The newly refurbished cameras of the H.E.S.S. gamma-ray telescopes in Namibia have detected their first signals from a cosmic particle accelerator: The new cameras recorded Markarian 421 as their first target, a well-known blazar in the constellation of Ursa Major. The active galactic nucleus, 400 million light years away, was detected during an active state and at high significance. After four years of development, testing, production and deployment, this is the last big milestone of the H.E.S.S. I camera upgrade project, which was led by DESY. The success is also an important test for the next generation gamma-ray observatory, the Cherenkov Telescope Array CTA, which will use the same camera technology.