Unlike analog IR sensors (such as the TCRT5000-based modules), the FC-51 includes an onboard comparator circuit (typically an LM393 chip) that converts the analog signal from the phototransistor into a clean digital output. This makes it extremely easy to interface with microcontrollers like Arduino, Raspberry Pi, ESP32, or STM32.
A multi-turn potentiometer (blue trim pot) is used to increase (clockwise) or decrease (counter-clockwise) the sensitivity and detection distance. Operational Notes Fc 51 Ir Sensor Datasheet