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.

• 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.
– user25450
Jun 20, 2013 at 2:42

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.

• 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. Sep 17, 2010 at 1:52
• Awesome, found it! Beautiful fade-in, beautiful fade-out. Sep 17, 2010 at 1:54
• Nicely done, sir! Sep 17, 2010 at 2:43

Here's an instructable on making a fade in and out circuit without an MCU