The series resistor R1 sets the operating current (and hence the brightness level) of IRED1. The infrared receiver IR1 is only sensitive to light modulated with a specific (38kHz) carrier frequency, ...

Here is the Arduino code for the Parallax sensor. ... PIR Pin “+” connects to Arduino +5. one side of the 10K Resistor ... This is a good way to not only test the sensor but to see if it is ...