Wednesday, October 28, 2009

Motion Detectors

Even with the long-recording facilities of DVRs and digital systems it is seldom desirable to continuously record. The most common method is to fit a PIR (Passive Infra Red) detector in the area to be monitored. Most people will be familiar with this concept as applied to security lighting.
The output of the PIR can be fed to a Video Remote Control module, hence the video recorder will only turn on when motion is detected. As a basic system it works well and wireless models are available that remove the need for additional cabling. As they are low-voltage devices battery life can be up to three years.

A VMD (video motion detector) is the modern alternative to the PIR. It works by analysing the video signal from the camera. When it detects changes in the signal it recognises this as movement within the camera's picture the output being used to switch on the recording device. The sensitivity can be adjusted which will overcome the problem of bright lights being turned on causing the VMD to activate.
The more sophisticated units feature an on-screen marker or rectangle. Only movement in the rectangle will trigger the output. By resizing and repositioning the rectangles it is possible to allow for pet activity within the observation area. In the example the blue car turning into the drive will activate the recorder but the grey car passing on the road will not. It is difficult to achieve this degree of accuracy with PIRs.

Most digital and PC-based systems have VMD built in to them. This can make multi-camera digital systems very cost effective as it alleviates the need to buy motion detectors for each camera or observation area

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