Add basic time-based light sensor code

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);