To carve the face, use a hobby knife with a sharp blade. If necessary, wipe the smile (and eyes) off of its face with some rubbing alcohol. (Newer Lego minifigure heads have a vent in the top.) (No, not your head, the head of your Lego minifigure.) This is particularly important if you’re starting with an older Lego head, since the suction can make it hard to remove the LED. It’s a good idea to carve your head before putting the LED in. It’s a sign from above: The little yellow guys are simply meant to have light-up noggins. Since LEDs were already coming in 5 mm packages (also called T 1 3/4 packages) by the mid 1970’s, it’s clear that Lego minifigures were made to fit LEDs in their heads. The modern Lego minifigure dates to 1975-8.
It’s that easy: grab an LED and a head, slip it in, and it fits perfectly. In what seems like a miraculous coincidence, a standard LED fits exactly into the head of a Lego minifigure.
It’s remarkably easy to hack LEDs into Lego minifigures to make awesome, creepy, and tiny decorations for Halloween.