MAGNETIC: The alarms work thanks to the magnetism between the alarm transmitter and the magnetic receiver. The 100 dB siren is triggered when an equipped door or window is opened (trigger distance: more than 10 mm). EASY: The alarm sensor and the magnetic receiver are equipped with adhesive tape. No tools are required, simply stick them on a door frame. TECHNICAL DETAILS: Transmitter dimensions: 9 x 3 x 2 cm; magnetic receiver: 5 x 1 x 1.5 cm Batteries: 2x AAA 1.5V per alarm (not included) Volume: 100dB MANY USES: Increase the security of your front door, terrace or balcony, your bedroom or the children's room - the set of 4 alarms and receivers gives you the choice. NOTE: Please make sure that your apartment or house has the right conditions for setting up the security system, without accidental triggering.