diff --git a/build-number.txt b/build-number.txt
index 065fd3e..964480f 100644
--- a/build-number.txt
+++ b/build-number.txt
@@ -1 +1 @@
-137
+213
diff --git a/src/light_sensor.c b/src/light_sensor.c
new file mode 100644
index 0000000..2d79442
--- /dev/null
+++ b/src/light_sensor.c
@@ -0,0 +1,119 @@
+#include "light_sensor.h"
+
+volatile unsigned int LightSensor_Measurement;
+volatile bool LightSensor_NewMeasurement = false;
+
+// Rolling average of the brightness measurement
+float LightSensor_AbsoluteBrightness = 0.5f;
+
+// Maximum and minimum encountered so far
+float LightSensor_MinimumBrightness = 1.0f;
+float LightSensor_MaximumBrightness = 0.0f;
+
+float LightSensor_RelativeBrightness;
+
+static void LightSensor_Measure(void)
+{
+    // Use light sensor pin in output mode, drive high
+    GPIOB->MODER = (GPIOB->MODER & ~(0x03 << PIN_LIGHT_SENSOR * 2))
+        | (0x01 << PIN_LIGHT_SENSOR * 2);
+
+    // Disable timer
+    TIM14->SR = 0x0000;
+    TIM14->CR1 = 0x0000;
+    TIM14->CNT = 0;
+
+    // Delay a bit to ensure capacitor is charged
+    for(unsigned int i = 0; i < 100; i++)
+    {
+        __asm__ volatile("nop");
+    }
+
+    // Re-enable timer
+    TIM14->CR1 = TIM_CR1_CEN;
+
+    // Switch to alternate function mode
+    GPIOB->MODER = (GPIOB->MODER & ~(0x03 << PIN_LIGHT_SENSOR * 2))
+        | (0x02 << PIN_LIGHT_SENSOR * 2);
+}
+
+void LightSensor_Init(void)
+{
+    RCC->AHBENR |= RCC_AHBENR_GPIOBEN;
+    RCC->APB1ENR |= RCC_APB1ENR_TIM14EN;
+
+    GPIOB->ODR |= (1 << PIN_LIGHT_SENSOR);
+    GPIOB->AFR[0] &= ~(0x0f << (PIN_LIGHT_SENSOR * 4));
+
+    // Enable input capture for light sensor pin, trigger on falling edge
+    TIM14->CCMR1 = TIM_CCMR1_CC1S_0;
+    TIM14->CCER = TIM_CCER_CC1E | TIM_CCER_CC1P;
+    // Set up TIM14 for an oveflow interrupt at the configured interval
+    TIM14->PSC = 48000 * LIGHTSENSOR_INTERVAL / 65535;
+    TIM14->ARR = 65535;
+    TIM14->DIER = TIM_DIER_UIE;
+    NVIC_EnableIRQ(TIM14_IRQn);
+    
+    LightSensor_Measure();
+}
+
+void LightSensor_Poll(void)
+{
+    if(LightSensor_NewMeasurement)
+    {
+        unsigned int measurement = LightSensor_Measurement;
+        LightSensor_NewMeasurement = false;
+        float brightness = 1.0f - measurement / 65535.0f;
+
+        if(measurement != 65535 && brightness < LightSensor_MinimumBrightness)
+        {
+            LightSensor_MinimumBrightness = brightness;
+        }
+        if(brightness > LightSensor_MaximumBrightness)
+        {
+            LightSensor_MaximumBrightness = brightness;
+        }
+
+        LightSensor_AbsoluteBrightness = LIGHTSENSOR_LAMBDA * LightSensor_AbsoluteBrightness
+            + (1.0f - LIGHTSENSOR_LAMBDA) * brightness;
+
+        // Slowly move maximum and minimum back to 0.0 and 1.0, respectively
+        LightSensor_MaximumBrightness *= LIGHTSENSOR_ALPHA;
+        LightSensor_MinimumBrightness = 1.0f - LightSensor_MinimumBrightness;
+        LightSensor_MinimumBrightness *= LIGHTSENSOR_ALPHA;
+        LightSensor_MinimumBrightness = 1.0f - LightSensor_MinimumBrightness;
+
+        // Scale and saturate to get relative brightness value
+        float range = LightSensor_MaximumBrightness
+            - LightSensor_MinimumBrightness;
+        float low = LightSensor_MinimumBrightness
+            + range * LIGHTSENSOR_LOW_BOUND;
+        float high = LightSensor_MinimumBrightness
+            + range * LIGHTSENSOR_HIGH_BOUND;
+        LightSensor_RelativeBrightness = (LightSensor_AbsoluteBrightness - low)
+            / (high - low);
+        if(LightSensor_RelativeBrightness < 0.0f)
+        {
+            LightSensor_RelativeBrightness = 0.0f;
+        }
+        if(LightSensor_RelativeBrightness > 1.0f)
+        {
+            LightSensor_RelativeBrightness = 1.0f;
+        }
+    }
+}
+
+void TIM14_IRQHandler(void)
+{
+    if(TIM14->SR & TIM_SR_CC1IF)
+    {
+        LightSensor_Measurement = TIM14->CCR1;
+    }
+    else
+    {
+        LightSensor_Measurement = 65535;
+    }
+    LightSensor_NewMeasurement = true;
+
+    LightSensor_Measure();
+}
\ No newline at end of file
diff --git a/src/light_sensor.h b/src/light_sensor.h
new file mode 100644
index 0000000..033b53f
--- /dev/null
+++ b/src/light_sensor.h
@@ -0,0 +1,25 @@
+#pragma once
+
+#include <stdbool.h>
+
+#include "stm32f030x6.h"
+#include "pinning.h"
+
+// ADC polling interval in milliseconds
+#define LIGHTSENSOR_INTERVAL    2000
+
+// 'Forgetting factor' of the rolling brightness average
+#define LIGHTSENSOR_LAMBDA      0.95f
+
+// 'Forgetting factor' for maximum and minimum brightness
+#define LIGHTSENSOR_ALPHA       0.999f
+
+// Bounds for converting absolute to relative brightness: Consider everything
+// near the minimum or maximum 0.0 or 1.0, respectively
+#define LIGHTSENSOR_LOW_BOUND   0.1f
+#define LIGHTSENSOR_HIGH_BOUND  0.9f
+
+extern float LightSensor_RelativeBrightness;
+
+void LightSensor_Init(void);
+void LightSensor_Poll(void);
diff --git a/src/main.c b/src/main.c
index 6a2ef80..5d21560 100644
--- a/src/main.c
+++ b/src/main.c
@@ -8,25 +8,26 @@ int main(void)
         __asm__ volatile("nop");
     }
 
