The Raspberry Pi Zero has pads on the underside that connect directly to the microSD card pins, and this allows for many different types of external slots and adapters to be used instead of the built-in one. This is very useful in situations where you cannot position the Pi to allow for access to the slot, but would still like the ability to remove the card to access or add data. The PSPi Version 2 is an excellent example.
One of the easiest implementations of this feature is simply using an SD to microSD adapter. The adapter has large pins available for soldering, and it can be attached inside of your custom device to give access to the slot.
SD to microSD adapter
To begin, let’s look at the Pi Zero.
It has many pins and pads available, and the pads we need are clustered together on the right side.The colorized image below shows the Pi Zero connection points.
The MicroSD adapter shown below gets soldered to the pads on the Pi as follows:
DAT1 goes to pad 17
DAT0/DOUT goes to pad 16
VSS top goes to GND
CLK goes to pad 14
VDD goes to 3.3v on the Pi
VSS bottom is unused
CMD/D1 goes to pad 15
CD/DAT3/CS goes to pad 19
DAT2 goes to pad 18
The image above is colorized to match the connection points on the Pi Zero. Only eight of the nine connections are used on this SD to microSD adapter because pins 3 and 6 are connected together.
Here is the final product. The microSD adapter is now fully attached.
All eight needed connections are made. On the diagram, the GND is connected to PP5 and the 3.3V is connected immediately above it. There is another 3.3V pin on the GPIO that would get the job done, but it is farther away and makes less sense to use. Just make sure you connect to 3.3V and not 5V. There are many GND connections on the Pi Zero and the one used here is not required, but again, it is the closest and makes the most sense.
There are a few important things to note. This was done with a microSD adapter, so I am unsure at the moment what modifications are needed to make a full SD card work in place of the adapter. An SD card has two ground connectors (pins 3 and 6), so please research the function of these connections before continuing in that direction. Also, the use of different color wires is highly recommended since 8 wires are being routed together. Last but not least, please do not try to use the internal microSD slot and an external one at the same time. I promise you will damage something if you try that.