Sorry I'll explain it more, be-warned its a bit of a wall of text.
I'm in the process of re-building/upgrading my RepRap 3D printer. Way back when the RasPi was first announced to the public I mocked up plans for using the RasPi in place of a laptop/desktop for slicing (generating g-code) and sending g-code to my printer.
On thing lead to another and my plans got lost and my printer gathered dust.
Fast forward about 3 years, I've rekindled my interest in 3D printing and dug out my plans.
To cut costs most DIY 3D printers use an ATX psu for power using the 12v rails (in the region of 20a), as the cheaper ATX psu's can have stability issues when just using the 12v rails most people attach something to the 5v lines which solves the issue of stability. Long story short a cheap ATX psu needs both 5v and 12v.
In my case I attach a 5v usb LED light which gives both the 5v rail load and lights up the print bed.
As you may or may not be aware some prints can take 12h+ to run and FDM printer involve 160c+ temperatures, so they aren't things you want to run with no-one in the house or something you want running when everyone is asleep.
This is were the Raspberry Pi comes in. I have a camera module which will allow me to remotely monitor the print and I can attach the RasPi to the printers electronics via usb which allows me to send prints.
Thats all fine as long as everything works fine however if something goes wrong I need to be able to remotely turn off the printer.
Which is were the Power Supply On relay is needed, breaking circuit instantly cuts out the psu and still allows you to start it back up by reconnecting.
The 5v and 12v relays are extras, but would allow me to remotely control the LED light and remotely power down/up the printer after/before a print.
The Power OK line is high when the psu is working fine, low then not. If I can read this line I can set it so it will never power on the printer if there is an issue.
Here's a rough image of the current setup (green board is the electronics for the printer). Red being the 5v line, yellow the 12v, green the power supply on and browns are earth/ground.
What I need help with is the wiring for controlling each of the non-brown cables via the RasPi as well as monitoring the power ok line (not sure) which is a 5v line.