Adafruit Photo Transistor Light Sensor

This Photo Transistor Light Sensor is a simple sensor that detects ambient light. It functions as the opposite of an LED; when light strikes the chip inside, it causes current to flow from the long pin to the short pin.

The sensor includes a built-in optical filter, likely an IR filter, to mimic light levels as perceived by the human eye. Connecting the long pin to a 3-15V DC power supply and the short pin through a 1K-10K series resistor to ground allows proper operation. When in the dark, minimal current flows, maintaining an analog voltage near ground. Exposure to light increases current through the resistor, elevating the voltage.

The series resistor can be adjusted to achieve the desired voltage range, which can be measured using a microcontroller. These sensors can effectively replace CdS photocells while adhering to RoHS compliance. If readings are not detected, try reversing the sensor orientation.