Elektor magazine recently ran an LED fading circuit contest, described as:
LEDs that slowly fade in and out to indicate stand-by mode or the availability of a new message have been made popular by smartphones and other gadgets. To do this is easy enough with a microcontroller and some programming, but can you do it without?
So here is the deal
Design an all-analog fading LED circuit and post it on Elektor Labs. Add a short video to show it off. Bonus points will be given for added functionality like adjustable fading speed and duty-cycle.
Each of the projects have a schematic and most of them have videos showing their effect – I’ve included a few of them below. You can see all 15 designs and read more about them here at elektor.
Seen in the lower-left, this fading circuit is based on the NE555:
This circuit can be adjusted while operating and uses the LM324 op amp:
Here’s an intriguing logarithmic fader described as:
Here is an analog dimming circuit for a LED which takes into account the light sensitivity of the human eye (Weber-Fechner-law). For this purpose, the current through which the LED’s rise logarhitmically and drops off again when switched off in reverse logarithm.
This video also went to the extra effort of including credits and a soundtrack. 🙂 Watch:
Here’s a fader circuit based on the LM358:
And lastly perhaps my favorite, a dual-transistor (NPN & PNP) design that can be easily adjusted with a single resistor swap:
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