Circuit board design Eye logic, LEDs oh my.

Do LEDs on the typical circuit boards we use operate on a DC pulse with a return signal
coming back to the IC's logic if it detects a high or low?

Im having problems with a board retrofit, the G6r boards used different LEDs
with an in-line resistor, so there may be either a difference on voltage requirement
or frequency that they operate within.

Or could it be the LED size? the current boards use a 3 MM LED, on the C eye
systems. the guns being retrofit use 5mm and cant read at the distance of
the breach of the guns. But can read when very close together.

I haven't seen any inverters on any of the circuit boards that I can recognize,
so I'm assuming its all DC logic rather than an AC signal.

I cant check at the moment without a scope. But I'm wondering if any might know ahead of time.