Working on a circuit to fade LEDs in/out

[alokw]

Hello there,

I'm a bit of a novice when it comes to electrical design, so I'm hoping to get some help.

I've purchased (12) MR11 LED lamps (with 12 individual LEDs in each). These run at 12V and draw approx. 40 mA.

I'll be wiring these with a switch inline directly to a 12V car battery. When the switch is turned on, all of the LED lamps should fade in at somewhere between 1 and 3 seconds. When the switch is turned off, all of the LED lamps should fade out at the same rate.

I did some research online, and came up with a potential solution, but I'd like to run it by you all, who know what you're doing.

**broken link removed**

Any thoughts?

Thanks!!!

 

[Grossel]

I guess you can make it very easy:

Put a capasitor in parallell with the leds. In adition, you can have a zenerdiode in series with the cap. It may work (little bit to tired to think right now):

 

[wayne2056]

If you want it to look like they are on a dimmer then you will have to use pulse width modulation. Google around....lots of samples out there.

Good Luck.

 

[rezer]

If you want it to look like they are on a dimmer then you will have to use pulse width modulation. Google around....lots of samples out there.

Good Luck.

PWM isn't necessary. Like Grossel suggested, you should be able to slow the rise of the supply voltage to the LEDs using a capacitor. Try a simple RC circuit using the current-limiting resistor. Tie the cap to ground. This will give a slower rise in voltage to your LEDs and give you the dimming effect you want.

C = 0.6/R

The 0.6 sec is for one time constant. This will give you full charge at the LEDs after 3 seconds. Your discharge rate will be different however, because your cap will discharge through the LEDs, which will be less than 3 sec. You use a DPDT switch and switch another resistor in-line with the LEDs to slow the discharge rate.

 

[Willbe]

When the switch is turned on, all of the LED lamps should fade in at somewhere between 1 and 3 seconds. When the switch is turned off, all of the LED lamps should fade out at the same rate.

LED brightness vs. current through them is well characterized; the voltage across LEDs is probably not controlled by the manufacturer.

If you're only concerned about average brightness of the group you might get away with voltage control, but you could get better control with a variable current source.

 

[rezer]

LED brightness vs. current through them is well characterized; the voltage across LEDs is probably not controlled by the manufacturer.

If you're only concerned about average brightness of the group you might get away with voltage control, but you could get better control with a variable current source.

True, the voltage is not controlled by the mfg, but by controlling the rise in voltage, your controlling the current as well, which does effect the brightness.

 

[Willbe]

True, the voltage is not controlled by the mfg, but by controlling the rise in voltage, your controlling the current as well, which does effect the brightness.

Try it. I think there will be a very nonlinear brightness variation with applied voltage, but that may be OK for this app..

If making a variable current source is 3x more complicated, then a voltage control should be tried if the chance of it succeeding is >1/3.
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