PoE for Raspberry Pi for under $2

Using a 1N4148 diode for reverse polarity protection might be a bit under-specced: the Pi-3 + peripherals can draw something near 800mA under heavy load and the 1N4148 is rated at 300mA max continuous current, 500mA max repetitive peak. Something like a 1N5817/1N5819 schottky diode would be better - and maybe a 3A 1N5822 if there's any power-hungry USB peripherals. That buck converter claims to be good up to 3A.

Ref: https://raspberrypi.stackexchange.com/questions/43285/raspbe...

I would put a suitable USB plug on it rather than wiring to the GPIO pins as I am certain in a rush I would plug it in wrongly damaging the Pi!

True, and a clear sigh that it was chosen by misinterpreting the data sheet [1].

The way the author trims the output voltage is also wrong: these small modules come by default with the trimmer feature enabled, that is, if you need a voltage different from those written near the pads just leave the pads alone and use the trimmer. But if you need one of the fixed voltages available, then before soldering the corresponding pads, the re's a thin track that needs to be cut in order to disable the trimmer so that a vibration or touching it by mistake won't affect the output (small trimmers like that one are the weakest point in every cheapo module). The thin track is just left of the "ADJ" pad; if you look closely it closes the trimmer pad to make the board adjustable by default so it has to be cut to use fixed voltages.

Also, the chip used in the regulator module is a MP2315 whose safe input voltage can be between 4.5 and 24V, which is ok for a 12V input like in the photos, just to add some data since it wasn't specified.

[1] Never, ever, ever, ever base any project on the absolute maximum ratings of any part. Especially when they're completely misleading as in this case: the 2A which the 1n4148 should sustain (it can't) are related to a peak surge of 1 microsecond, and not even close to the continuous current which is a fraction of what a Raspberry PI can draw.

Thats the reason why diode is used for protecting the non-PoE switch's Port when raspi is accidentally powered through micro-usb port(or usb-c for pi-4)

There are some switches that supply passive PoE, such as many of Ubiquiti's products.