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LED fade in/fade out

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1-3-2-4

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I was looking around and I was thinking about doing this.. I have a LED light strip behind my TV anyways From what I hear using a trimmer pot and a capacitor wont work?

I was looking for something to be like 30 secs fade in and 30 sec to fade out..

Right now I'm running the LED strip at 12V at 1A I guess it has maybe 60 LED's that go around the 40" tv.

Hmm it seems like it's not possible to do it without some type of IC be it a 555 timer or an op amp?
 
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audioguru

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You need an oscillator driving a powerful current amplifier to fade in/fade out LEDs.
A trimmer pot, a few resistors, a few capacitors, two opamps and a power transistor will do it.
 

alec_t

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What controls do you plan? Push button to start fade-in? Same or another button to start fade-out?
 

audioguru

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33k into 100uF has a charging or discharging time of only about 7 seconds, not the 30 seconds that is needed.
 

1-3-2-4

Member
wow sorry I did not get any notifications for the thread do I need some type of push button for fade in? 30 seconds for the fade in/out is not critical but nothing lower then 7 seconds.

how I had planned for it was right now the LED's (154 I counted) are being powered at 12 V @ 1A I switch it on and it turns on.. And when I mean switch on it's on a switchable light switch.
 
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ljcox

Well-Known Member
A PWM solution would be better than an analogue one.

The duty cycle would have to be made to vary from 0% to 100% over about 30 sec then reverse.

A microcontroller would be the simplest solution.

Alternatively a pair of hardware oscillators. One would generate a sawtooth waveform that would control the duty cycle of the other.

Or it could be done with an up/down counter and a D/A conversion to control the duty cycle of an oscillator.
 

colin55

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We started with a poster wanting to dim a set of LEDs and finished up with someone suggesting he use a MICROCONTROLLER!!
 

alec_t

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Let's start off right. Presumably you don't want the LEDS to repeatedly fade in/out continuously (which would soon become boring)?
When you switch on you want the LEDs to fade in over 30 s. When you switch off they should fade out over 30 s. Confirmed?
 

alec_t

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Ok. This should do it.
U1b/R2/C3 form a ~23kHz oscillator whose output is integrated by R3/C4 to give an approximation of a triangle wave. When the switch is closed C5 charges via the pot to generate a rising voltage ramp. The wave and ramp are summed, at the input of U1c, by R5/R6, to give a PWM output which switches Q1 and Q2 to drive NMOS FET M1 and hence the load (your string of LEDs plus current-limiting resistors). The PWM signal has a duty cycle which increases over the ramp period (settable with the pot) to fade-in the LEDs.
When the switch is opened the ramp voltage falls and hence the PWM duty cycle decreases over the same set period to fade-out the LEDs.
N.b C5 should be 220μF in practice, but to make the simulation run faster it is 22nF.
 

ljcox

Well-Known Member
Here is a circuit that I designed for someone else some time ago.

It does not do exactly what you want but it could be adapted.
 

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alec_t

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@colin55
The circuit in post #15 will need adapting to get a 30 sec fade period. It currently gives a 3-4 sec period.
Re Ohm's law, take a look at the stated voltage at point X in the schematic and work things from there.
 

1-3-2-4

Member
Is it possible to make it adjustable? not a huge deal if it requires a whole change.

also for post #15 mine is a LED strip so I don't know how much that has in common with the luxeon LED's
 
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alec_t

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Is it possible to make it adjustable?
If you mean the post #11 circuit it is already adjustable, using the pot labelled 'Set time'.
 
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