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Circuit for a up/down fade led with solid on option


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Simple one here for yall im sure but I am looking to make a little circuit for a custom power button for a computer that I am building that will up/down slow fade while off and then stay solid when the PC is powered on. Standard ATX power voltages / signals available and I think I will need an additional power supply to keep the lights on without having to use the PC power supply for power. Button operates at 3.3V , 5V, 12V and I was thinking of using a simple 555 circuit to do the fade up/down. But what I was wondering was could some one here help me with the switching of the "light mode" and how to best do that. I was thinking of using the PCs Power LED (3v i think) as a signal to switch from pulsing mode to solid mode. I used to be able to do stuff like this easily but like all things, if you don't use it you lose it. :)

555 circuit.JPG

Button to be used: Link Here for Button
Additional power supply: Link Here for Pwr Supply

Any help would be great and thank you


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Welcome to ETO!
The first link is broken.
Not sure what you mean by 'light mode'?


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To clarify, you want the light to fade from solid on, to solid off when you power down. And from solid off to solid on when you power up?


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A 555 circuit is not a good fit, since there's no easy way to vary the PWM duty-cycle without a pot.

Below is the LTspice simulation of a PWM generator using an LM339 or LM393 that varies the duty-cycle from an RC ramp voltage to go from full-off to full-on and then back to full-off.

Most standard PC ATX supplies have a 5V VSB output that is always on to allow the PC power button to turn it on.
This can be used to power the circuit (+5VSB).
The 5V main power rail (V_5V) is used to provide the voltage to charge and discharge the R7C 2 circuit to provide the varying PWM signal to the LED when the PC is turned on and off.

The time to reach full brightness and to turn off is determined by the R7C2 time-constant.
Varying C2 will vary the time as desired.

Last edited:


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Food for thought:

I created a logarithmic ramp for something I did many moons ago. Essentially, I monitored a voltage and when that voltage was exceeded, I amplified the voltage across the capacitor in an RC circuit. This drove a LED through a resistor. I also added an LM334 constant current source at the output that would be active when the ramp finished.

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