With this simple code, you can transform your Arduino + Ultrasonic sensor + some LEDs in a presence light system. Everytime someone or something pass in front the sensor, the lights will turn on.
Just build your light system (using LEDs) and connect the Arduino Pin #10 as a COMMON CATHODE.
PS: Don't forget to check the maximum current per pin for your arduino board!
The code was originally created to the Arduino NANO, so I had to create a PWM-like function, to make the lights turn on/off smoothly.
Change the line below to monitor the desired distance (in centimeters):
#define distance 120
If you need to see some Debug messages, remove the comment mark // in:
//#define DEBUG