As it seems my components interfear somehow and influence each other… I recently added a button to shutdown the mirror. It works by constantly setting a GPIO-pin to 3.3V, pressing the button grounds the pin and draws the voltage to 0V. A Python script registers this change and shuts down the OS. Strangely this also happens if the motions sensor enables the monitor. At first I assumed there might be something wrong with my circuits, so my uncle and I thought about how the 230V from the monitor could influence the Pi over the relays. Another guess was, that the monitor drains to much power at startup and with that collapses the 3.3V for a really short amount of time. Later I realized that this was also happening if I connected the monitor directly to a power outlet without the relay in-between.
I just picked up this old lamp, because I needed some light to correctly judge how reflective the mirror actually was when used normally. Well, I didn’t quite expect what happened next… Turning on (and off) the lamp (when placed a certain points near the other components) turns of the monitor for about two seconds. It does not seem to influence the Pi though but probably something similar is going on with the shutdown triggers.
I experimented a bit further with the lamp or to be precisely with its power cable. If I place the lamp’s cable over the Pi, the same thing happens, as if I turn on the monitor, a shutdown gets triggered as soon as the lamp is turned on or turned off. The problem therefore probably really is some kind of electromagnetic radiation.