Research into no-IC LED fade in/out
While you can use either a 555 IC or a microcontroller to fade one or more LEDs, for various reasons this project is looking at how to fade two to three LEDs with only discrete components (e.g. resistors, capacitors, maybe transistors).
15 August 2011¶
Put together the two circuits from the "RC Circuits (Resistors & Capacitors)" section of the Making Electronic Circuits page. The first circuit worked as expected but the second circuit did not.
After consulting with local experts I realised that it appears that in the second circuit diagram the position of the LED and capacitor have been reversed. This means that the LED will turn on immediately when power is applied (because the charging of the capacitor doesn't affect it) and fade out when power is removed (because the capacitor does at least get charged). The correct behaviour would be for the LED to fade on when power is applied and fade out when power is removed.
Here are the original and fixed diagrams:
16 August 2011¶
Yesterday I also tried a multi-LED version of the circuit.
With three LEDs and various resistors I could make the bottom LED glow brightly then each other LED placed above it glow a little less.
It was a bit of a pain to fiddle with the resistor values so today I've rewired it to use 3 potentiometers temporarily to work out the best resistor values.
It sort of very quickly fades the lights in but has a noticeable delay on the fade out. Because we only have one capacitor the "waterfall"/"decay" fade out isn't really accurate but might be good enough fakery.
The effect would be much more impressive/controllable with a microcontroller but it's not clear at the moment if the effect we have is "cool enough" given the goal of a minimal number of discrete components.