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Easy LED fade on - hold - fade off circuit? Answered

Is there an easy way to make an led fade on and then stay at a constant brightness then fades off when the circuit is broken?

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steveastrouk

8 years ago

What kind of period ? 

Steve

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tsdgensteveastrouk

Answer 8 years ago

Sorry, I think I should have given more details on where I'll be using this. I plan to make a simple six LED flashlight but I want it not just to turn on or off. I have this idea to 'sophisticate' the on and off effect by fade on and fade off. A 1 sec fade on (and fade off) is sufficient for the effect.

And as a flashlight, the least batteries needed and the simplest circuit is appreciated.

Thanks in advance.

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steveastrouktsdgen

Answer 8 years ago

For a flashlight, I'd like to use a decent switch mode current controller rather than a simple RC circuit - the resistors' ll waste power. The current mode controllers might have a brightness input which you could put an RC circuit on like I showed - 1Meg and 1 microfarad would be 1 second time constant.

Steve

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steveastrouk

8 years ago

Play with this a bit. I'd put  a current limiting resistor (R2), but you can keep that to a small-ish value. On 12V rails, it'll work better than on lower rails. R1+R2 limit the diode current to safe levels when the supply is on, only R2 limits the diode current when the supply switches off

cap led.GIF
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cyberpageman

8 years ago

 Clever idea!
 I'll add that I think you'll need to put a resistor in series with the capacitor to decrease the rate of charging and a resistor in series with the LED to prevent too much current flow from the source.

If the LED doesn't fade properly, then you can have a capacitor-resistor combination control the fade on by means of a transistor, though I can't think of how to work the fade off when you switch off the source.

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Re-design

8 years ago

You could design or find a circuit that would ramp up the power then slope it down.

I've not tried it but it sees to me that the right size elec. capacitor in parallel would do the same thing.  It would absorb most of the voltage at first then as it filled the led would light and get brighter, then when you turn off the cap would supply voltage to the led then fade. 

Try it and see.  I won't be working in my shop today or I'd try it myself.