This project have been tested by myself, hoping you having same lucky :D
I made this item to using with my 3d printer using marlin firmware, it is bored what ever you want to print something and go to take out sdcard to copy file into it and then replug it in to 3d printer machine. ofcouse you can upload files to printer by the USART line but you will need to spend half of day when uploading a file around 10 mega bytes, which being limit by speed of USART.
I have search hackster for some similar product and not lucky, when asking google I found a project guide to create one, it seem it is worked, but with my limit knowedge about electric it still having some risk with that driver, now, i'm intro another version which remove all limit of his driver.
Before making things, let us talk about sdcard. The first is sdcard being controlled by the mcu or other device, so to communication with sdcard, mcu using SPI technique, and if we want to interfere that process we will extend it with a fake sdcard or simply cut the wire connect from mcu to sdcard (the way this project did). why ? as you know, the SPI line on the 3d printer board is the Master, if we implement another Master to read sdcard then it will cause conflict between two master because both of them send signal to the sdcard to control it, which may lead to a short circuit. even when the 3d print dont using SPI line, then still having a random electric signal go throunght wire to the sdcard pin. To cut the we must using something similar relay, but the size of relay is too big, and to implement "wire cut" you will need 8 relay, ofcouse you could use relay K6, it is around 1cm, but we having a better choice, it is 74HC4066.
Here is schematic:
https://oshwlab.com/tungbuivn/sd-card-marlin-esp8266
Now, let me explain schematic and workflow. At the boot time SWLine2 will be pull down to GND, this will make signal of the SWLine is 3V3, this will allow 3d printer to sdcard and prevent our mcu access to the sdcard. What ever we want to upload file to the sdcard we will need to cut the wire connect from other device connected to sdcard, this will be done by trigger SWLine2 to posible signal which will cause MCU_LINE short and BOARD_LINE will open, easy then.
At this time, we can control the signal we want to send to sdcard, but the 3d print is direct connect to it, and at the time we switching line, it is not instant, so still having electric going between BOARD_LINE and MCU_LINE, that is why i'm adding some resistor to output pin of the 74HC4066 to prevent shorted circuit. but this will lead to create a fake sdcard to prevent any device direct connect to it before trigger 74HC4066 switching the line, so you can buy a extending sdcard line, or create one like this:
now, assembly everything and we have final product:
ofcouse, this will need you to implement custom software to upload file to it, I have modify ESP3D project to fit it:
https://github.com/tungbuivn/ESP3D-direct-sd/tree/2.1.x
Here is pcb I have made.
Here is pcb to aid you assembly electric component:
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