I'd like to use several LEDs on a project, with brightness control (it's a scale model of a street with houses, with LEDs of different brightness in each house, which I've been working on with my son). However, I'd want the brightness of the LEDs not to change when I turn some of them on and off (or equivalently, if I add or remove LEDs).
I know I can control dimness of LEDs either by directly varying the current applied to them as in this project from someone else or by using PWM as suggested here -- but in both cases (I implemented the two projects listed above as a test), the current source is the same so their brightness will vary depending on the number of connected LEDs (which make sense, since the full LED set is plugged on a single transistor's collector).
What would be the simplest way to achieve what I'd want in this case? I'd rather avoid using, for example, an Arduino (would be too physically large and would involve programming -- I'd like this to be as simple, small, cheap as possible, without microcontrollers). Is there some simple way to decouple each LED from the others, and still have central control of their brightness?
Connecting the LEDs in serial would not be good either, since I would not be able to remove those in the middle (and this would require a too high voltage to work, because of the accumulated voltage drop of the LED string).