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The sensor shouldn't "see" any other light source, so we need to take measure for this. A tube around thesensor protects it from any incoming light from other angles.
Quite another problem: the sending light source should be camouflaged in the layout. An overhanging lantern post, aligned to send it's light towards thesensor, is a good solution.
These measures still aren't sufficient, however. Other light sources, e.g. on the ceiling, should be filtered, and that's done with some electronic synchronisation filtering.
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The sender is build first. It is, in fact, a modulated power generator that will provide the leds in the lantern posts with short spikes of light. This schematic is therefore useable to control light in general. The small circuit is build with one IC and a few extra components.
The generator at the left
hand side is made and amplified with schmitt-triggers. The exit "A" gives the synchronisation signal for the receiver. A power transistor gives the nescessary "juice" for lots of leds.
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