Keep individual LED brightness steady when other LEDs are added or removed
Update: Op has noted his issue is with using a NPN transistor as a high-side controller. It would not be working in a saturated mode and as the load changes it will have odd voltage and current properties. The proper simple solution is to use a PNP transistor suitable for the load. Or switch to a NPN low side setup, noting that the PWM period will be inverted.



I'm looking at the PWM circuit, a simple high-side transistor + 555 timer setup. I'm not seeing why this would change the brightness if you remove a series string from it. Unless you are removing a single led from a series string.







simulate this circuit &ndash; Schematic created using CircuitLab

Here, all the leds and resistors are the same. So the voltage between Node1 and Node2 are the same, as is the voltage and current going through all the strings.

If you turn off string 2 (R2, D3, D4 via SW1), the other strings should not see any change in voltage, and thus, current and brightness. If you turn off D6 in string 3, via SW2, then the voltage across D5 goes up, and thus current and brightness (if it doesn't burn out right away). But String 1 and String 2 are still the same.

If you add leds to any of the strings, you will see a brightness change as the voltage is divided among them. You would have to adjust the resistor value to make sure the same amount of current still goes through.

The only problem you may face is if your voltage source is a constant current source instead. Or if your voltage source is very load sensitive. If the source has a high equivalent series resistance, then as your load increases in current, a higher voltage drop is seen, affecting everything. Use a high current capacity and/or quality power supply. Also keep in mind that your transistor needs to be sized for your load. A 2n3906 will only handle 200mA, so if your load is more than that you will see problems.