LED fading when driven by 4017

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LED fading when driven by 4017

Post by bigkim100 » Sat Sep 06, 2003 8:05 pm

On the output of a 4017 Led chaser, is there any way to have the LED's slowly fade on and off?
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Chris Smith
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Re: LED fading when driven by 4017

Post by Chris Smith » Sat Sep 06, 2003 8:36 pm

Through a cap and resistor charging and discharging. The "R" value will determine the time value. [so too will the cap] Its called R/C

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Re: LED fading when driven by 4017

Post by desterline » Sat Sep 06, 2003 8:41 pm

Not directly (at least not to my knowledge) But there is always a way.<p>One that leaps to mind is put a small switching diode in series with the output of the 4017 and a rather large cap in parallel with your LED. The thinking being that the 4017 would charge the cap when each output is on and the switching diode prevents the cap from discharging back into the 4017.<p>I don't think you'll like the results much though. An LED is a fairly curent hungry device and you'll need big caps to get very much fade time.<p>You could also expand the concept and use an opamp as a variation on the typical sample-and-hold. That would seperate the time delay and the current draw - i.e. smaller caps. But that would require an opamp and support circut for each LED. Lots of hardware.<p>Another consideration is to just use small filiment lamps. There are lots of small bulbs available that only draw a few ma and are smaller than some LEDs. Since a filiment is heat related the light emitted builds and falls in relation the the heat, providing a slower off.<p>My personal choice though would be to look into a PIC microcontroler. They can drive LEDs diectly and the blink patern is totaly under software control.<p>-Denny

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Re: LED fading when driven by 4017

Post by Calcbert » Sun Sep 07, 2003 9:23 pm

Something I worked on a little bit at one time...I can't guarantee how well it will work, but it might be worth a try.<p>Use a combination of an NPN transistor (i.e. 2N2222, 2N3904), a capacitor, and a resistor. Use something like a 10 or 100uF capacitor and perhaps 470 or 1K ohm resistor (it's very likely that you'll have to adjust those values some...experimenting shouldn't hurt much of anything).<p>Connect the output of the 4017 to the positive lead of the capacitor (since it will be an electroylytic capacitor you use). Connect the negative lead to ground. Connect the resistor in parallel with capacitor, so that it has one lead each to the 4017 and ground also. Finally, connect the output of the 4017 to the base of the transistor also. Connect the collector to the positive voltage supply, and the emitter through the LED and current limiting resistor for the LED to ground.<p>The idea of this is that the resistor constantly drains the capacitor, making it 'fill up' slower and 'drain' faster. The transistor provides some isolation for the LED from the capacitor side of the circuit.<p>Disclaimer: I don't remember actually testing this arrangement with a digital chip like the 4017, so it could actually damage the chip. It could also shorten its lifespan. No guarantees, but I definitely think it's worth a shot.

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Re: LED fading when driven by 4017

Post by Ron H » Mon Sep 08, 2003 5:36 am

If you want the fade rate to be much slower than the chase rate, modulate (vary) the supply voltage that powers the LEDs.

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Re: LED fading when driven by 4017

Post by sundancer87 » Mon Sep 08, 2003 9:20 am

I've spent a few hours attempting to do what you want to do. It's just not there, without the glow of a filament you can't get that slow loss of light. Because the LED is dependant on current, when it gets below the thresehold, it just goes out.
I wanted to do the same thing by shining ultra brights into a bundle of clear fiber optic strands. The idea was to light the optics up and slowly let them fade while the other optic bundle came on. I had more R/C circuits on the board than anything else. I never accomplished what I wanted. In order to do that I would have to go the old filament lamp route.

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Re: LED fading when driven by 4017

Post by algaeman » Mon Sep 08, 2003 3:42 pm

Sundancer87, what you said about LEDs is not true. The LED doesn't just go out when the current drops below a certain level. Just think about it. If you connect an LED in series with a potentiometer, the light will vary as you turn the pot. Perhaps there is a thresehold, but if there is, then it's very low.

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Re: LED fading when driven by 4017

Post by N9AOK » Mon Sep 08, 2003 6:17 pm

I'm experimenting with an LED now and I can get light out from about .5ma up!! with a fairly linear light ouput.<p>My voltage range is from 6.5V to 4 volts to go from 20ma to 1ma<p>I'm using a 3.8V LED with a 47ohm Resistor in line.<p>Todd

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Re: LED fading when driven by 4017

Post by toejam » Thu Sep 18, 2003 2:49 pm

I have faded led's using pulse width modulation. they fade nicely as you decrease the pulse width and because the voltage is constant they dont experience a threshold. if you connect the anodes to a pwm and the cathodes to inverters connected to the outputs of the 4017 you can get the desired effect. you will also need a way to ramp down the pwm and trigger the ramp with each pulse to the input of the 4017.This can probably be done with a couple of 555s.

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