The GREEN light on the front of the base near the bottom indicates that an audio signal is present. This light will go on and off depending on whether an audio signal is present. NOTE: The base can still charge the battery in the headset even if an audio signal is not present and hence this light is off.
The crucial light to indicate that the battery is charging is the light directly below the Sennheiser name as this light indicates that charging is taking place. If the headset is not in the charging cradle this light will remain unlit. If the headset is in the charging cradle and the battery is charging then the light will be RED. Once the battery is charged the light will turn GREEN.
NOTE: The headset of the Set 840 S and Set 840 TV look different but the concept is the same.