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This is probably a very easy question, but I've been trying for several weeks now to create a simple circuit that "fades in" some LEDs using only discrete components like resistors, capacitors, inductors, (photo) diodes, transistors, and a voltage source.

Using only discrete parts (no ICs), how do I "fade in" a few LEDs over the course of a second or two?


I've managed to create the opposite, the "fade away":

  • +Volts connected to "RC" and then to "PL" and then to ground in series
  • RC is a resistor and capacitor in parallel
  • PL is roughly some leds in parallel: PL1 and PL2 in parallel
  • PL1 is a LED and resistor in series
  • PL2 is three LEDs and resistor, all in series

When powered up (at +6V with the RC using 1K ohm and 2200uF) all LEDs immediately light (no fade in at all), but over 2 seconds the triple LED (PL2) fades to black.


I can of course make this using an Arduino and the PWM output pins to make intricate fade in and fade out patterns, but the circuit is to demonstrate using a simple transistor to switch a system on, and if I'm using a microcontroller to drive that system, I might as well have the transistor trigger my laptop to play a youtube video.

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  • \$\begingroup\$ Someone you might want to look into is an artist named Bjoern Schulke. His kinetics sculptures produce discrete actions using, simply, transistors, motors, and capacitors. If you replace the motor with a light its a similar concept. The problem is that its hard to discern which component is which when looking at his work. But the concepts are elegant, simple and highly functional. \$\endgroup\$ – user25450 Jun 20 '13 at 2:42
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Put the capacitor in parallel with the LEDs and leave the resistor where it is.

When you power up, the voltage across the cap is 0 V. It will slowly charge up to match the voltage at the top of the LEDs.

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  • \$\begingroup\$ I'm still working on finding the right numbers, but the fade-away effect on this one is just awesome. I'm still getting an instant power on (with a low "R") or the triple never turns on (with a high "R"). Hopefully the fade-in is somewhere in between. \$\endgroup\$ – Jack Schmidt Sep 17 '10 at 1:52
  • \$\begingroup\$ Awesome, found it! Beautiful fade-in, beautiful fade-out. \$\endgroup\$ – Jack Schmidt Sep 17 '10 at 1:54
  • \$\begingroup\$ Nicely done, sir! \$\endgroup\$ – pingswept Sep 17 '10 at 2:43
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Here's an instructable on making a fade in and out circuit without an MCU

http://www.instructables.com/id/No-CPU-MCU-led-pulse-fade/

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  • \$\begingroup\$ Thanks, this seems like a good way to adjust the fade-away rate. Right now it is working perfectly without the transistors, but I assume the numbers will get harder to juggle the more LEDs are involved. \$\endgroup\$ – Jack Schmidt Sep 17 '10 at 19:51
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I've tried to do this before and the problem is the shape of the exponential RC curve. Your eyes are very sensitive to changes at low light levels, so going from OFF to low current, it immediately looks bright because that part of the curve is rising quickly. I think that an inverted curve (logarithmic) would give you what you want, but it's harder to do with just analog components.

I ended up throwing a microcontroller at the problem since it was simpler to do that than build a log amplifier. Even then, I found that I had to increase the current very very slowly at the low end (was trying to simulate sunrise).

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