TL;DR
Clem Mayer from element14 Presents revived a vintage LED matrix display using an Arduino UNO R4 WiFi. The sign, which still worked and displayed its last programmed message, posed a safety risk due to a poorly wired AC-to-DC adapter. Mayer first replaced the risky power supply with a correctly wired external one that outputs a safe 5V DC, ensuring it was safe to use.