septillion: 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+.
These are the cheap Christmas tree lights which are in parallel/series. I'm in the US so 50 +50. I'm in the US so 120AC/170RMS. I believe the voltage drop/working voltage is 1.5 per light but can't remember at the moment.
I connected the blubs to a variable power supply. At 1.5/2 volts DC about the same brightness as at 120vac. As I increased the voltage to 4 or 5 they appear to be the correct brightness. I suspect these bulbs are made for 220VAC and also sold in 120VAC market.
To my surprise the blubls would consistantly blow at about 8 volts. So I'm thinking driving them at 5VDC should not be a probelm. And for the most they've been working fine.
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.