sensor_t prst_adc_photo_read(double battery_voltage) {
-  nrf_saadc_value_t raw_photo_output = sample_adc_channel(PRST_ADC_PHOTO_CHANNEL);
+  nrf_saadc_value_t raw_photo_output =
+      sample_adc_channel(PRST_ADC_PHOTO_CHANNEL);
   prst_adc_photo_sensor_t ret;
   ret.raw = raw_photo_output;
   ret.voltage = (3.6 * raw_photo_output) / (1 << PRST_ADC_RESOLUTION);
-  ret.lux = (uint16_t)ret.voltage*100000/(battery_voltage-ret.voltage);
+  // This value needs to be calibrated.
-#if PRST_ADC_SOIL_DEBUG
+  // The photo resistor forms a voltage divider with a 10 kOhm resistor.
-  NRF_LOG_INFO("[adc] Read lux level: %d (raw); %d (lux)",
+  // The voltage here is measured in the middle of the voltage divider.
-               ret.raw, ret.lux);
+  // Vcc ---- (R_photo) ---|--- (10k) ---- GND
+  //                      Vout
+  // So we can estimate R_photo = R * (Vcc - Vout) / Vout
+  const double photo_resistance =
+      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);
+
+#if PRST_ADC_PHOTO_DEBUG
+  NRF_LOG_INFO("[adc] Read lux level: %d (raw); %d (lux)", ret.raw, ret.lux);
 #endif
   return ret;
 }

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

@@ -33,8 +33,9 @@
 | 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                               |
 */
-#define SERVICE_DATA_LEN 16
+#define SERVICE_DATA_LEN 18
 static uint8_t service_data[SERVICE_DATA_LEN];
 
 // Stores the encoded advertisement data. As per BLE spec, 31 bytes max.
@@ -135,7 +136,8 @@ 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, uint8_t run_counter) {
+                              uint16_t soil_moisture, uint16_t lux,
+                              uint8_t run_counter) {
   // 4 bits for a small wrap-around counter for deduplicating messages on the
   // receiver.
   service_data[1] = run_counter & 0x0f;
@@ -152,6 +154,9 @@ 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;
+
   // Encodes adv_data_ into .gap_adv_data_.
   uint32_t err_code = ble_advdata_encode(
       &adv_data_, gap_adv_data_.adv_data.p_data, &gap_adv_data_.adv_data.len);

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

@@ -13,6 +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, uint8_t run_counter);
+                              uint16_t soil_moisture, uint16_t lux,
+                              uint8_t run_counter);
 
 #endif  // _PRST_BLE_H_