Main Question: What is the best way to convert PWM voltage generated by an external (i.e. 'blackbox') power source to a constant DC voltage while minimally affecting the load it is connected to?
Background: I built this PWM switch circuit to dim a 12V, 2A LED strip. This would be installed into a boat, and would be connected to the onboard 12 volt battery. Attached to this battery is an inverter and a battery charger, which is powered by the shore power (mains).
Everything worked fine in testing whilst the board was powered by a benchtop power supply, but after being installed into boat it was discovered that when the battery charger is connected, the LED strip begins visibly flickering erratically.
What I believe is happening is that the battery charger converts the shore power into pulsed DC, which it modulates depending on what it decides is required to charge the boat's battery. Because my board is connected to the battery at the same time, this modulated voltage becomes the input voltage to my board, which resets the 555 timer on each cycle of the battery charger modulation, leading to the flickering. I also presume that even if it didn't reset the 555, beat frequencies would be produced and could be visible due to the difference in the power source modulation and my board's PWM switching.
If this is indeed the problem, how can I stop the modulated DC power source from affecting my downstream switching circuit or the load? Thank you in advance.