Dimming LEDs

[synergy]

Hi, I need to make a circuit which will make 3 LEDs (I'm using those 3mm blue LEDs) slowly dim off when I open the car door. Any idea or there is any schematic where I can see?

Cheers!

 

[SMUGangsta]

dimming LEDs has been convered loads of times, try the search box at the top, or search for LED pwm

 

[davidbball13]

couldn't you just use a big cap. to dim them when you are turning them off.....

 

[Mikebits]

couldn't you just use a big cap. to dim them when you are turning them off.....

huh? .............

 

[Seven71ne]

huh? .............

Normally when you have a capacitor, it keeps the charge after the power is off, and it slowly goes away, creating a dimming effect.

If im wrong sorry.. thats how i always looked at it.

 

[Mikebits]

Normally when you have a capacitor, it keeps the charge after the power is off, and it slowly goes away, creating a dimming effect.

If im wrong sorry.. thats how i always looked at it.

Ok, I buy that.

 

[Hero999]

A large capacitor will work.

If you know the voltage drop of the LEDs and the series resistor then it's easy to calculate the current decay.

You'll need a pretty large capacitor though, assuming the LEDs are in series and the resistor is 150R for a current of 10mA, a 6800:mu:F capacitor will give an RC constant of just over a second meaning it'll take about 5 seconds to turn completely off.

 

[ecerfoglio]

A large capacitor will work.

If you know the voltage drop of the LEDs and the series resistor then it's easy to calculate the current decay.

You'll need a pretty large capacitor though, assuming the LEDs are in series and the resistor is 150R for a current of 10mA, a 6800µF capacitor will give an RC constant of just over a second meaning it'll take about 5 seconds to turn completely off.

You may use only one or two leds in each series (and a higher resistor), that will give yoiu more time with a smaller capacitor. Of course, you will have to connect those series strings in parallel

The capacitor will discharge from the supply's voltage to the led's foward voltage, with fewer leds you have more voltage difference and more discharge time.

[The extra power and current that will be used putting the 3 leds (and resistors) in parallel will be no problem - a few mA is little current for a car's barttery]

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Also, it would be a good idea to add a small resistor (like 10Ω) between the supply and the capacitor to limit the current surge when you turn on the leds

 

[Hero999]

The problem with putting the LEDs in series is that they willl draw more current so you'll need a larger capacitor to provide the current for the same length of time.

 

[audioguru]

You don't need a huge capacitor if you use a high gain transistor or darlington transistor as an emitter-follower to drive the LEDs.

 

[Hero999]

That depends on whether he has a permanent live connection available.