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I was discussing with a co-worker about a led driver for our project, something that is really simple and it could be irrelevant, but it now has a major relevance from my point of view for: for a huge increase in the price for multiple unnecessaries components. I believe he wants to show a "slow" fading (which is not necessary) led if he wants that. so drive the led to just turn on/off it could be easy to do with the design (2) (mine) instead of (1) with 4 extra components or 5 if I remove all the caps.

note: the pulse is coming from un microcontroller, not from a push button

enter image description here

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  • \$\begingroup\$ Neither of those circuits will give you a 'slow fade time' for any realistic C values - at least not if you insist on using an NPN BJT. Change it to an N-MOSFET and you might have some success there depending on the R and C values you choose. \$\endgroup\$
    – brhans
    Commented Apr 23, 2018 at 17:11
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    \$\begingroup\$ Can't you just use PWM from the micro to cause the slow fade? It'll be easier than passive filtration. \$\endgroup\$
    – Reinderien
    Commented Apr 23, 2018 at 17:16
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    \$\begingroup\$ Also, a linear slow fade of the LED will imply an exponential decay rate of the current, because of how humans perceive intensity. \$\endgroup\$
    – jonk
    Commented Apr 23, 2018 at 17:34
  • \$\begingroup\$ Can you quantify what is a "slow"? 1us, 1s, or 3 weeks? \$\endgroup\$
    – Paul Uszak
    Commented Apr 23, 2018 at 21:22

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If you can't drive the LED directly from the micro (maybe spend a few cents more for a bright LED and increase the 150 ohm resistor a bit), then just PWM it through a transistor (like your diagram but minus the 0.1uF).

There's no need for any of the pF capacitors or the 10K to ground in his diagram. His 10K in series is a bit on the high side, your 1K value is better.

Assuming it's just used as an indicator, the 150 ohm value implies either a really crappy cheap LED or a blindingly bright decent LED. Maybe you should ship sunglasses along with it?

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