Various forms of simple motion sensors are known but previously known forms of motion sensors cannot detect both sudden shock movements and gentle motion movements.
Sudden shocks can be detected by means of a sensor comprising a piezo-electric crystal which is arranged to be hit by a freely suspended weight when a sudden movement occurs. The resulting electrical output from the piezo-electric crystal is used to detect the shock but such a detector may not react to gentle movements. Another form has a weight at the end of a spring arm or spring. An example of such a sensor is shown in U.S. Pat. No. 3,649,787. Sudden movements will cause the spring to bend because of the inertia of the weight and this causes closing of an electrical switch for a short while which is detected electrically. Such a motion sensor may not be effective in detecting gentle movements however, since the spring has to have reasonable stiffness to support the weight and restore it to its centre rest position and the stiffer the spring the less sensitive the device is to gentle movements.
Another somewhat similar device is shown in U.S. Pat. No. 4,418,337. Here a small magnet is mounted on a spring and the magnet is positioned within an electrical coil. Movement of the magnet is detected by monitoring the emf generated in the coil by the movement of the magnet. This device however is designed to detect lack of movement rather than movement. Thus it is designed to be carried by say a fireman and, should he be overcome by fumes or the like and rendered unconscious, then the magnet must quickly be damped, in other words the spring must be quite stiff, so that the lack of a generated emf in the coil can be detected and the alarm given. The teaching of this Patent is therefore quite contrary to the detection of slight and gentle movements.
An example of a motion detector which will detect gentle movements but not always a sudden shock comprises an electrically conductive ball rolling over a printed circuit board, movement of the ball causing it to form a conducting path between different conductive areas on the printed circuit board which can then be detected electrically. Alternatively, a mercury switch where a drop of mercury can move within a glass bulb can be used, motion causing the drop to move to a position where it completes a conductive path between two electrical contacts. Sudden shock may however fail to be detected since the conductive path may well fail to be completed.
All these types of motion sensor therefore will only be effective in detecting either sudden shock motion or gentle continuous motion. It is therefore an object of the present invention to provide a simple motion sensor which is capable of detecting any sort of motion, whether it be a continuous, gentle motion or whether it be a sudden shock movement.