diff --git a/code/b-parasite/config/prst_config.h b/code/b-parasite/config/prst_config.h
index f0370b8..2fc7349 100644
--- a/code/b-parasite/config/prst_config.h
+++ b/code/b-parasite/config/prst_config.h
@@ -7,13 +7,15 @@
 // the the version you're programming. The version can be found on the
 // b-parasite board.
 // #define PRST_VERSION_1_0_X
-#define PRST_VERSION_1_1_X
+// #define PRST_VERSION_1_1_X
+#define PRST_VERSION_1_2_X
 
 // Built-in LED.
 #define PRST_LED_PIN NRF_GPIO_PIN_MAP(0, 28)
 
 // Deep sleep.
-#define PRST_DEEP_SLEEP_IN_SECONDS 300
+// #define PRST_DEEP_SLEEP_IN_SECONDS 2
+#define PRST_DEEP_SLEEP_IN_SECONDS 2 * 3600
 
 // Analog to digital converter (ADC).
 // Prints out ADC debug info, such as the values read for battery and soil
@@ -29,7 +31,7 @@
 // 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:01:01"
+#define PRST_BLE_MAC_ADDR "f0:ca:f0:ca:00:10"
 #define PRST_BLE_ADV_NAME "prst"
 // Total time spend advertising.
 #define PRST_BLE_ADV_TIME_IN_MS 1000
@@ -65,6 +67,16 @@
 // Whether to produce debug messages for the LDR
 #define PRST_ADC_PHOTO_DEBUG 0
 
+// #endif  // End of version-specific configuration.
+#elif defined(PRST_VERSION_1_2_X)
+
+#define PRST_HAS_PHOTOTRANSISTOR 1
+
+#define PRST_PHOTO_V_PIN NRF_GPIO_PIN_MAP(0, 29)
+#define PRST_PHOTO_OUT_PIN NRF_GPIO_PIN_MAP(0, 2)
+
+#define PRST_ADC_PHOTO_DEBUG 1
+
 #endif  // End of version-specific configuration.
 
 #endif  // _PRST_CONFIG_H_
diff --git a/code/b-parasite/src/main.c b/code/b-parasite/src/main.c
index 5c8f055..99549df 100644
--- a/code/b-parasite/src/main.c
+++ b/code/b-parasite/src/main.c
@@ -26,7 +26,7 @@ static void log_init(void) {
 static void gpio_init(void) {
   nrf_gpio_cfg_output(PRST_LED_PIN);
   nrf_gpio_cfg_output(PRST_FAST_DISCH_PIN);
-#if PRST_HAS_LDR
+#if PRST_HAS_LDR || PRST_HAS_PHOTOTRANSISTOR
   nrf_gpio_cfg_output(PRST_PHOTO_V_PIN);
 #endif
   NRF_LOG_INFO("GPIO pins inited.");
@@ -67,11 +67,16 @@ static void rtc_callback() {
   nrf_gpio_pin_clear(PRST_FAST_DISCH_PIN);
 
   uint16_t lux = 0;
-#if PRST_HAS_LDR
+#if PRST_HAS_LDR || PRST_HAS_PHOTOTRANSISTOR
   nrf_gpio_pin_set(PRST_PHOTO_V_PIN);
-  prst_adc_photo_sensor_t photo_read = prst_adc_photo_read(batt_read.voltage);
+
+  // Just while debugging.
+  while (1) {
+    nrf_delay_ms(1000);
+    prst_adc_photo_sensor_t photo_read = prst_adc_photo_read(batt_read.voltage);
+    lux = photo_read.brightness;
+  }
   nrf_gpio_pin_clear(PRST_PHOTO_V_PIN);
-  lux = photo_read.brightness;
 #endif
 
   prst_ble_update_adv_data(batt_read.millivolts, temp_humi.temp_millicelcius,
diff --git a/code/b-parasite/src/prst/adc.c b/code/b-parasite/src/prst/adc.c
index de57482..c46e09a 100644
--- a/code/b-parasite/src/prst/adc.c
+++ b/code/b-parasite/src/prst/adc.c
@@ -8,9 +8,16 @@
 #include <stdint.h>
 
 #include "prst_config.h"
+#include "sdk_config.h"
 
 // 10 bits resoltuion.
+#if NRFX_SAADC_CONFIG_RESOLUTION == 1
 #define PRST_ADC_RESOLUTION 10
+#elif NRFX_SAADC_CONFIG_RESOLUTION == 2
+#define PRST_ADC_RESOLUTION 12
+#else
+#error "NRFX_SAADC_CONFIG_RESOLUTION does not have a valid value"
+#endif  // NRFX_SAADC_CONFIG_RESOLUTION
 
 #define PRST_ADC_BATT_INPUT NRF_SAADC_INPUT_VDD
 #define PRST_ADC_BATT_CHANNEL 0
@@ -71,6 +78,7 @@ void prst_adc_init() {
 
   nrf_saadc_channel_config_t photo_channel_config =
       NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(PRST_ADC_PHOTO_INPUT);
+  // photo_channel_config.gain = NRF_SAADC_GAIN1_4;
 
