Discussion:
Dimming led's ?
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mkr5000
2021-04-03 15:54:12 UTC
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Here's a brain teaser (for me) that I'd like to hear opinions on.

I ran into this video this morning and I'm puzzled as to what may be going on.
After all, in modern day electronics, if you are using any sort of digital device you can only output a high/low so I wonder how the trailing led's in the video are "dimming" out rather than showing an abrupt on/off. If you watch this starting at :24 sec and stop it. you'll see what I mean.

I mean I know there are A/D inputs but is there such a thing as a digital to analog output? Maybe there is some capacitance on the output that is feeding the led causing the fade out?

It's a cool effect. Can't recall seeing anything like it.


mkr5000
2021-04-03 16:02:48 UTC
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Post by mkr5000
Here's a brain teaser (for me) that I'd like to hear opinions on.
I ran into this video this morning and I'm puzzled as to what may be going on.
After all, in modern day electronics, if you are using any sort of digital device you can only output a high/low so I wonder how the trailing led's in the video are "dimming" out rather than showing an abrupt on/off. If you watch this starting at :24 sec and stop it. you'll see what I mean.
I mean I know there are A/D inputs but is there such a thing as a digital to analog output? Maybe there is some capacitance on the output that is feeding the led causing the fade out?
It's a cool effect. Can't recall seeing anything like it.
http://youtu.be/bokWXzVXsyc
or I guess, maybe it's just the video "creating" it?
j***@highlandsniptechnology.com
2021-04-03 16:07:47 UTC
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Post by mkr5000
Here's a brain teaser (for me) that I'd like to hear opinions on.
I ran into this video this morning and I'm puzzled as to what may be going on.
After all, in modern day electronics, if you are using any sort of digital device you can only output a high/low so I wonder how the trailing led's in the video are "dimming" out rather than showing an abrupt on/off. If you watch this starting at :24 sec and stop it. you'll see what I mean.
I mean I know there are A/D inputs but is there such a thing as a digital to analog output? Maybe there is some capacitance on the output that is feeding the led causing the fade out?
It's a cool effect. Can't recall seeing anything like it.
http://youtu.be/bokWXzVXsyc
LED brightness is usually PWM. I suspect that circle case is PWM on
all the LEDs with some software doing the patterns. Leds can be wired
in a matrix to reduce the number of drivers.

That TI capacitive touch thing is cute, but conductive plastic
resistive touch sensing would be easier.
--
John Larkin Highland Technology, Inc

The best designs are necessarily accidental.
Rick C
2021-04-03 18:28:02 UTC
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Post by mkr5000
Here's a brain teaser (for me) that I'd like to hear opinions on.
I ran into this video this morning and I'm puzzled as to what may be going on.
After all, in modern day electronics, if you are using any sort of digital device you can only output a high/low so I wonder how the trailing led's in the video are "dimming" out rather than showing an abrupt on/off. If you watch this starting at :24 sec and stop it. you'll see what I mean.
I mean I know there are A/D inputs but is there such a thing as a digital to analog output? Maybe there is some capacitance on the output that is feeding the led causing the fade out?
It's a cool effect. Can't recall seeing anything like it.
http://youtu.be/bokWXzVXsyc
Digital outputs are one or zero but can be flash LEDs very quickly, faster than your eye can see. The eye does perceive the integral of the flashing, so the duty cycle can be varied to adjust the brightness. That's all.

An issue with this is if the flashing is not fast enough, you won't see anything while the LED is stationary, but in motion you can see multiple images as it flashes on and off. They take advantage of this in toy wands that can spell out a message as you wave the wand in the air and the multiple LEDs are flashed on at the appropriate times to show letters and words. It's like the opposite of the moving message displays. The wand moves and the message stays still. Car tail lights can create multiple images as you turn your head making it look like there are many cars when there is only one or two.
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Rick C.

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whit3rd
2021-04-03 19:55:26 UTC
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Post by mkr5000
Here's a brain teaser (for me) that I'd like to hear opinions on.
I ran into this video this morning and I'm puzzled as to what may be going on.
After all, in modern day electronics, if you are using any sort of digital device you can only output a high/low so I wonder how the trailing led's in the video are "dimming" out rather than showing an abrupt on/off.
An easy way to do it, is with a transistor having a capacitor from collector to
base; the base resistor and that capacitor (Miller capacitor) make a slow-slewing
output voltage. It also buffers a logic output (sub-milliamp) up to full LED
drive (if you really want that, nowadays; thirty years ago, you definitely did).

