@The-Bean
In my case the RaPi takes less than 0.4A with very short peaks just above that. So the consumption is around 2-2.5W incl. keyboard and mouse. Add this to your screen consumption and add 20-30% on top of that. So 45W is the minimum, 50W and above should be the better choice. But it also depends how long the screen will be on. If it’s just for a couple of minutes every hour or so, 45W will do.

The 12V/3A of your screen, are they measured values, or are they from your manual? What screen are you using?

@The-Bean
They say the power supply should be able to do 20-30% more than what you need. So better aim for 45W. Something like this should do. Just check Amazon or eBay in your local country. They are usually very cheap and do the job just fine. I use 2 of those (30W) to drive LEDs for years now.

You could use something like this:

You plug in the wall outlet and then you plug the power supply into this device. It will show you voltage, amperage, wattage, etc. They are very cheap. I think I paid for mine around 5€ 5 years ago.

Or, if you know how much ampere is used at a known voltage, you can easily calculate the wattage by multiplying amere A with voltage V, e.g. 3A at 12V would result in 36W.

This is how my setup looks like at the moment:

Relay: Black = GND, white = Vcc 3.3V, green = IN
PIR: Black = GND, red = Vcc, green = out

Gray, blue, puple = ds18b20 temperature sensor.

I actually haven’t had a false trigger yet and the Pi is running for more than an hour now. I will keep it running the whole day. Have a multimeter connected to the relay, set to continuity test, so I should hear a beeping sound every time the relay gets triggered.

What independent PIR are using now? Can give me link? I was looking into it too, but couldn’t find one that runs on 12V only. They all have 230V input and 12V output. That’s not working for me.
When you used the PIR with the GPIO pins, did you use a ferrit ring with it? Apparently that can help. I also had phantom triggers when I only used the PIR (without relay) and the MMM-PIR-sensor module. The ferrit ring resolved the phantom triggers for me 100%. Just now with the python script and the relay I get them very randomly. Sometimes after 2 minutes, sometimes after 2 hours.

I did that in the very beginning but then relay isn’t working at all. It only works with 3.3V. No idea why.

You should check how much power your mirror, incl. screen and RaPi, consumes and buy an appropriate LED power supply. Especially the IP67 ones are very slim. My 60W power supply is only 33mm high. My whole system uses max. 30W so I decided to go bigger (46W was out of stock, 50W has the same dimensions). If you’re at max. below 30W you could aim for a 36W power supply that are even smaller (I think 20mm high).

You could use buck-boost-converters to adjust the voltage you need for the Pi or the LCD. In my case the LCD is driven by a V59 universal controller board that runs on 12V, so it can be powered directly from the supply. The Pi will get the power also from the power supply but with a buck-converter in-between, that reduces the 12V to 5V.

The is connected to pin 15 (GPIO 27), 17 (3.3V), and 20 (GND). I tried to post a link to the it, but then this post is flagged as spam. It’s just a cheap relay from Aliexpress.

No idea if that relay is active high or low, but does it matter? It just switches between to connections, so if the relay port 1-2 or 2-3 are active shouldn’t matter, right?

I am using a 3.1A power supply for the Pi. Later I will connected it via a 8A buck-boost-module to a 12V/5A power supply that will power the screen. The Pi will get the power from before the relay so it won’t switch itself off ;-)

Please send a picture, I will gladly try, once I have the extra party I might need (transistor or resistor).

@__Tom__
It’s one hour per direction. Not worth for 12.50€ but I would probably do for bigger mirrors as the shipping costs can get quite expensive.

Thanks a lot to cruunnerr, who has the patience of an angel!

I think there were two problems in my case and I write them here in case someone else has trouble getting the PIR+relay to work.

First: By installing the MMM-PIR-sensor module I used the wrong wiring-pi. npm install wiring-pi installs an old version (2.25) which - so I’ve read - doesn’t work with the Raspberyy Pi 3. It’s best to install wiringpi according to this site.

Second: I used a relay which was supposed to run on 3V to 7V, but apparently that’s not the case. I guess the seller shipped the wrong relay. So I had an old relay lying around which should only run on 5V but it works nicely on my RPi.

Now everything is working, except I get some phantom triggers of the relay every couple of minutes. But I will look into it once everything is set up completely. Maybe I have something around here that can trigger the PIR sensor even if it’s covered by a cloth.