Flicker free LED flasher

[newInRobotics]

I should aim to have LED flashing (PWM) at 500Hz to make sure it is flicker free. (thanks to Soeren and corrado33)
----------------------------------------------------------------------------------------------------------------------------------------------

Ordinary TV provides 24 to 25 FPS and human eye cannot detect any flickering, does it mean that LED flashing at 25Hz would be flicker free as well? Or are these different things?

[corrado33]

The frame rate may be 25 frames per second, but the Tv flashes at 60 or 120 Hz.  Sometimes it could display the same frame twice or "merge" two adjacent frames.

And, just passing along the info I got in my thread a while ago, if you want it to be definitely flicker free, aim for 500 Hz.

May I ask what you're trying to do?  LED dimmer?  When you say LED "flasher" it makes me think you're just trying to make an LED turn on and off and some .... noticeable... frequency. 

[newInRobotics]

I'm thinking of a small projects that should help me to get into uC programming. I was thinking something not too difficult, however useful at the same time. I came up with an idea to make myself an LED torch based on ATtiny13V; torch would be dimmed or brightened using 2 mini push buttons to adjust PWM duty cycle.

I'm interested in flashing that is flicker free for obvious reasons, however I do not want to overkill it with frequencies that are way too high in order to make torch as efficient as it's possible.

I recon that 60Hz would be flicker free, however, I'll have to run some tests to find out that minimum after which it becomes visible.

[Soeren]

Hi,

Ordinary TV provides 24 to 25 FPS and human eye cannot detect any flickering, does it mean that LED flashing at 25Hz would be flicker free as well? Or are these different things?

Ordinary TV used to be CRT's, meaning they had a phosphor coating that retained light for a short time - The higher frequencies of the flat screen TV is not just to produce a better picture, but more a necessity to NOT produce a worse picture - just look at a horizontal panning and you'll see why I still prefer a good old CRT.

Flashing an LED at 25Hz will make it appear constant as long as viewed from the same angle, ie. without moving either LED or your eyes, but if you eg. turn your head, it will appear like a string of dots.
White LEDs may be different (haven't tried), as most of them rely on UV from a blue LED die exiting a yellow phosphor, so they have some lag (not much though).

Eyes are different and even the same pair of eyes will be different as well - slow reacting when you just wake up, then faster and as you grow tired, they slow down again.
Age makes your eyes slower as well and the peak speed at any person will be around 16..22 years of age, around midday and with no illness and in a medium lit scene with no extreme contrasts.

As regular LEDs are very fast switching (some as low as ~15ns), they have a very well defined light/darkness contrast, which makes the flickering more noticeable.

You might get good results with 100..200Hz, but if it isn't a problem, my advice is to go for 500Hz, as you're then certain that you won't see flicker (unless you shoot the flashing LED from a gun as a tracer or something similar - you can actually buy GOLF balls with LED flashers).

Like corrado33, I am a bit curious about the purpose of a "flicker free flasher", as it sounds a bit strange, unless you're modulating the ON pulses of the flashing.

[newInRobotics]

Thanks for replies guys.

[corrado33]

So you're going to use PWM to dim the LED? 

I'd say it's a great starter project.  Easy to implement.  You just have to learn PWM, and that's not that bad.

I've dimmed LEDs for a project I did a while ago.  It was a cool learning experience, and is a great way to learn PWM without having to mess with motors and h-bridges or oscilloscopes or servos etc.  And if you break the LED, so what, they're cheap. 

unless you're modulating the ON pulses of the flashing.

I thought something like this too (since I was messing around with my 2 digit 7 segment display recently), but I didn't know how to say it.   

Maybe the OP thinks the way to dim it is to turn it on, then flash it, then turn it off? 

Wait, maybe he is making a blinking LED dim-able.

I dunno. 

[newInRobotics]

In essence what I want to build is an LED flasher, however it will make LED flash so fast that it will seam like continuous light. 

[rbtying]

Shouldn't you just... leave the LED on all the time if you want it to seem continuous?  The terminology "LED flasher" is rather confusing as well.

[corrado33]

In essence what I want to build is an LED flasher, however it will make LED flash so fast that it will seam like continuous light. 

He wants to build an LED flashlight with 2 brightness settings from what I can understand in his second post.

[newInRobotics]

He wants to build an LED flashlight with 2 brightness settings from what I can understand in his second post.

No, I want to build LED flashlight with 255 brightness settings  

Shouldn't you just... leave the LED on all the time if you want it to seem continuous?  The terminology "LED flasher" is rather confusing as well.

Then I'd miss on learning how to program uC, how to create PWM using uC and I wouldn't have flashlight with 255 brightness levels  

Why LED flasher is a wrong term if PWM flashes LED say 3000 times per second? My first and second post clearly states what I plan to build and my question was answered straight away. What's the point to discuss about irrelevant things such as the name of the topic even though it does make sense; it's not like I named the topic "How to calculate torque of a motor" and then asked a question about PWMing LED.

[corrado33]

No, I want to build LED flashlight with 255 brightness settings  

Then I'd miss on learning how to program uC, how to create PWM using uC and I wouldn't have flashlight with 255 brightness levels  

Why LED flasher is a wrong term if PWM flashes LED say 3000 times per second? My first and second post clearly states what I plan to build and my question was answered straight away. What's the point to discuss about irrelevant things such as the name of the topic even though it does make sense; it's not like I named the topic "How to calculate torque of a motor" and then asked a question about PWMing LED.

Ah we were just confused.  No need to get upset!  See when you said "flasher" we automatically think you want an LED to be noticeably flashing, meaning it doesn't look continuous.  That's all.  

Anyway, I think you're project is cool.  Although I don't know if you'll be able to do 255 brightness settings (cause under a certain number it just looks off, and above a certain number it just looks as bright as it can be), but you'll get close.  If you need any help on the programming end feel free to ask.

[Soeren]

Hi,

[...] I want to build LED flashlight with 255 brightness settings  

Like corrado33, I think you should shoot for something lower, since you won't be abele to tell the difference between eg. 18mA and 19mA (I'm thinking a 20mA LED here) and that's around a 5% difference.
In the low end, say from 1mA to 2mA you'll be able to tell the difference of course, but that's a 100% increase.
The best way to make smooth transitions in the least number of steps, is to step "in percentages" and with eg. a 10% step increase, you'd need a bit more than 30 steps to go from 1mA to 20mA.

It may sound enchanting with 255 levels, but in daily use... I have a LED torch with LASER pointer that I rarely use, since it annoys me having to tap over the LASER each time I turn it off. The same goes for a couple of torches I have with 3 brightness settings, where it's tap tap tap before it gives you usable light - the less interfacing the better