Is it possible to run a red LED with a 555, say 20% duty cycle, but two or three times the current, to get a brighter light? Ive googled "LED PWM" but only come up with PWM drivers, and for now, Id like to stick with a 555 if possible.

 

You can but you don't really gain anything unless you're using it to transmit data.

The response of the human eye is like a low pass filter, it takes the time average of the light level.

An LED running at five times the current at 20% duty cycle will look no brighter than an LED that's always turned on running at the normal curent.

If you're using it for flashes then you're still better turning the LED on for longer and using a lower current than doing brief pulses of very bright light.

Transmitting data is a totally different ball game as the receive has a much faster response than your eye, allowing you to use less than half the power to get the same distance.

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Your eye responds to peak brightness, you could overdrive the LED but it's still not going to want to exceed it average current rating. So 100ma through a 20ma LED with a 1/5 duty might work it may simply appear to be the same brightness.

Why not just use ultra bright LEDs?

Interesting question though.

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Bill
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Hmm, interesting. Thanks for the timely answers!

 

I have an LED flashing light that I purchased more than 10 years ago. I recall doing some research and found that this particular device supplies an initial peak voltage that is somewhat higher than could be applied continuously - the intent being to have a moment of peak brightness to improve attention-grabbing capability. I do not know if someone actually did experiments to prove that it was worth doing. I am recalling that the old LM3909 might do something similar.

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stevez

 

Pulse-width-modulation dims LEDs because your vision sees brightness from the average current in an LED, not the peak current unless it is present for a fairly long time.

Cheap "ultra-bright" LEDs are just a dim one in a tightly focussed plastic case. You can't see them unless they point directly at you.
Modern bright LEDs have a wide angle.

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Uncle $crooge

 

Hmm I thought peak brightness was how the eye responds.

http://www.onsemi.com/pub_link/Collateral/AND8067-D.PDF

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Bill
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Hi Bill,
I disagree with ON Semi who quote Siemens about how our vision works.
1) Multiplexed displays are very dim unless the LED current is increased a lot so that the average current is equal to DC continuous current.
2) The LEDs driven by an LM3915 are very dim with music unless a peak detector keeps each LED turned on for at least about 30ms.
3) LED dimmers use PWM. The narrower are the pulses then the dimmer they are.

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Uncle $crooge

 

I don't know, have any tests been conducted on this?

The problem is the eye has a logarithmic response to light so this needs to be taken into account.

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Shine a brightly blinking light in somebody's eyes and see if the iris closes to reduce the brightness. I think the iris responds to the average brightness, not the peak.

My Ultra-bright LED Chaser projects use PWM for dimming, the peak current of the pulses remains the same and they dim perfectly.

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Uncle $crooge

 

Yes and No. Say you were sleeping deeply and somebody turns on the light and wakes you up. In that case you might get blinded by the apparent bright light. The eyes however adapt to light by closing the retina and the result is seing average light. (just comfortable enough for the eye.)

The same thing happens driving from daylight into a tunnel. You'll see NOTHING for a few split seconds.

The time it takes to see "normal" is called adaption time and it is very individual. Observe yourself driving a car at night. Opposite traffic shouldn't bother a healthy and fast adapting eye. Many people however try to avoid night driving for that reason.

Btw, Thais should never drive at night. They're night blind 99.9%! They do and creep like snails at an average speed of not more than 25km/h.

Regards

Hans

 

I don't like seeing the new extremely bright LED traffic lights at night. A light turns red so I put on the brakes and nearly stop then notice that the extremely bright red traffic light is miles away!

I think they should be dimmed at night.

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Uncle $crooge

 

Found an interesting article light and the eye.

The eye is fast. The entire sequence, from photon absorption to nerve output, is completed within 50 microseconds.

Visual response to light is analog.
Rhodopsins strongly absorb light, which gives them a characteristic dark, opaque color called visual purple. The photoisomerization causes a bleaching from purple to transparent yellow, which allows light to pass deeper into the outer segment and strike photopigment molecules below.

Given the speed and analog nature of the eye itself one would think that the eye is capable of peak detection.

However the true organ of vision is not the eye but the brain. The eye is but a sensor used by the brain to produce vision. The question then is does the brain do average or peak detection? Both?

If the brain does peak detection alone how is POV accomplished? It seems we do not know that either.

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