# LED Flash light ..Pulsed LED, underdamped oscillations

Discussion in 'General Electronics Chat' started by williB, May 24, 2005.

1. ### audioguruWell-Known MemberMost Helpful Member

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2.2F should power a very bright LED for about 1 minute, at which time it will be very dim. :lol:

2. ### williBNew Member

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i think that i can get 16 minutes. of light out of it..here is my reasoning..
Jouls/Jolues per second(Watts) = time.
33J/0.064W=515 sec.
11V-3V(diode)=8V across resistor or constant current source , whatever..
8 * 0.008 A = 0.064 W
515 sec /60 = 8.5 minutes (constant current)..
now with a varible PWM at say 100HZ starting out blinking on for 1/256 of the time.( i am going to hav to see how many intervals i can break the PWM into)
should at least double the light ON time...
therefore 16 minutes of light..

3. ### audioguruWell-Known MemberMost Helpful Member

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You can't get something for nothing.
A bank of series-parallel-connected caps of 1.1F/11V will power the LED continuously at only 8mA for about 18 minutes using a constant current source.

With PWM reducing the average current in the LED to only 8mA/256, it will be so dim that you will see it only in absolute darkness, and it won't shine on anything enough to light it.
I use PWM to dim my chaser circuits, and without having an on-off switch for the circuit, the LEDs are considered to be off with a tiny average current about the same as your calculation.

I am sorry to say that you will have an extremely dim LED "lit" for a very long time. :cry:

If you find an LED that can withstand current pulses of 2A, PWM will make it brighter and be lit longer.

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5. ### williBNew Member

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its the blinking that saves power because your eyes cant tell if a stationary LED is running constant current OR is blinking at a fsat rate..
its that simple.

6. ### audioguruWell-Known MemberMost Helpful Member

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Your vision isn't a peak detector. It responds to average brightness when a light is blinking faster than about 50Hz.
Try it. :lol:

7. ### williBNew Member

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so you agree that your eyes cant tell the difference between a blinking LED and one that is being driven by constant current.??
i'll take that as a yes..
so say that the LED is ON for only half the time (conservitivly), all other things being equal , it should last twice as long..
16 minutes !!

8. ### audioguruWell-Known MemberMost Helpful Member

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Then it will look like it is being powered continuously with only 3 or 4mA.
I thought that you want a flashlight, not just an indicator light. :cry:

9. ### williBNew Member

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i guess that you dont agree that your eyes cant tell the difference between a blinking LED and one that is being driven by constant current.??

10. ### williBNew Member

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and that one driven by a pulse (of lesser average current) can look as bright as one being driven by constant current..

11. ### Nigel GoodwinSuper ModeratorMost Helpful Member

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As you say, it's that simple - BUT, you are drawing the completely wrong conclusion.

If you pulse an LED at 50/50, it will only be as bright as being ON all the time at 50% of the current, that's what PWM is used for!. Likewise, pulsing it ON for only 10% of the time will only be 10% as bright - you never get something for nothing!.

Where it DOES come in handy is in IR remote control, by pulsing the LED at very high currents for brief periods you extend the range and reduce battery consumption - it's usual to drive the LED at ten times it's maximum rated current!.

12. ### williBNew Member

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i may be wrong , but i dont think thats true, because your eyes have a persistance factor , like a scope screen , the darker it is outside (or inside) the more persistance and a blinking LED will appear to be on all the time , when it is actually blinking..

13. ### Nigel GoodwinSuper ModeratorMost Helpful Member

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Try it and see - bear in mind it's EXACTLY how a light dimmer works in your house!.

14. ### williBNew Member

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i hope to try it out , soon..
as soon as i can get a handle on PWM , and my caps get here..
i am currently going over the Enhanced PWM mode section 11.3 of the 16F684 manual..

15. ### Nigel GoodwinSuper ModeratorMost Helpful Member

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Check my PIC PWM tutorial, it gives code for two channel PWM on the 16F876/7 - but it applies to pretty well any PIC PWM, it's simple to change for only one PWM channel.

16. ### audioguruWell-Known MemberMost Helpful Member

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Persistence of your vision is complicated by the AGC effect of the iris. But persistence occurs for fairly long pulses of light, maybe about 20ms or 1/50th of a second and longer.
I flash the LEDs in my Chasers and Plants Watering Watcher-2 projects for 15ms for them to appear very bright, and about 15us for them to appear very dim. If I move the LED or my eyes when the LED is modulated with about a 50-50 duty-cycle at 3kHz, I see a pattern of lighted dashes across my vision.

17. ### williBNew Member

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Ok what is this doing out here??

:roll: :roll: :roll:

18. ### eblc1388Active Member

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I have taken a look at the Plant Watering Watcher-2 project mentioned above which flashes a LED when the plant needs watering.

It completely solved the nagging question of whether to water a plant when it shows sign of wilting, or have already wilted. As recommended by one user:

"It even flashes through wilted leaves that might hang in front(of the LED)!" :lol:

I'm sorry, just can't help it.

19. ### audioguruWell-Known MemberMost Helpful Member

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Hi L. Chung,
My wifey gets mad at me for letting her plants wilt a bit. I don't water them when I should because I like seeing the LEDs flash brightly. I have a few and wait for their flashing to sync. Some still have their original two AAA alkaline battery cells and have been flashing for about 18 months.

I can't help it if all my projects are useless! :lol:

20. ### eblc1388Active Member

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This is not what I meant in the post. Reads it over and over again. :lol:

OK. Try this. "This tester measures the forward voltage across a diode. It even shows you zero volt for a shorted diode."

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