Welcome, this is my first Instructable and hopefully it is easy to understand and you can make your own. If you have any questions please feel free to contact me and maybe I can help you. I will try to make this extremely simple to follow with this step by step process provided by Instructables.
Step 1: Obtaining Parts
Below is a list of parts that you will need to get for this project. Most you can get cheap on Ebay but there is a long delay time on shipping on it. If you can not find the part you just need to do some research for a replacement part. I will also have the datasheet for the STN 1110 below as well, this is what i used to design this. Most of the parts listed are either the surface mount version or are hard to find. I will also have some comments to help find the parts for some of the general parts.
Step 2: Circuit Board
Here is the circuit board and the Gerber files to make the board it can be a bit confusing so that is why I'm including the Gerber file. If you view the datasheet for the STN 1110 it'll help you understand where all the connections go. The Bluetooth module that is listed in the parts has a logic level converter circuit built in. If you are using a 5V logic level Bluetooth version the UART out of the STN1110 can be directly connected to the Bluetooth module.
Step 3: Assembly
Now it's time to assemble everything and test. mine is the original prototype and is a bit of a mess but if you view ScanTools website about their STN 1110 you will see it works with all ELM327 software. Go ahead and test yours out, there are some test ports for the different voltages to verify that they are correct. they aren't labeled but if you follow the traces you can figure out what they should be.