This configuration (using a logic gate with TTL compatible output) uses an amplifier transistor to obtain greater sensitivity. The capacitor acts as a filter, preventing the action of spurious signals that may appear when the incident light on the sensor is cut. The 15 kΩ resistor at the base of the transistor determines the sensitivity and can eventually be changed in the range of 4k7 to 47-k ohm, depending on the application. The circuit is shown in the figure and any common phototransistor can be used as a sensor.
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