Visibility-duty cycle relation with LEDs

I want to make a 1W red LED blink at ~1 Hz. Providing the best possible visibility from a distance is vital and takes precedence over consideration of power consumption. Nevertheless, I'd like to reduce the average power as much as possible.

Is there a generally accepted optimum figure for the duty cycle and/or pulse duration to that end? Adaptability of the human senses being what it is, I don't want to rely solely on my own observations with a test setup.

Reply to
Pimpom
Loading thread data ...

The only thing I can suggest is a buck switcher for a current regulator and maybe a CMOS version of the 555 timer with a driver tranny to switch the LED in at 1p/s///

btw, The part number is like ICM7555, and a LMC555 I think it is. The quiescent current is around 60uA, so that shouldn't be too much of a drain on you.

Jamie

Reply to
Maynard A. Philbrook Jr.

Wild guess: pulse it for 50 ms, at 1 Hz, with as much current as your power budget allows. Check that the LED is still reasonably linear at that current.

But your own eyeball testing is important.

Is red necessary? Other colors would be more efficient.

--
John Larkin         Highland Technology, Inc 

jlarkin att highlandtechnology dott com 
 Click to see the full signature
Reply to
John Larkin

Greetings John, When learning to program microcontrollers the first program was the typical blinking LED. I meesed around with the timing and as I recall

50 ms on time was easy for me to see. I fiddled around with the timing to see just how short a pulse of light I could detect reliablly and 50 ms was a lot longer than I needed. I'm sure that the pulse duration will need to be much longer if the LED isn't very bright when viewed from a distance, but when looking at a standard LED from no more than 3 feet your 50 ms time was more than adequate. Eric
Reply to
etpm

A flash of some given time*current product will look about the same below (very) roughly 50 ms. So 10 mA at 50 ms will look about like 100 mA at 5 ms. The limit (1 amp for 500 us, 10 amps for 50 us) is that the LED efficiency will start to drop off somewhere at high currents. The LED data sheet should have the graphs you need.

--
John Larkin         Highland Technology, Inc 

jlarkin att highlandtechnology dott com 
 Click to see the full signature
Reply to
John Larkin

LED datasheets indicate that linear scaling of current with reduced duty cycle is not permissible. For example, while 1W LEDs are typically rated for a maximum continuous current of 350 mA, the max peak current for 100usec at 1kHz is 500-700 mA. None of the datasheets I've looked at give ratings for pulse lengths in the tens of milliseconds.

Moreover, circumstances force me to use generic LEDs. So I want to be conservative with respect to peak current and limit it to

400 mA or thereabouts. Given that and the repetition rate of about 1 Hz, what I have to determine is pulse length versus visibility over the greatest possible distance. Instinct tells me that a high duty cycle like 0.5 will not have any significant advantage and will just be a waste of power.
Reply to
Pimpom

for that, 500ms is a good starting point, although you may have to reduce the current a bit.

2 to 4 Hz at 50% duty cycle may be more visible,
--
umop apisdn 


--- news://freenews.netfront.net/ - complaints: news@netfront.net ---
Reply to
Jasen Betts

It works up the the point that the current gets high enough that efficiency starts to drop off. For basically all LEDs, you can pulse them and increase visibility. In a recent thread on s.e.d., we discussed pulsing white LEDs as a warning indicator, with only microamps of current available.

For example, while 1W LEDs

For a given budget of average current, low duty cycle flashing does make an LED more visible.

Try it.

--
John Larkin         Highland Technology, Inc 

jlarkin att highlandtechnology dott com 
 Click to see the full signature
Reply to
John Larkin

Then, try to arrange a good, black, background: a hood over the lamp, and a red filter, will help. One can also use a circular polarizer for a window (light from outside shines in, becomes right-circular-polarized, reflects, becomes left-circular-polarized, and can't get out).

There's tens of percents available with electronic tricks, but there's orders of magnitude available with stray-light management.

Reply to
whit3rd

I think your bst bet is to use multiple LEDs and not push them so hard.

I do see where you were misunderstood, you want to flash at one Hertz.

If you want 50 % on and 50 % off, that does not men you can go to double th e current. Your illustration proves that. What is wrong with tewo LEDs ? Mu ch easier for the bang for the buck. Putting them in series should work ju st fine.

One thing though, put Zeners or something across them if you want reliablit y. If one opens up, the other will still ligh but at a lower level and ale rt you to the fact that you need to replace one.

Of course the option of just using one means if it goes out you got nothing . But putting them in parallel is even worse when it comes to drive.

You oculd use resistors across them. They go what like two volts ? So at fo ur volts choose it so it is not a significant load, but is enough to light the other LED in case of failure. Just enough. Then you take your bagfull o f generics and stick one in.

When you push LEDs hard they do not last forever. And I thinkk that is wha t you were asking. "If I only pulse this LED, how much harder can I pump it ?". Well maybe not...

I would go with the lower power thart it can handle continuous and blink it OFF. If you really want huans to see it and now it is not a natural phenom enon or whatever, blink it off three times every other second, and two or f our times on the opposite seconds. Very unikely to occur in nature. Therefo re unnatural enough to catch the attention.

So what is htis all about ? pople kep hitting your mailbox out front ? I go t a guy fixs that. He builds to a brick mailbox type of thing. A regualr ma ilbox can be put on it or custom. but they would brign a bunch of construti on debris and fill the walls, ;up to where the mailbox goes. They put light s on some of them, but the drunks owuld still hit them.

But NOW with that brick and mortar at least the f***ed up their cars ! mwah ahaha.

Anyway, just curious as to the application here. AND, are you trying to get this thing seen during the day ? If so, well it gets pretty bright out the re so you are fighting alot. In the US, here anyway, there are traffic ligh ts that are damn hard to see, and what latitude is Ohio ?

Over in design there is a guy named Jim Thompson who ight know more about t his. He designed a bunch of boards that display on those signs I guess outs ide stadiums and shit, and maybe when there is road work. I'm sure you've s een them if in the US, or probably a bunch of other countries.

Also, in the US, buses and trucks are now (not all) equippd with LED brae l ights with "enuncitaors". Every time the driver hits the brake pedal, when the array of LEDs goed on, it purposely flickers, I think twice. that is de signed to attract attention. Nothing in nature goes on and off like :

I think the DOT requires that on new big trucks and shit. Not sure, but if they don't, the only reason to put them in is a break from the insurance co mpanies and when they speak, people listen.

So maybe :

Something like that.

In other words, you can only get so much attention out of brighness. soetim es it is the timing.

Reply to
jurb6006

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.