Instructions may change as needed.
Step 1: Introduction
This is my first instructable so you'll have to bear with me.
The Monoprice select mini is a great machine but in being a low priced 3D printer is has its drawbacks and in my experience it's been the main board and the user interface. Although the LCD is great the turn dial is very hard to control. I was not only having control issues but was experiencing some odd problems with my mini's main board. The replacement mini mainboard is $69 (January this year) so I wanted something a bit better.
If you have read on the mini you will know it uses and ARM 32 bit processor. Most of the open source 3D printers use the ramps 1.4 which is an 8 bit processor. I felt by going to the ramps setup I would essentially be downgrading the hardware so that's why I decided on the smoothieboard because it uses a 32 bit processor. In searching I found the MKS Sbase 1.3 board that uses smoothieware which is open source firmware specifically designed for the 32 bit processor.
Step 2: Steppers
The connectors for the stepper motors use the JST-XHP which is the standard and are the same ones the mini uses. You can just plug in the wires to the corresponding X, Y, Z and extruder (E0) connectors on the new board. Some of the directions of the steppers will need to be reversed but what's great about smoothieware is the firmware works in conjunction with a text file located on the sd card. The direction of the stepper can be reversed by editing the text file on the SD card so there is no need to re-flash firmware or re-pin the wires. Only two of mine needed to be revered as they were going the wrong direction.
Step 3: End Stops
The smoothieboard uses a three pin connector for the end stop. Signal (S), Ground (G) and Voltage (V) which is labeled directly on the board with a (S, G & V). This is for an optical end stop setup which would use all three wires. The mini uses a two pin mechanical end stop, Signal (S) and Ground (G). Do not connect to the voltage pin as it may burn the board. I made my own pigtail connector wires as you can see in the picture which goes from a two pin to a three pin connector so I could just plug in the mini end stop wires to my pigtail harness and then into the X-, Y- and Z- connectors otherwise known as X min, Y min and Z min. I found a connector kit on ebay for around $8 which had all the connectors and pins and more than I needed for this project. Depending on how you wire them they may or may not be reversed for the firmware on the smoothieboard. Go on the smoothieware web site (http://smoothieware.org/) it shows you the M code on how to test the end stops for correct operation using pronterface. If reversed this can be edited in the config file on the SD card also. All of mine needed to be reversed in the config file.
Step 4: Thermistors and Remaining Connectors
Most of the remaining connectors have to be cut and connected to the proper screw connectors on the smoothieboard with the exception of the bed and hot end thermistors. There are four locations for thermistors, I connected my hotend thermistor wire to the white connector located next to the green screw in connector and my bed connector to the black connector. The locations can be edited in the config file on the SD card. A sample config file can be downloaded from smoothieware.org and you can edit as necessary.
The STL files I used for this project were a side plate for my modified 8mm gantry, which had provisions for a panel mount usb connector as well as microSD card slot, a mount plate for the 3.2 MKS touchscreen and some board and fan spacers.
The hardware I used was M3 x 16mm screws and M3 nuts.
Well that's pretty much it. Your setup and choice of additional hardware may be vastly different from mine and some may choose to use the ethernet port on the board (I use Octoprint) so I didn't upload any of my printed files. If anyone is interested in the 3D files I used I can certainly upload them to thingiverse.
Also wanted to mention Is that this board is capable of a dual extruder and hotend setup. E1 or extruder one is disabled in the config file I used. The smoothieware web site is a great reference for the settings or changes.
Step 5: MKS 3.2 or 2.8 Touchscreen
I wanted to add this section because I've been asked a few times. There is a package to download for the screen itself, this is copied onto an sd card or usb and inserted into the screen sd card slot or usb port. The MKS-config shows my settings I used for the LCD to communicate to the board. My LCD was V2.0.
Step 6: Files
I posted the files I printed for the conversion on Thingiverse. None of these are "perfect" remix or modify as needed.