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dwyl / smart-home-security-system

Smart Home Security System
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Controlling an electric strike from RPi w/ PoE #14

Closed th0mas closed 3 years ago

th0mas commented 4 years ago

Currently, I have been able to connect up a RPi and the NFC development board using PoE and a USB-C splitter. I've found no issues with this.

PoE is 48V which is far too high to power our 12v door strikes. We need to work out a way to step down the power to 12v, and, if possible, work out a way of getting power from the same cable as the Raspberry Pi.

12v splitters for PoE do exist: image

So it would be possible with two ethernet cables, but that defeats the point.

It also seems possible to power two low-power devices by splitting a PoE cable: https://networkengineering.stackexchange.com/questions/32560/multiple-poe-over-a-single-cable image

Which could be our best option as long as both of our PoE splitters support "Mode A" operation

th0mas commented 4 years ago

Apparently, PoE switches require some negotiation before they supply power. This means we can't just plug the power rails into a transformer and expect power. This means just buying the transformer as you suggested in #15 won't work.

We can either buy a microcontroller and start building our own PCBs, or we can buy a splitter made by someone that knows what they're doing: https://www.amazon.co.uk/DSLRKIT-Active-Splitter-Ethernet-IEEE802-3at/dp/B01H37XNHE

image

We can easily cut the plug off this and split the wire directly into our relay.

nelsonic commented 4 years ago

@th0mas yeah, I was thinking the same thing. (Good detective work BTW!) 💭 Sadly it's not as simple as grabbing the power from the POE. 🙄 🤦 Thankfully all the external doors have a 240v AC socket next to the Ethernet socket. 💡 ethernet-power-socket (we added this "just in case" powering the doors via POE wasn't an option ...) So if we have to, we can always place a 12V DC power supply for each Door controlled by a relay from the RPi0. I think it might actually be the best course of action for now as the hardware side isn't the most important part. (provided we can get a relay working to activate a 12V "thing" we can work out the rest later!)

nelsonic commented 3 years ago

@th0mas definitely continue with your current line of development as we will deploy it @home as soon as it's working end-to-end. :shipit:

What I'm proposing below is related to the hardware side of things, the software will not need to change.

This is probably a crazy idea, but it might work so please bare with me ... 💡 As you have noted above, converting POE 48V to 12V for each of the 20 doors might be a bit tedious. 💭 (we will do it where there is no other option, but maybe we need to think laterally on this ...)

What if we could sidestep the POE 48V 600mA -> 12V 3A (power stepdown) thing entirely (for the doors) and just use a single 12V 3A power supply to power all the Doors directly using a central relay board in the server rack? We would still use 2 pairs of wires in the the CAT6 cable for the 12v 3A to the Electric Strike and use the other 2 pairs for the UART connecting the RPI0 to the NFC/RFID reader.

TBC. (requires more research)

Note: Edit this comment to reveal more notes. I need to finish drawing the diagram/schematic for clarity before sharing.