The invention concerns a pressure switch, especially a hood switch, according to the introduction to claim 1.

Stringent requirements are placed on pressure switches which are intended for use as hood switches in, e.g., cars. They have to be functionally reliable and watertight, should not be affected by dust and impurities, in addition to which they must be easy to instal. A second object of the pressure switch according to the invention is that it should be capable of being employed, e.g. in cars for retrofitting of alarms without height adaptation. A further object is to provide a pressure switch which can be placed in doors or window frames when installing house alarms.

The above-mentioned advantages and objects are achieved according to the invention with a switch which is characterized by the features which as disclosed in the claims.

The pressure switch according to the invention will now be described in more detail with reference to the drawing.

Fig. 1 illustrates the pressure switch according to the present invention.

Fig. 2 illustrates how the pressure switch in fig. 1 is installed on a base.

Fig. 3 illustrates the pressure switch in fig. 1 in the contact position and with the contact arm 4 fully open.

Fig. 4 indicates the angular movement of the switch arm in a closed switch.

Fig. 5 illustrates the pressure switch in fig. 1 in an open position and with the switch arm fully down.

Fig. 6 illustrates the cable output from the switch in fig. 1.

Fig. 7 illustrates the installation of the switch arm in the switch housing.

Fig. 8 illustrates the location of a magnet in the switch housing.

Fig. 9 illustrates schematically several pressure switches according to the present invention installed in an alarm circuit.

Fig. 1 illustrates the pressure switch according to the invention with an exploded view of the components. The switch comprises a plastic housing 1 on to the side of which there is moulded a reed relay 2 in the section 8 on the side of the plastic housing and with a watertight cable outlet, thus preventing corrosion damage to the cables and contact legs. Ordinarily this is a major problem when using hood switches in cars due to road salting in winter, which results in the deposition of salt solution in the engine compartment. The switch is mounted on to a base with a self-tapping screw 7 which penetrates the housing 1. In a recess 9 in the switch housing 1 there is provided a permanent magnet 3. It is kept in position, e.g., by a spring snap element which is formed in one piece with the side of the recess 9 and thereby the housing 1. A steel switch arm 4 is mounted on the housing 1 in such a manner that it can cover the section of the housing 1 which does not include the section 8 at the side. The switch arm 4 pivots about a shaft 5 mounted in the housing 1 and is raised to the open position by a stainless steel spring 6 mounted on the shaft 5. In the figure it can be seen that the central section of the switch arm 4 on the side is provided with a downwardly projecting lug or section 10 which, when the switch arm 4 is located in the lowered position and in contact with the switch housing 1, enters a longitudinal slot 1 1 between the recess 9 and the section 8 which contains the reed relay 2. The polar axis of the permanent magnet 3 is substantially parallel to the switch arm's pivotal plane and thereby the section 10. The section 10 shields the reed relay 2 from the magnetic field of the permanent magnet 3 and the contact is broken. The switch arm 4 with the downwardly projecting section 10 is designed in such a manner that a rather small angular downward movement of the switch arm 4 will cause the switch to be opened.

The installation of the pressure switch according to the invention on a base is illustrated in more detail in fig. 2. It is secured by connecting the housing 1 to a self-tapping screw 7 and is quite simply mounted with a screwdriver. Fig. 3 shows the switch according to the invention in the closed position and with the contact arm raised. It will be seen that the downwardly projecting section 10 no longer provides a shield from the magnetic field. In fig. 4 the switch arm 4 has been moved from the position illustrated by a dotted line to the position illustrated by a solid line. The switch is now open, but from the

last position the downwardly projecting section 10 on the switch arm will block the magnetic field and keep the reed relay 2 open. Fig. 5 illustrates the switch according to the invention with the switch arm 4 fully down and resting on the housing 1 in such a manner that the switch is still located in the open position. It will be seen that the height of the switch in this position can be exceptionally low. The pressure switch is open over a great part of the switch arm's 4 angular movement from its contact with the housing 1, and this means that the switch 1 can be mounted even in a location where there is a substantial clearance between, e.g. hood and base. In fig. 6 it can be seen how the wires 2a from the reed relay 2 (not shown) moulded into the switch housing 1 are passed in a watertight fashion out of the switch housing 1. Fig. 7 illustrates as shown in fig. 1 the switch arm 4 removed from the housing 1 and shows the rotary shaft 5 and the spring 6. The spring 6 locks the rotary shaft 5 in the switch housing 1 and is fastened in such a manner that it also raises the switch arm 4. Fig. 8 illustrates the switch housing 1 with the permanent magnet 3 removed. Fig. 8b illustrates how the permanent magnet is inserted from beneath into the recess on the switch housing 1 and is locked there by a snap device 13 which forms an integral part of the housing 1.

Finally fig. 9 shows how the pressure switch according to the invention can form part of an alarm circuit either in a car or in a house alarm. When installing house alarms it is a very simple matter to adapt the switch according to the invention for use in door openings and window frames. A number of switches S can then be mounted in series. In this case the switch according to the invention can be equipped with a small resistor R in parallel over the reed relay. If the alarm cable is now cut at some point between the switches or between the switches and an alarm central monitor, the resistance value detected and set in the central monitor according to the number of switches no longer applies and shows that the alarm circuit has been tampered with.

If, on the other hand, doors or windows are opened, the alarm switch concerned will be closed and a reduction will be obtained in the resistance value detected in the central monitor. Thus it will be seen that the alarm will be activated either when the alarm cable is cut or when windows and doors are opened when the alarm is set. An alarm which employs this version of the pressure switch according to the invention thereby provides a much more reasonable wiring arrangement since all the switches are connected via only

one single cable, i.e. one cable branch outgoing from the central monitor and one ingoing from the central monitor.

Since the pressure switch according to the invention is mounted on the surface of a base, has a very low height when the switch arm 4 is located in abutment with the housing, and permits a large angular movement of the switch arm 4 before the relay is closed, a highly advantageous and flexible installation solution is obtained, especially when used as a hood switch in a car.

In addition to the fact that the sensitive, exposed electrical components are moulded into or inserted in the switch housing 4 in a watertight manner, there is provided on the side of the switch which is attached to the mounting base a drainage opening 12 which effectively drains the recess 9, this being impor¬ tant when, e.g., the switch is mounted as an armoured switch in a car.

Furthermore the pressure switch according to the invention is particularly suited to being mounted close against a steel plate or another material which is influenced by magnetism, without the switch's function being affected. Moreover the pressure switch according to the invention, as it is designed, is sabotage and tamper-proof and in general is particularly well suited to use as an hood switch or alarm switch in cars.