Biggest question, what was the rated voltage of the lights? Just connecting them to 8V and see they don't blow right away isn't very exact And is that wall wart AC or DC? Did you measure it? A low loaded AC wall wart (real transformer) can easily output 10V+.
Wall wart is not a transformer, not heavy enough, there's a chip in there.
I'm wondering if maybe I should add a resistor in series to drop the voltage a bit?
Or a zener?
Or maybe switch to 12v, and use a bcuk voltage reducer. And that will give me 12 to power the Arduino.
If the string is working with 120V, pull-out one lamp. All of the bulbs in series will go out. If any stay-on they are in a separate series-string. If you have a series string of 50, that's 120V/50 = 2.4V per bulb. The energy/wattage from 120V RMS is the same as 120VDC. (Don't worry about the peak AC voltage.)So with 5VDC, two in series would be slightly over-voltage but probably OK.
Or would a better approach be to use a 9v or 12 v power supply with a 12 to 5 volt regulator.
To put the expected life of an incandescent lamp into perspective, the life changes rapidly with changes in voltage.In general the life changes with the 12th power of the voltage ratio.So if a bulb is rated for 5V and you are applying 8 V the expected life would be (5/8)^12 or 0.3% or the original expected life.The same is true for reduced voltages (but increasing the life)
LEDs are cheap and very reliable....Allan
Not sure what your point is? LEDs just like incandesent blubs burn out too from over voltage.