"Soft" LED Chaser w/ acceleration?

[CRE]

Hello,

This is something I've been trying to figure out the logic of.... I'm lost. :-\

There are two additions I'd like to make to a LED chaser similar to those which commonly us a 555 and a 4017. The first is I'd like to make one where each LED slowly fades on and then slowly fades off (without using 20 large caps, which I'm not sure would work anyway). So basically I want the power running along a triangle wave as can be managed with a couple op amps so the LED get .2v up to 3.5v back to .2v and off... gradually. I've got it

The second part (I should probably focus on one thing at a time) would be to add a variable acceleration to the circuit, so that depending on how a given pot is set the pulse width will span perhaps 1 second peak to peak and gradually move faster with each pass. I'm thinking something along the lines of using a second decade counter with different resistances on the output affecting the primary 555's input... Does that make any sense? I think this would work, but is there an easier way?

Thanks,
Christoph

 

[audioguru2]

Hi Christoph,
I have made many chaser projects.

Firstly, the output current of an ordinary 4017 is too low to drive LEDs brightly, unless you use a high supply voltage that will cause a 4017 to exceed its max output power. Therefore you will need to use output transistors. The transistors have a high input impedance so you can use small capacitors to fade the LEDs.

Secondly, LEDs operate with current, not voltage so will be at 3.5V/25mA and be bright, or 3.3V/0.5mA and be dim. They are usually dimmed by changing the voltage across their current-limiting resistors or with Pulse-Width-Modulation. Transistors can be used as emitter-followers to feed a ramping voltage to the LEDs and their current-limiting resistors and the transistors can be fed by small caps which are charged and discharged by the outputs of the 4017.

Thirdly, the outputs of a 4017 "jump" from one to the next. Their output pulse width must be shortened to smoothly fade with capacitors. On my 3V LED Chaser project,
http://www.electronics-lab.com/projects/games/003/index.html
I use a diode and resistor to shorten the pulse of the clock oscillator, and use an inverter driven transistor to "gate-on" the LEDs with that pulse. Maybe you can shorten the clock's pulse like that and use diodes to gate the pulse to charge the fading capacitors.

Finally, in my circuit I use a pot to manually change the chaser's speed. You could use the VCO portion of a 4046 PPL as a clock for the 4017 that changes its frequency with the increasing voltage from a charging cap. It might be difficult to make the pulse from the VCO remain at a reduced width and the fading effect to keep up with the increasing speed. You could even make a Roulette Wheel by a discharging cap.

 

[CRE]

Thanks for the reply Audioguru,

I don't have all the parts to experiment with yet (gotta order some more LEDs). I'm ordering from DigiKey and they list two "series" of 4046; CD4046*, and 74HC4046* does it matter which one I use?

Now back to the fading, how do I go about using a cap to produce a fade on and fade off effect? More specifically, if I'm using an NPN transistor, between where do I connect the cap? I could imagine it going from base to collector, or from base or emitter to ground (although I suspect that'd require a larger cap). What's most efficient?

Thanks again.

EDIT: Oh, BTW the voltages I referred to at the start of this thread were merely for example... the LED's I'm ordering are 2.2v (2.4max) @ 20mA (40mA max)... Although I suspect that a transistor may still be needed to efficiently fade the LEDs on and off.

 

[audioguru2]

Hi CRE,
It makes a big difference which IC you use. You need to select the IC to fit your application and supply voltage.
An ordinary CD4xxx IC operates with a supply voltage from 3V to 18V but very poorly below about 5V and can use signals up to around 2MHz. A high-speed 74HCxx IC operates with a supply voltage from 2V to 7V and can use signals up to around 30MHz.
Use google.com to see the manufacturer's datasheets and "family characteristics".

You will have an output from the 4017 (or gating diodes to shorten its pulse width), that feeds a resistor that connects to a fading cap to ground. The voltage across the cap will be a ramp up and another ramp down.
The base of an NPN transistor connects to the junction of the resistor and cap, its collector goes to the supply voltage and its emitter connects to the current-limiting resistor which has a grounded LED.

Your 2.2V LEDs are probably red and not too bright, something like the 1.9V LEDs I use in my 3V Chasers. I thought you would use 3.1V blue, green or white ultra-bright LEDs like I use in my "6V" Chasers.
Click on the link at the bottom of my project to see a discussion of my 3V Chaser with a 6V battery and ultra-bright LEDs. They are awesome.

There is a pic in that discussion showing my Fantastic Colourful Fader project that projects very slowly fading 16M colours on the ceiling. I haven't posted that project yet. ;D

 

[CRE]

Yup, I saw the "mood light" I'm familiar with basic concept, I've made a couple circuits that'll fade an LED using a dual or quad opamp, but what I want is to fade 3 or more in sequence. I'll try out the cap and transistor when I get the parts. This circuit would most likely be used for lighting effect inside a PC so a supply voltage of 12 and lower isn't a problem.