I dislike the PWM-direct approach, because it requires logic, and because
it generates EMI, and causes light output artifacts. Muxed LED displays,
ditto.
j***@highlandsniptechnology.com
2021-04-03 20:30:23 UTC
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Post by whit3rd
Post by mkr5000
Here's a brain teaser (for me) that I'd like to hear opinions on.
I ran into this video this morning and I'm puzzled as to what may be going on.
After all, in modern day electronics, if you are using any sort of digital device you can only output a high/low so I wonder how the trailing led's in the video are "dimming" out rather than showing an abrupt on/off.
An easy way to do it, is with a transistor having a capacitor from collector to
base; the base resistor and that capacitor (Miller capacitor) make a slow-slewing
output voltage. It also buffers a logic output (sub-milliamp) up to full LED
drive (if you really want that, nowadays; thirty years ago, you definitely did).
I dislike the PWM-direct approach, because it requires logic,
Let's avoid that.
--
John Larkin Highland Technology, Inc

The best designs are necessarily accidental.
Clifford Heath
2021-04-03 23:13:50 UTC
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Post by whit3rd
Post by mkr5000
Here's a brain teaser (for me) that I'd like to hear opinions on.
I ran into this video this morning and I'm puzzled as to what may be going on.
After all, in modern day electronics, if you are using any sort of digital device you can only output a high/low so I wonder how the trailing led's in the video are "dimming" out rather than showing an abrupt on/off.
An easy way to do it, is with a transistor having a capacitor from collector to
base; the base resistor and that capacitor (Miller capacitor) make a slow-slewing
output voltage. It also buffers a logic output (sub-milliamp) up to full LED
drive (if you really want that, nowadays; thirty years ago, you definitely did).
I dislike the PWM-direct approach, because it requires logic,
The logic is available already built-in to almost every MCU
Post by whit3rd
and because
it generates EMI, and causes light output artifacts.
So use a series inductor

CH
j***@highlandsniptechnology.com
2021-04-03 23:44:07 UTC
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Post by Clifford Heath
Post by whit3rd
Post by mkr5000
Here's a brain teaser (for me) that I'd like to hear opinions on.
I ran into this video this morning and I'm puzzled as to what may be going on.
After all, in modern day electronics, if you are using any sort of digital device you can only output a high/low so I wonder how the trailing led's in the video are "dimming" out rather than showing an abrupt on/off.
An easy way to do it, is with a transistor having a capacitor from collector to
base; the base resistor and that capacitor (Miller capacitor) make a slow-slewing
output voltage. It also buffers a logic output (sub-milliamp) up to full LED
drive (if you really want that, nowadays; thirty years ago, you definitely did).
I dislike the PWM-direct approach, because it requires logic,
The logic is available already built-in to almost every MCU
Post by whit3rd
and because
it generates EMI, and causes light output artifacts.
So use a series inductor
CH
If you scan an LED matrix at 1 KHz, the AM band is the 1000th
harmonic.
--
John Larkin Highland Technology, Inc

The best designs are necessarily accidental.
Don Y
2021-04-04 06:13:15 UTC
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Post by whit3rd
An easy way to do it, is with a transistor having a capacitor from collector to
base; the base resistor and that capacitor (Miller capacitor) make a slow-slewing
output voltage. It also buffers a logic output (sub-milliamp) up to full LED
drive (if you really want that, nowadays; thirty years ago, you definitely did).
The problem with that approach is it "hard-wires" the visual response.
Post by whit3rd
I dislike the PWM-direct approach, because it requires logic, and because
it generates EMI, and causes light output artifacts. Muxed LED displays,
ditto.
Agreed. But, with large numbers of lamps, it's often the most practical way.

I have ~500 indicators on a "summary status display". Driving each of them
directly would be costly (esp if you want to control intensity -- which is
almost essential if you want to deal with color-blindness in observers;
that information has to be encoded on another "display channel"!).

OTOH, the visual artifacts tend to be minimized -- because you're not expecting
"motion" in the indicators and are typ not moving your head side to side or
up and down while viewing them.

And, you're not spending lots of time *studying* the display ("Gee, what's
happening with THAT one?")
Paul Hovnanian P.E.
2021-04-08 02:14:51 UTC
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Post by mkr5000
http://youtu.be/bokWXzVXsyc
I suspect that the trailing LEDs are dimmed by a programmed PWM with a
decreasing duty cycle based on their distance or time since the
corresponding point has been touched.
--
Paul Hovnanian mailto:***@Hovnanian.com
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