John Graham-Cumming needed to log when a light switched on and off during the night as part of debugging an oddly behaving movement sensor. To do that a really simple light sensor logger was built using an Arduino Leonardo, a large LED and a resistor or three.
That light sensor via LED relies on the photovoltaic effect: when light strikes the PN junction inside the LED it creates a voltage (albeit a small one). But there’s another way to measure the light hitting an LED. Ultrasonic Sensor Range Companies
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