movement detector - optical guard

The special feature of this movement detector is that it is a passive device: it does not use a transmitter like a light barrier. The fundamental principle of its operation is very similar to that of the human eye. movement detector elektor november 1983 movement detector The sensor consists of two photo diodes which have the same task as the rods in the retina of the human eye. These diodes are mounted close together in a light-proof box and a lens in the box projects an image of the space being watched onto the diodes (see figure 1). When the brightness of the environment changes, for instance, through increasing clouds, twilight or the like, the light falling onto both the diodes is reduced equally. If something moves within reach of the lens (and it may only be the budgerigar), the light onto the diodes is reduced in dis- similar proportion. The circuit is extremely sensitive to very small differences in light intensity. In view of this characteristic, the device is particular...