FairyPocket Voltage Sensor Tester Pencil Safe No Contact Test Detect Pen, Sound Light Alert Alarm Non-contact, need not to contact steel parts of electric circuit. So it is very safe. It can sensor volt and alarm with in 1 to 20 cm decide by voltage. Across the cable wrap. Very sensitive, It can alarm even close to network cable and USB cable. Require 2 AAA batteries, Not include Sound and LED light alert When it sensed voltage.