The red light shows you when the Electro-Vibrawave technology sends vibrations through your wiring. The green light blinks when the Ultrasonic sound is emitted. The yellow light represents the Ionic feature. The red and green lights will blink intermittently as the ultrasonic and Electro-Vibrawave technologies are used at random intervals to constantly startle the pests and prevent them from getting used to these sounds. The yellow light will be on all the time because the AT is constantly emitting negative ions.