Intro I wanted to detect my Siedle doorbell press using an ESP32 so I could expose it to Home Assistant. At first glance this sounds trivial—until you actually put a multimeter on the Siedle line. Siedle’s 1+n system is partially documented here: Systemhandbuch 1+n-Technik (2019) On my installation, the doorbell line sits at ~18 V DC when idle and rises to ~22 V DC when the button is pressed. There is no clean contact closure, and the line is clearly current-limited.