How do they do that? The tech behind fireworks shows

SAN FRANCISCO (KGO) -- They've been around for centuries, but fireworks are a lot more high-tech than they used to be.

Lighting up the San Francisco sky is a way of life for Jeff Thomas.

We've been doing it over 40 years, probably closer to 50 years," he told ABC7 News.

Back when he started, the whole 20-minute fireworks show was lit by hand with a road flare.

But now, they use a computer firing controller. Using radio signals, it syncs up to a master clock, and lights each firework at precisely the right moment, with an electric spark.

Gone is the guesswork of timing fireworks to music. The computer calculates how early to shoot off the shells, so they explode right on time.

"PFT is pre-fire time, so that's saying how long it's going to take, 5.7 seconds, 3.9 seconds," Thomas explained.

Of course, the computers can't do everything. Getting those smiley faces right side up still has more to do with luck than anything else.

But some things about the fireworks haven't changed. even with all that technology, someone still has to stuff 10,000 explosive projectiles into all racks and make sure every one of them is rigged up to fire.

For two whole days, it's tough work with dangerous cargo.

"We have to keep them dry, we have to keep them away from static electricity, we have to be careful we don't drop them," pyrotechnician Jon Erickson said. "Any of these shells alone if you hold them in your hand and it goes off, you can kiss your hand goodbye."

They take safety seriously, and thanks to the computers, their hands don't always have to be so close anymore.

"Most all of us have all of our fingers, and...hearing maybe not," Thomas said.