One of the two LEDs on the P-03 alerts you to the presence of voltage on a wire pair. The other LED alerts you to current flow in a wire pair when the Line Driver switch is operated. The Line Driver switch applies a voltage to the wire pair under test to turn on voltage activated microphones. The A/B switch reverses the polarity of the voltage for detecting polarity sensitive devices. You can also monitor the wire pair through the TD-62 amplifier and speaker to determine if room audio is present on the pair. Any of the above conditions can indicate a microphone attached to the wire pair under test.