+    LightSensor_Init();
     LED_Init();
 
     int ct = 0;
     uint8_t *data = (uint8_t*)LED_PixelData;
     while(1)
     {
+        LightSensor_Poll();
+        uint8_t brightness = LightSensor_RelativeBrightness * 255;
         if(LED_FrameFlag)
         {
             LED_FrameFlag = false;
-            data[ct]++;
+            data[ct] = brightness;
             LED_Commit();
-            if(data[ct] == 255)
+            
+            ct += 3;
+            if(ct >= LED_COLUMNS * LED_ROWS)
             {
-                ct += 3;
-                if(ct >= LED_COLUMNS * LED_ROWS)
-                {
-                    ct -= LED_COLUMNS * LED_ROWS;
-                    ct += 1;
-                }
+                ct -= LED_COLUMNS * LED_ROWS;
+                ct += 1;
             }
         }
     }
diff --git a/src/main.h b/src/main.h
index d51f46d..3f1bfc6 100644
--- a/src/main.h
+++ b/src/main.h
@@ -3,6 +3,7 @@
 #include "stm32f030x6.h"
 #include "buildid.h"
 #include "led.h"
+#include "light_sensor.h"
 
 int main(void);
 
diff --git a/src/pinning.h b/src/pinning.h
index c10a259..7527e61 100644
--- a/src/pinning.h
+++ b/src/pinning.h
@@ -14,6 +14,9 @@
 #define PIN_LED_G_3             13
 #define PIN_LED_B_3             14
 
+// Port B
+#define PIN_LIGHT_SENSOR        1   // ADC_IN9
+
 // Port F
 #define PIN_ROW_DATA            0   // Shift register data in
 #define PIN_ROW_SCK             1   // Shift register clock