Alarmed Vehicle Hoods Use Reed Sensors REED RELAYS REED SENSORS REED SWITCHES Introduction The most costly items of an automobile are under the hood of the car. Thieves, once the hood of the car is open, can unfasten the engine, battery, etc. very quickly. Having a sensor that is tied into the on-board computer could sound an alarm or the car horn. That is exactly what auto designers have done, using MEDER’s reed sensors that sense the movement of the hood sending a signal to the on-board computer. This in turn initiates the alarm. Figure 1. MK14 Sensor physical layout Features • The reed sensors reliably operate between 50˚C to 150˚C • Magnet and Reed Sensor are isolated and have no physical contact by typically having the magnet mounted on the hood and the Reed Sensor mounted and positioned to accurately pick up the motion of the hood • The reed switch used in the Reed Sensor is hermetically sealed and is therefore not sensitive to rough, wet, dirty environments Figure 2. Reed Sensor is mounted inside engine compartment and magnet is mounted to the underside of the hood. A spring loaded ferromagnetic shunt is positioned between the sensor and magnet to shunt magnetic field. • The magnet is not affected by its environment • Tens of millions of reliable operations • Cylindrical mounting and screw fastening mounting • Contacts dynamically tested Applications • Ideal for sensing the motion of a car hood when an auto is sitting idle and not in use • Ideal for applications sensing any kind of motion in a host of different configurations Figure 3. When the hood is opened the magnetic shunt springs out of position causing the magnetic field to actuated the sensor and signal the onboard computer to alarm. Reed Sensors accomplish the task of initiating an alarm once the hood is moved when locked All of us who own autos have on many occasions left our vehicles unattended in areas that were unprotected from the potential of theft. Thieves have become expert in opening the hood of an auto even when in the locked position. They are also adept at taking under-the-hood prizes and very quickly. The only way to deter this action is to have the hood armed with an alarm system. MEDER’s reed sensors have provided a very secure reliable approach to initiate the alarm action. MEDER electronic Application MEDER electronic AG | Germany | tel.: +49(0)7731/8399-0 | Email: [email protected] MEDER electronic Inc. | USA | tel.: +1 508.295.0771 | Email: [email protected] Page 1 of 2 www.meder.com Alarmed Vehicle Hoods Use Reed Sensors REED RELAYS REED SENSORS REED SWITCHES Specifications Operate specs Min Max Units Must close distance 5 25 mm Must open distance 5 25 mm Hysteresis Load Characteristics Switching voltage Switching current Carry current Contact rating Static Contact resistance Dynamic contact resistance Breakdown voltage Operate time Release time Operate Temp Storage Temp Typical 50% Min Max Units 200 0.5 1.5 10 150 200 320 0.5 0.1 -20 85 -35 85 Volts Amps Amps Watts mΩ mΩ Volts msec msec °C °C Reed sensors generally use a magnet that is brought into the vicinity of a reed sensor activating the reed sensor. The reverse can also occur, the magnet is moved away from the vicinity of the reed sensor which will also initiate an action. In this application, MEDER has taken a different approach. Working closely with the auto designers MEDER engineers came up with a very reliable and very effective approach. The magnet is mounted to the hood as is typical, but there is a spring loaded thin ferromagnetic rectangular piece of metal that is placed in between the magnet and the reed sensor. This metal piece acts as a magnetic shunt. The magnetic field of the magnet is literally shunted from the reed sensor. The reed sensor essentially does not experience any magnetic field in presence of the shunt. When the hood is opened even slightly, the spring loaded metal shunt springs away from it’s position between the magnet and the reed sensor. Now the full strength of the magnet is incident on the reed sensor closing the contacts. The contact closure sends a signal to the on-board computer, which in turn, turns on the car alarm, beeps the horn and/or flashes the high beam lights. Generally, one and/or all three are generally enough to scare the most ardent thief away. The reed sensor is an excellent choice because it can operate reliably from 50˚C to 150˚C and represents an economical way to carry out the sensing function. Because MEDER’s sensors use hermetically sealed reed switches that are further packaged in strong high strength plastic, they can be subject to rough treatment and environmental concerns such as dirt, grease, and moisture without any loss of reliability. MEDER has cylinder packages and well as screw fastening packages having lead wires for remote attachment to the electronics. Cylindrical Panel Mount Series Series MK03 MK03 Dimensions mm Dia 5.25 L 25.5 Illustration inches 0.207 1.004 MK14 MK14 Dia L 4 25.5 0.157 1.004 MK18 MK18 Dia L 5 17 0.197 0.669 MK20/1 MK20/1 Dia L 2.72 10 0.107 0.394 Rectangular Screw Flange Mount Series Series MK04 MK05 MK05 Dimensions mm W 13.9 H 5.9 L 23.0 W 19.6 H 6.1 L 23.2 Illustration inches inches 0.547 0.232 0.906 0.772 0.240 0.913 W 14.9 0.587 H 6.9 0.272 L 32.0 1.260 **Consult our factory for your specific design requirements. MK12 MK12 MEDER electronic Application MEDER electronic AG | Germany | tel.: +49(0)7731/8399-0 | Email: [email protected] MEDER electronic Inc. | USA | tel.: +1 508.295.0771 | Email: [email protected] Page 2 of 2 www.meder.com