Obstacle sensor, based on TCRT5000, uses infrared diode and photo-receiver to detect obstacle in front of it - emitted light just reflects back to the sensor. Onboard, you'll find voltage comparator, ...

I redrawn the circuitry and found that the 4.7K resistor could be positioned like this: I don't understand why this resistor is there and can I replace two 10K and 4.7k resistors with one 3.3K ...