Naked Science Forum
Non Life Sciences => Technology => Topic started by: chris on 18/12/2008 04:09:29
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Does a flashing bike light consume less power than a light in which the LEDs are continuously lit?
Chris
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How long is a piece of string?
You presumably imply "for the same visibility at a given distance".
Because the eye perceives rapid flashes better than a continuous light, you will need less power (average) for a given level of visibility. (So yes!)
I don't think the overheads / losses in the electronics of the lamp circuit would make any significant difference to the power consumed for either system.
BUT this only applies when there are not many kamikaze cyclists on the road after dark (or equivalent flashing sources). Eventually, the scene could be so confusing that the power needed would be the same.
There is still some doubt about the legality of these lights n'est pas?
But when has that bothered cyclists?
(Don't get defensive about that statement - I, of course, don't mean anyone with enough sense to be reading this forum but cyclists and one way streets, traffic lights and pavements are frequent sources of annoyance / risk to every other road user.)
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I was parked on Gt Portland St when I noticed a cyclist emerging from Riding House Lane, he was going the wrong way on a one way street. A motorist turning right from Gt Portland St the right way into Riding House Lane collided with the cyclist and knocked him off his bike. It was entirely the cyclists fault. The motorist instantly stopped and got out his car. Helping the cyclist to his feet he seemed to be asking him if he was OK. When the cyclist had recovered his composure and indicated to the motorist that he was alright, the motorist decked him!
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There are dummy alarm kits: an empty alarm box with a flashing LED powered by a battery to deter thieves.
The life of the battery in these devices is inversely proportional to the flash frequency of the LED.
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Flashing lights are more eye-catching than ones which are constantly on, (a changing stimulus),
so the flashing may not be just to conserve battery power.
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If the light has the same instantaneous brightness as a permanently-on lamp, but only 50% duty-cycle (flash) then it will take 50% (or fractionally over for the electronics overhead) of the power.
But you might choose to run the lamp at higher instantaneous power while it's flashing, and maintain the same battery life you would have had with a less-bright steady lamp.
i.e. "How long is a piece of string?"
Cycle lamps of different models and manufacturers vary enormously in their useful light output and their battery life. If would be naive in the extreme to take two lights off the shelf and say this one will last longer because it flashes.
My understanding was that the present law (probably reluctantly) allows flashing cycle lamps only when used in addition to steady ones.
Some people have hypothesised that the brighter your (steady) rear light, the wider the clearance cars will give you when overtaking at night...
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A flashing LED light will consume less power than one that is constantly lit - there is relatively little overhead in changing the state of the LED. Bicycle lights are not intended to illuminate anything and the legislation that requires a continuous light on a bicycle achieves nothing. The only purpose of lights on a bicycle is to be seen by motor vehicles, to reduce the chances of them driving in to the cyclists. In this respect, a flashing light is not only more economical but also much more effective - they actually make the motorist think (this is not a general slur upon motorists but the fact is that most people drive on auto-pilot)
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You'd better not do any slurring on me, sunshine!
Autopilot? Speak for yourself
Damn - I just missed my turning.
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...Bicycle lights are not intended to illuminate anything and the legislation that requires a continuous light on a bicycle achieves nothing. The only purpose of lights on a bicycle is to be seen by motor vehicles, to reduce the chances of them driving in to the cyclists. In this respect, a flashing light is not only more economical but also much more effective - they actually make the motorist think (this is not a general slur upon motorists but the fact is that most people drive on auto-pilot)
In lit urban areas maybe, but in my experience, on an unlit road, as a motorist it can be harder to judge the exact location of a flashing light than a steady one.
A bright light coupled with good reflectors is good.
Modern urban environments have a lot of visual clutter which may in turn drive the need for a cyclist to have flashing lights for visual contrast.
Which reminds me, I need to buy a better, brighter rear light for my bicycle. Why do they mostly make rear LED lights run on AAA batteries when you'd get twice the energy for the same price from AAs?
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In lit urban areas maybe, but in my experience, on an unlit road, as a motorist it can be harder to judge the exact location of a flashing light than a steady one
If you're uncertain of something should you not slow down as the uncertainty obviously represents a risk?
In practice, sadly no. Many drivers resent having to slow down and the vast majority assume that where there are uncertain factors things will work out ok. I think this is illustrated by the fact that hardly anyone drives at a safe distance from the car in front because of the assumption that nothing will go wrong. Well, accidents occur when something does go wrong and although I don't have the statistics I suspect that something is going wrong somewhere every few minutes or so. It's then just down to luck as to whether you're involved.
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Same power but less energy.
A 2000 Watt kettle is much more powerful than a 100 Watt bulb but the later may use much more energy if it on 8 hours a day compared to the kettle being on for only a few minutes a day.
8 x 100 Watt = 800 Watt-hours. 0.125 (total of 7.5 mins) x 2000 = 250 Watt-hours
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"Which reminds me, I need to buy a better, brighter rear light for my bicycle. Why do they mostly make rear LED lights run on AAA batteries when you'd get twice the energy for the same price from AAs?"
I guess for the same reason they use things like aluminium and titanium rather than steel. On the whole cyclists prefer lightweigh stuff.
It's been a while since I did any cycling but I can assure everyone that if it's a dark night and you are going home along a canal towpath, having a front light that actually stays on and shows you where you are going is a good thing.
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I bet this ensemble was created by a cyclist who had heard the line "I didn't see you mate" once to often
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I bet this ensemble was created by a cyclist who had heard the line "I didn't see you mate" once to often
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http://en.wikipedia.org/wiki/File:Lighted_bicycle_jacket.jpg
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