diff --git a/main.c b/main.c
index a9bc986..785217a 100644
--- a/main.c
+++ b/main.c
@@ -52,6 +52,7 @@
  */
 volatile bool button_flag = false; /**< flag set when board user button has been pressed/released */
 volatile bool time_flag = false; /**< flag set when time changed */
+volatile bool photoresistor_flag = false; /**< flag set when ambient luminosity is measured */
 /** @} */
 
 #define TICKS_PER_SECOND 256 /**< the number of ticks in one second */
@@ -96,6 +97,10 @@ char command[32] = {0};
 uint8_t command_i = 0;
 /** gamma correction lookup table (common for all colors) */
 uint8_t gamma_correction_lut[256] = {0};
+/** photo-resistor measurement of ambient luminosity */
+volatile uint16_t photoresistor_value = 0;
+/** factor to dim LED of the clock, depending on the ambient luminosity */
+float clock_brithtness = 0;
 
 int _write(int file, char *ptr, int len)
 {
@@ -225,6 +230,11 @@ static void clock_show_time(uint32_t time)
 	clock_leds[led_second*3+0] = 0xff-brightness_second;
 	//clock_leds[led_second*3+1] = 0; // clear other colors (minutes/hours indication)
 	//clock_leds[led_second*3+2] = 0; // clear other colors (minutes/hours indication)
+
+	// adjust brightness
+	for (uint16_t i=0; i<LENGTH(clock_leds); i++) {
+		clock_leds[i] *= clock_brithtness;
+	}
 }
 
 /** @brief set the LEDs
@@ -417,6 +427,12 @@ int main(void)
 	}
 	printf("battery voltage: %.2fV\n", battery_voltage);
 
+	// now use interrupts to only measure ambient luminosity
+	channels[0] = PHOTORESISTOR_ADC_CHANNEL; // only measure ambient luminosity
+	adc_set_regular_sequence(ADC1, 1, channels); // set now group
+	adc_enable_eoc_interrupt(ADC1); // enable interrupt for end of convertion
+	nvic_enable_irq(NVIC_ADC1_2_IRQ); // enable ADC interrupts
+
 	// get date and time
 	printf("current time: %02lu:%02lu:%02lu\n", rtc_get_counter_val()/ticks_hour, (rtc_get_counter_val()%ticks_hour)/ticks_minute, (rtc_get_counter_val()%ticks_minute)/ticks_second); // display time
 	clock_animate_time(rtc_get_counter_val()); // set time with animation
@@ -464,6 +480,9 @@ int main(void)
 		while (time_flag) { // time passed
 			time_flag = false; // reset flag
 			action = true; // action has been performed
+			if ((rtc_get_counter_val()%(ticks_second/10))==0) { // one tenth of a second passed
+				adc_start_conversion_regular(ADC1); // start measuring ambient luminosity
+			}
 			if ((rtc_get_counter_val()%ticks_second)==0) { // one second passed
 				led_toggle(); // LED activity to show we are not stuck
 			}
@@ -477,6 +496,20 @@ int main(void)
 				rtc_set_counter_val(rtc_get_counter_val()%ticks_midday); // reset time counter
             }
 			clock_set_time(rtc_get_counter_val()); // set time
+			//adc_start_conversion_regular(ADC1); // start measuring ambient luminosity
+		}
+		while (photoresistor_flag) { // new photo-resistor value has been measured
+			photoresistor_flag = false; // reset flag
+			action = true; // action has been performed
+			float photoresistor_voltage = photoresistor_value*1.2/ref_value; // calculate voltage from value
+			if (photoresistor_voltage<1.7) { // high ambient luminosity
+				clock_brithtness = 1; // set highest brightness
+			} else if (photoresistor_voltage>2.1) { // low ambient luminosity
+				clock_brithtness = 0.3; // set low brightness
+			} else { // intermediate ambient luminosity
+				clock_brithtness = 0.3+(0.7*(1-(photoresistor_voltage-1.7)/0.4)); // set variable brightness
+			}
+			//printf("%f, %f\n", photoresistor_voltage, clock_brithtness);
 		}
 		if (action) { // go to sleep if nothing had to be done, else recheck for activity
 			action = false;
@@ -489,6 +522,7 @@ int main(void)
 }
 
 #if defined(BUTTON_ISR) && defined(BUTTON_EXTI)
+/** @brief interrupt service routine called when button is pressed of released */
 void BUTTON_ISR(void)
 {
 	exti_reset_request(BUTTON_EXTI); // reset interrupt
@@ -496,8 +530,16 @@ void BUTTON_ISR(void)
 }
 #endif
 
+/** @brief interrupt service routine called when tick passed on RTC */
 void rtc_isr(void)
 {
 	rtc_clear_flag(RTC_SEC); // clear flag
 	time_flag = true; // notify to show new time
 }
+
+/** @brief interrupt service routine called when ADC conversion completed */
+void adc1_2_isr(void)
+{
+	photoresistor_value = adc_read_regular(ADC1); // read measured photo-resistor value (clears interrupt flag)
+	photoresistor_flag = true; // notify new ambient luminosity has been measured
+}