How do SD card readers detect write protection?

[iOutatime]

So an odd question I had: "How do SD card readers work to know if an SD card has the write-protection switch on or off?" I'm currently modding a case to house a reader and I'm wanting to rig it to my own switch, but I want to know how the SD card reader currents work and how it activate/deactivates the reader. (Please feel free to use the pictures below to demonstrate or correct me in anyway. )

[JoostinOnline]

The magic elves that live inside look at the switch. Duh. 

[manikyath]

the SD card itself has a switch under the slider thingy, it's not handled trough the reader like it was in the magnetic carrier days.

[LunaP0n3]

The SD card actually reports to the reader (and thus the OS) that it's write protected.

Wikipedia has more:

Quote

The user can designate most full-size SD cards as read-only by use of a sliding tab that covers a notch in the card. The miniSD and microSD formats do not support a write protection notch.

When looking at the SD card from the top, the right side (the side with the beveled corner) must be notched.

On the left side, there may be a write-protection notch. If the notch is omitted, the card can be read and written. If the card is notched, it is read-only. If the card has a notch and a sliding tab which covers the notch, the user can slide the tab upward (toward the contacts) to declare the card read/write, or downward to declare it read-only. The diagram to the right shows an orange sliding write-protect tab in both the unlocked and locked positions.

The presence of a notch, and the presence and position of a tab, have no effect on the SD card's operation. A host device that supports write protection should refuse to write to an SD card that is designated read-only in this way. Some host devices do not support write protection, which is an optional feature of the SD specification. Drivers and devices that do obey a read-only indication may give the user a way to override it.

Cards sold with content that must not be altered are permanently marked read-only by having a notch and no sliding tab.

 

[iOutatime]

7 hours ago, manikyath said:

the SD card itself has a switch under the slider thingy, it's not handled trough the reader like it was in the magnetic carrier days.

The switch on the SD card doesn't have anything inside of it. The reader however, does have this piece of bent metal that is pushed inwards to detect what position the SD card write protection switch is in. It is also just a piece of bent metal that is attached to the shell of the SD card reader. That means when the bend metal is touching the SD card switch and is pushed back out, it cancels the power from the board that was carried across the shell? If so, how can touching a plastic tip defuse the power all together that was going across the SD card reader shell?

[manikyath]

2 hours ago, iOutatime said:

The switch on the SD card doesn't have anything inside of it

the SD card i ripped apart for science did..

 

i'd imagine if there's an actual electronic contact behind it, it does at least *something*