And yes, red LEDs of medium intensity.... they're cheaper than the 1.9v red ones I've got set aside for finished projects. : Kinda got tired of blowin' pricey LEDs (It really only happened twice).

Thanks!

 

[audioguru2]

Hi CRE,
My "mood light" circuit smoothly dims the LEDs from almost off to full brightness, not with a linear ramp but logarithmically using a bootstrap circuit.

I recently bought some "ultra-bright green" LEDs selected from a catalog and passed over the counter in a sealed bag. They were actually dim yellow!
A local Indian surplus parts store and their neighbouring Chinese parts store have inexpensive ultra-bright LEDs that I test before purchasing. (They are mixed-up sometimes since they all look the same, OOps, I'm talking about the LEDs). ;D

I have never burned-out an LED and I'm not lucky, I just follow the rules (current-limiting less than their max rating). But I smoked some 2N3904 and BC547 transistors before discovering that their E and C are opposites. ;D

I'll have to buy a Lumiled soon and see if it is bright enough to wake my neighbours far across the street! ;D

 

[Yevgenip]

Boy, If I had a transistor for every transistor I burned-out... ;D

 

[CRE]

You know oddly enough I have yet to burn a transistor out... As for the LEDs well, that's what I get for not paying enough attention.

Thanks for all the suggestions Audioguru, and I look forward to more on that circuit of yours. I was interested in doing something similar using a 3 LED IC I found online.... ordered 2.
They're not ultrabright by any stretch but I think they'll be fun to tinker with.

 

[audioguru2]

Hi CRE,
The RGB 3-colour LEDs that I've seen point their colours in different directions. I don't know what type of lense would combine them. Using separate LEDs, my fader has all colours together and I can drive them harder because they remain cooler.

I have nearly finished a "6V" ultra-bright green LED Chaser. Our vision is most sensitive to green so it should be very, ultra bright. My blue one lights-up the other end of my house at night! If you look at it for a moment then look away, you see phantom yellow dots for a while. ;D

Last night I got some "free" strings of outdoor LED Christmas lights. My electrical utility traded them for old incandescent ones to save energy. I don't like them, they flicker very fast on my 60Hz, and must be very annoying on 50Hz. My chasers "flicker" at about 2KHz due to my PWM brightness control but you can't see the flickering at such a high frequency.

 

[CRE]

Well, my thoughts on the LEDs in an IC was that since the light isn't straight as in a laser passing it through an acrylic rod/lens should serve to blend the colors sufficiently.

hmmm...... free string o' LEDs? Nice.... too bad about the flicker.

When you post the info on the 16M projector let me know.... I wanna see how it's done!

 

[audioguru2]

Hi Christoph,
Wow, is this site ever slow! I've been watching TV shows and building my projects while it loads.

I don't like narrow-angle LEDs that "cheat" on their intensity by focussing into a narrow beam. I try to select bright LEDs with at least a 30 degree angle.
The RGB LEDs I saw project the colours in 3 different directions with a small overlap in the center when fairly close.

I have in my hand as we speak (type?) a Chinese clear LED that has bright red and bright blue alternating about 3 times per second (built-in multivibrator). At a distance of 1m, the projection on the wall or ceiling is a flashing red ball about 10cm diameter on one side and a flashing blue ball on the other. A 10cm space between the balls is dark. It is weird to look at, when the distance is correct so that one eye sees 1 colour and the other eye the other. I would make a chaser with them if the beams were wider, but then they wouldn't be bright enough.

I don't know whether to search for the lost plans of my 16M projector or just reverse engineer it. I'll post it either way.

 

[CRE]

I agree that the narrow angle of projection can be a real PIA, but on occassion I don't mind if it gets me the color wavelength I want. I just ordered some blue 3mm LEDs which I hope come out nice and deep in color (430nm as 450nm is "true blue"). They're not the brightest only about 50mcd typ. and the angle is ~20º.... but, if the wavelength is as the datasheet says I'll be quite happy.

And I know what you mean about the site being slow.... wow.... just that much holiday traffic? or is it always slow?

 

[audioguru2]

Hi Christoph,
The 4 years old, expensive, 5mm, 630mcd (but today new ones are much brighter), 30 degree HP/Agilent ultra-bright blue LEDs that I've used are at 472nm. They are as blue as blue can be, I guess.
The cheap Indian or Chinese copies that I got recently look the same. Good stuff!

This site is extremely slow for nearly a week now. There is chat about it under "Feedback" in the index. I got half of another chaser project built while it loaded!