How long bright flashing LED last on AA cells?

wrote:


Nice. Judging by the combination of visceral response to physical action, and the fact that few people other than the builders of such things can separate the practical requirements for that kind of machine vision, and that needed for recording images, that thing should deter criminals both smart and dumb.
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Lostgallifreyan wrote:

LOL, you might think so. However, around here, all of these fake cameras are the same make and model and crims can easily identify one and all.
One idea is to take some of these fake cameras and retrofit real camera modules. The crims will treat them as fakes and operate quite openly in front of them, rather than try and find somewhere not covered by a camera..
Of course, all you have to do is spread the rumour that this has been done..and then not bother..
etc..
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Sue





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No. It looks completely fake and will drain the batteries quickly with it's prolonged motorised sweeping movement.
Although it uses light level movement sensing it does trigger easily. I got a couple cheap and took 'em to bits. (As I tend to do.)
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Clive Mitchell
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The price of real cameras seems to be falling so much that a very basic "starter" real camera is not all that much more expensive than a dummy.
It's perhaps true that the cheaper sort of real camera look even more fake than the fake cameras do.
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As OP I can say that's not quite it but it is reasonably close in principle to what I had in mind.
In fact, the flashing is not for humans to notice though but for animals. That's why I am vague about the correct flash rate.
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Jaxx wrote:

You might want to consider some CMOS low power internally linked shift registers, to give a pseudo-random variable flash rate.
It puzzles the heck out of my squirrels, when connected to a motorised table. They can manage up to 3 bits of this "Simon" memory game, correctly jumping to the table at the correct time. But not 4 bits.
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Another task begging for a cheap microcontroller..... A PIC 12F629 comes to mind. Eight pin chip and it does the LOT.
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Clive Mitchell
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As OP, I can say there is some truth in that. I want the flash rate to be one that is noticed but I would have to try it out in real life to see which rate worked best for me.
What I really wanted to know was whether an NIMH was going to give me such a short life (say under 4 days) that I would be better using a mains PSU. This is an outdoor application so a battery setup has quite a lot of advantages from portability and wider choice of location.

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Victor Roberts wrote:

Did you read the answers? Which one(s) did you find surprising in view of the fact that the op did not tell us the current requirement?
I saw only 1 answer that would have required knowing the op's current requirement to support the respondent's conclusion. So I wonder what I am missing?
Ed
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On Tue, 04 Jul 2006 02:11:29 GMT, ehsjr

Yes.
All of them - because the OP did not tell us the current.

The answers that didn't require knowing the OP's current requirements were probably correct for the assumptions made by the responder, but not specific to the OP's question.
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Doesn't matter though, there are two ways to go, either demand exact specs from someone who might not have a clear idea yet, or post a response based on your own carefully judged assumption. The latter is usually easier that waiting and second-guessing your response before you've made one, and it gives the OP something to think about. If the result is confusion and an apparent grasping at each offered straw in turn, then it is probably wise to demand some kind of focussed assessment of the original situation. At least this way the OP is not considered stupid until proven intelligent. :)
My own thoughts on the topic are: you can have a 'super-bright' LED and long life, but a few things should be managed to do it. Use a lithium battery preferably, then alkaline, and avoid Nimh types. Use a power converter to efficiently make use of the entire battery charge. Use a narrow viewing angle on the LED, so that it projects strongly for only a limited point of view. Use a clear LED not a coloured plastic. Use a short duty cycle, less than 1% and pulse length no longer than 50 milliseconds even when the flash period is longer than 5 seconds, with that period never being longer than 10 seconds unless you're really going to be staring at that LED for 30 seconds or more regardless of what it might be doing. These are not hard and fast rules. :)
Lastly, consider a low power laser diode, UNLENSED. These are eye safe at distances beyond a few inches, (take care NOT to use a microlensed version though) as they diverge strongly. Their output is highly monochromatic, specular, very eye-catching, and their conversion of electricity to light is second to none, so they can make ideal beacons. They're much harder to use though, easily damaged, and initial power requirements are higher. Also, while you can make an LED stand much higher currents than their nominal rating, often tenfold increase, with a laser diode, the pulse rating for a CW diode is usually less than twofold higher.
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A third and the easiest way to go is ignore dumb questions like "I have an application for some string, what length do I need?".
Dumb questions like this often generate long threads of, frankly, dumb answers.
The OP remains notable by his absence. If the OP can't be bothered to ask properly why do so many feel inclined to answer?
Perhaps it is because letting the reader choose the question allows them to pick one they have an answer for.
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Thank you for your input.
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I was going to thank him for his output. :-)
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Yeah, what "Lost" said.