   APP_ERROR_CHECK(nrf_drv_saadc_channel_init(PRST_ADC_PHOTO_CHANNEL,
                                              &photo_channel_config));
@@ -142,7 +150,9 @@ prst_adc_photo_sensor_t prst_adc_photo_read(double battery_voltage) {
   prst_adc_photo_sensor_t ret;
   ret.raw = raw_photo_output;
   ret.voltage = (3.6 * raw_photo_output) / (1 << PRST_ADC_RESOLUTION);
+  // ret.voltage = (2.4 * raw_photo_output) / (1 << PRST_ADC_RESOLUTION);
 
+#if PRST_HAS_LDR
   // The photo resistor forms a voltage divider with a 10 kOhm resistor.
   // The voltage here is measured in the middle of the voltage divider.
   // Vcc ---- (R_photo) ---|--- (10k) ---- GND
@@ -161,9 +171,55 @@ prst_adc_photo_sensor_t prst_adc_photo_read(double battery_voltage) {
   ret.brightness =
       MAX(0, MIN(mult_value / powf(photo_resistance, pow_value), UINT16_MAX));
 
+#elif PRST_HAS_PHOTOTRANSISTOR
+  // The ALS-PT19 phototransistor is a device in which the current flow between
+  // its two terminals is controlled by how much light there is in the ambient.
+  // We measure that current by calculating the voltage across a resistor that
+  // is connected in series with the phototransistor.
+  // Some infor:
+  // - Not all lights are the same. The ALS-PT19 has different current
+  // responses for incandescent and fluorescent lights and it shows no values
+  // for sunlight. We have to make some compromises here, aiming for a
+  // "reasonable" middle ground through calibration.
+  // - ALS-PT19' minimum voltage is 2.5 V. Our CR2032 battery has a theoretical
+  // range of 2.0 V to 3.0 V, but in our usage pattern it seems to spend most of
+  // its life around 2.6 V (https://github.com/rbaron/b-parasite/issues/1). In
+  // my manual tests it seems we can usually go lower than that. So while we're
+  // pushing it a bit, we should be okay most of the time.
+  //
+  // In order to design the value of the series resistor, we assume Vcc = 2.5V,
+  // and we want the upper limit of light intensity to correspond to a value of
+  // Vout < 2.5 V - 0.4 V (saturation) = 2.1 V. Let's call this the "direct
+  // sunlight" voltage across our resistor. This direct sunlight voltage will
+  // appear when the phototransistor outputs the direct sunlight current.
+  //
+  // In short, what we want:
+  // A value of R_L such that Vout is < 2.1 V (but close) when the sensor is in
+  // direct sunlight. While Vcc is 2.5 V, R_L = 470 Ohm outputs Vout ~ 1.7V, so
+  // there's still some wiggle room for even more sunnier days.
+  //
+  // Another caveat: the datasheet shows that the current in the transistor is
+  // relatively constant when we vary Vcc (Fig.4). This seems to be true for low
+  // currents (the datsheet uses 100 lx in Fig.4), but not for larger currents.
+  const float phototransistor_resistor = 470.0f;
+  const float current_sun = 3.59e-3f;
+  // Assuming 10000 lux for the saturation test. Calibration with a proper light
+  // meter would be better.
+  const float lux_sun = 10000.0f;
+
+  const float current = ret.voltage / phototransistor_resistor;
+  ret.brightness = MAX(0, MIN(lux_sun * current / current_sun, UINT16_MAX));
+
 #if PRST_ADC_PHOTO_DEBUG
-  NRF_LOG_INFO("[adc] Read brightness level: %d (raw); %d (lux)", ret.raw,
-               ret.brightness);
+  NRF_LOG_INFO("[adc] Phototransistor current: " NRF_LOG_FLOAT_MARKER " uA",
+               NRF_LOG_FLOAT(1000000 * current));
+#endif  // PRST_ADC_PHOTO_DEBUG
+#endif  // PRST_HAS_PHOTOTRANSISTOR
+
+#if PRST_ADC_PHOTO_DEBUG
+  NRF_LOG_INFO("[adc] Read brightness level: " NRF_LOG_FLOAT_MARKER
+               " mV %d (raw); %d (lux)",
+               NRF_LOG_FLOAT(1000 * ret.voltage), ret.raw, ret.brightness);
 #endif
   return ret;
 }
diff --git a/code/b-parasite/src/prst/ble.c b/code/b-parasite/src/prst/ble.c
index 6d74835..c5b9209 100644
--- a/code/b-parasite/src/prst/ble.c
+++ b/code/b-parasite/src/prst/ble.c
@@ -114,7 +114,7 @@ static void init_advertisement_data() {
 
   // Bit 0 of byte 0 specifies whether or not ambient light data exists in the
   // payload.
-#if PRST_HAS_LDR
+#if PRST_HAS_LDR || PRST_HAS_PHOTOTRANSISTOR
   service_data[0] |= 1;
 #endif
 
@@ -171,7 +171,7 @@ void prst_ble_update_adv_data(uint16_t batt_millivolts,
   service_data[8] = soil_moisture >> 8;
   service_data[9] = soil_moisture & 0xff;
 
-#if PRST_HAS_LDR
+#if PRST_HAS_LDR || PRST_HAS_PHOTOTRANSISTOR
   service_data[16] = brightness >> 8;
   service_data[17] = brightness & 0xff;
 #endif