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Streetlamp switched on by a Bird
Even a Pigeon can operate street lighting, so it seems! Never mind if it's daytime, On with the lamp!
Streetlamps are designed to come on at night when it's dark outside. However, the design of public lighting to fulfil this purpose is tricky. Of course the efficiency of the lightbulbs is of great importance, as any saving makes a big difference when multiplied by the number of streetlamps. For this reason, streetlamps aren't like household bulbs which come on at the flick of a switch. The bulbs are even more economical than an economy lightbulb. Also, the design of the lampshade is important, so the light is cast upon the street and not up into the sky causing light pollution. But also, the timing of the coming on of the light is a matter of design. Some streetlights have internal clocks, but this is not a perfect solution. The clocks can go wrong and leave the lamp timing skewed, and also in temperate climes the onset of night is at a variable time according to the time of year.
So, the designers of streetlamps had a brilliant idea to have the lights come on when it gets dark. Now that's great, because at any time of year the timing is right. With the long nights of winter and the short nights of summer, the streetlights manage to be on during the hours of dark and off during the day. Also, the lights might even come on during a total solar eclipse!
Although many people may be aware of their own electricity usage, because of having an econometer, and choosing their electric company to offer the best tariff, most people are unaware of the efficiency of public streetlighting and how that saves public money and the environmental impact etc.
Systems are not foolproof, though, as can be seen in these amusing pictures. Here, a perfectly functional streetlamp has come on in the daytime. The reason for this can be seen in the pictures, as the lamp has atop it a bird. The pigeon involved may be unaware that it can control streetlighting, but there it is. A bird, perched on top of the lighting sensor by which the streetlamp determines whether it's night or day, has caused the light to start up. As well as being amusing to see a bird perform such a trick as to fool a streetlamp, it's also informative, as we can now see how it is that streetlamps manage to turn on at night and off at daybreak. The bird has revealed how the trick is done. It's done by having a light sensor on top of the streetlamp. It's a streetlamp with a built-in light sensor. Very clever!
The situation with streetlights is much better than it was many years ago when it was not uncommon to see a lamp on in the daytime (and presumably off at night) because its timer had been skewed. These days we tend to take for-granted that streetlights get it right almost all the time.
Incidentally, if you're wondering what happened when the bird flew off, the lamp remained on for about 30 seconds and then suddenly turned off. Notice how there is a delay. This also implies that at night, flashing lights, lightning, flying objects, and other illumination which is fleeting, won't cause the streetlight to stop. Another clever feature.
It may not matter that birds can switch on streetlamps, but if you wanted to design a perfect streetlamp, there are a few things that could be done:
It would be possible to add a set of flexible plastic bird-deterrent spikes, possibly four around the sensor. Another possibility would be to have the sensor on a slope so it couldn't be perched-on quite so comfortably by a bird. If the sensor was at a slope, it would need to be adjustable at install time so that it could point in a compass direction (West for example). Otherwise, different streets would have the lights go on/off at different times because of the orientation of the lamps in relation to dawn and dusk.
There may even be a way of having individual streetlight control by a combination of light sensor and clock. For example, it could be that light could synchronise the timing and that clock time could be advanced to have a virtual day and night based on solar time (ie changing length of day and night according to season). This sort of thing would require either a set of gears to emulate the earth's axis, or it would require a digital model of the changing day and night, which is astronomically well documented.
However, it's hard to improve on the basic light and dark influenced control of streetlights. Night and day aren't the controlling influence; dark and light are.
If you're looking for a light sensor, they come in various types: LDR (light dependent resistor) which is slow changing, and the photodiode which is fast changing. These are available from electronic catalogues such as Maplin , Farnell , and CPC
Update: There's another picture, taken on another day, in which two birds are perched on the streetlamp. One of them is on top of the light sensor, and so the streetlamp has come on in the daytime.
A cute pair of pigeons, but Why Do You Never See Baby Pigeons?