This sounds interesting, do you have any sources?
Thanks!
[I think the discussion engendered by this particular "How long a piece of string?" query has been useful and enlightening [sic].]
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wrote:

Agreed. :) 'Nospam', we do it for us. It amuses, interests... There are ters questions with terse answers with immensely high SNR for those involved, but they are often so specific that they offer nothing to anyone but those who ask and answer. When a few people get to put in ideas threads get interesting to many at once. Some verge on the inane but I don't care, I take as I find it, I don't have to read them all. :)
William, there's laser diodes from roithner-laser.com, intellite.com, and if you're in the UK, photonic-products.com have some especially nice ones made by Opnext (Hitachi). Sometimes the most cost effective way is eBay, seller milasers (Meredith Instruments) was selling boxes of 100 5 mW 635 nm 5.6 mm packaged diodes. They want around 45 to 70 mA to get full output, but pulsed, these could form excellent small beacons, extremely visible as the wavelength is a lot shorter than most red LED's. Even unlensed, a distance of a few feet is wise if you're going to look into them directly. Lensed to collimate parallel, that should not be done at all. Nice advantage: collimated, you can project the beacon on a high wall or post without the expense of wires. Totally weatherproof too, that way.
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Many thanks!
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Victor Roberts wrote:

IME,YMMV, almost all OPs contain insufficent information to be certain of the requirement.
Now in many cases, where safety and/or the law is involved, all a respondent can do is to ask for more information. AS an example, when someone asks about domestic electrical wiring and how to do something - it cannot be answered without knowing where they are and what regulations apply. So, if uncertain, the only response can be "Where are you?"
With posts where safety and/or law aren't immediately involved, responding to everything with "insufficient information, please clarify" makes sense but is a tad boring - treating the information as clues to a puzzle is more fun..It is never possible to eliminate the "ass" in assumptions - but no harm is done. Putting the clues together correctly can be a bit like doing the Times crossword..
I would imagine that most posters here enjoy solving puzzles and trying to produce the answer that they think the OP would have got, had he given enough information to be certain...
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The OP did not provide enough information. In fact, he asked how long an LED would last - He did not ask how long the cells powering the LED would last.
He did not specify duty cycle. His interpretation of "flashing" could range from 1/2 second on, 1/2 second off (think traffic signal on flash), which would likely average 10 ma (20 ma LED / 50% duty cycle) + power consumption of whatever you are driving it with , down to sub millisecond pulses every 10 seconds, which could result in the cell(s) lasting for their self discharge life.
He did not say what he would be flashing it with. He could be using a relay with a capacitor to provide a time constant, which would likely make for a very short battery life, or, with enough cells in series, and no current limiting for the LED, could answer the question as to how long the LED would last (not very long), or he could be using an LM3903.
He could be talking about something like this http://www.lc-led.com/View/itemNumber/144 Even reading the datasheet for that device does not provide enough information where you could provide an accurate answer to either interpretation of the OP's question.
You could change the wording of the original question to address a different technology, and have similar questions:
Very approximately how long would a lawn mower engine last? The power source would be one (or more but how many?) gas cans each with a capacity of 20 liters?
None of this is meant to put down the original poster. He just did not provide enough info to answer his question without assuming just about everything about it.
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Bob wrote:

Yes but it is fun guess-timating what he had in mind... Incidently, indefinately - I can never start the bloody thing unless I have someone to hold the thing down whilst I pull on the cord with both hands...
--
Sue






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