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LED Fader

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Stealth

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For years I have looked for a way to have an LED fade in and out (like a beacon), and I have finally found a circuit to do just that.

**broken link removed**

I construced the circuit and it worked as I wanted. But I am having a small problem with it. I would like to use a blue LED with it, but when I tried using my blue LED (5v, 30mA), the LED was not brought to its full brightness by the circuit. I tried decreasing the values of the resistors, and increasing the size of the capacator, but the LED was not brought to its full brightness.

What needs to be done to bring a blue LED (5v, 30mA) to it's full brightness?

Also, if anyone has a better idea on how to make this, it would be greatly appreciated!

EDIT: Found LED's exact mA requirement
 
Last edited:
Increase the supply voltage to 12V or so.... You'll have to recalculate the LED resistor though.
 
Stealth said:
For years I have looked for a way to have an LED fade in and out (like a beacon), and I have finally found a circuit to do just that.

**broken link removed**

I construced the circuit and it worked as I wanted. But I am having a small problem with it. I would like to use a blue LED with it, but when I tried using my blue LED (5v, 30mA), the LED was not brought to its full brightness by the circuit. I tried decreasing the values of the resistors, and increasing the size of the capacator, but the LED was not brought to its full brightness.

What needs to be done to bring a blue LED (5v, 30mA) to it's full brightness?

Also, if anyone has a better idea on how to make this, it would be greatly appreciated!

EDIT: Found LED's exact mA requirement
Click to expand...


Hi Stealth..

The circuit is working well but having a problem as you said.It was happened to me.With some low current LED's like RED LED'S its works fine.The Display on time and the Display off time is nicely balanced.But for some LED's like Superbright BLUE & GREEN the off time is More than on time.not like 50% - 50% the output pulse is not increasing to its maximum state.Thats why you see the LED is not light brightly,suddenly its decreasing the brightness.Even you change the supply & resistors you wont get the result well.I also wonder why its hapening like this?
 
kchriste said:
Increase the supply voltage to 12V or so.... You'll have to recalculate the LED resistor though.
Click to expand...
Yes, that is one of the first things I tried, but no luck. I even changed the chips input voltage to 5v (by removing the 2 47K resistors), and still no luck

Gayan, thanks for the reply, it's great to know I'm not the only one with this problem
 
I would be prepared to help if it was drawn as a circuit diagram, not as a wiring diagram.

I really don't feel like looking up the data sheet of the LM324.
 
ljcox said:
I would be prepared to help if it was drawn as a circuit diagram, not as a wiring diagram.

I really don't feel like looking up the data sheet of the LM324.
Click to expand...
It isn't my diagram, so I have no control over the information it contains without redrawing it.

This is the pinout for the LM324:
**broken link removed**
 
Last edited:
Here is another way that fades.But will it be like earlier one????I haven't try this?
 

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Gayan Soyza said:
Here is another way that fades.But will it be like earlier one????I haven't try this?
Click to expand...
That one didn't seem to work, with a single LED it is the same as the only I originally posted, just with an added capacitor.
 
Try this. I have shifted the range of the sawtooth waveform and moved the LED to the collector so the trasistor acts as a current source.

You need to make some minor changes to your circuit and add a few components.
 

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    LED Fader.gif
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The output of the LM324 goes to a max voltage of +3.8V with a 5V supply. Then the emitter-follower transistor reduces it to +3.1V. A white, blue or superbright green LED needs about 3.4V to have enough current to be bright.

If you use a PNP transistor to drive the LED then the output voltage of the LM324 will go close to ground then there will be enough voltage for a 3.4V LED to have enough current to be bright.
 
Audio,
That's a good idea, but I estimate that the ramp will be from about 1.8V to 3.8V. So there is not enough voltage for the LED since 1.8 + 0.8 = 2.6V

5 - 2.6 = 2.4V

So you need to shift the ramp down so it runs from 3.8 to 0V.
 
In the circuit I posted above, the maximum LED current is set by the 150 Ohm resistor. So if you want to be able to set the maximum brightness, replace the 150 Ohm with a 100 Ohm in series with a 100 Ohm pot.
 
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