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Led Fade on Fade off


Steve T
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Hi Everyone,

Im fairly new to electronic i do have some knowledge but not a great deal.

What I would like to achieve is when power is put across a led it fades on then when the power is cut is fades off.

How can this be achieved?

I know that if you run a cap in parallel with the led you can make it fade off but I don't know how to make it fade on.

Is there a better way of doing this?


Steve

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Hi Steve,
Welcome to our forum. ;D
If a resistor feeds a capacitor then the voltage on the capacitor ramps slowly up as it charges then ramps slowly down as it discharges.

An LED needs about 20mA so a ramping capacitor to power it must be huge.
A transistor can drive the LED and the input current to the transistor is only about 0.1mA so the capacitor feeding the input can be small. The transistor is connected as an emitter-follower and the LED is in series with a current-limiting resistor at the emitter of the transistor.

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