This project is suitable for cars with OBD2 protocol ISO 15765 11 bits 500 Kb/s, most cars have this protocol. The application reads data of the ECU using CAN. The MCP2515 module drives the CAN BUS. The circuit supply of 5V is from the USB, the Arduino is plugged to the PC USB. The app reads 17 most common PIDs. The app calculates the data from the ECU.
The PID button is for reading ECU results for MODE and PID numbers typed in the TextBox next to the button (ie 010D). Button SAVE saves text file of readings in the app folder.
For compiling the Arduino code you need to install a lib from GitHub. Lib for the Arduino can be downloaded from GitHub:  https://github.com/coryjfowler/MCP_CAN_lib
OBD2 plug https://www.ebay.co.uk/

This project is suitable for cars with OBD2 protocol ISO 15765 11 bits 500 Kb/s, most cars have this protocol. The application reads data of the ECU using CAN. The MCP2515 module drives the CAN BUS. The circuit supply is from the car 12V , the Arduino generates the 5V for the LCD and MCP2515. The app reads 12 most common PIDs. The app calculates the data from the ECU. The code requests data when pressing button PID.
For compiling the code you need to install a lib from GitHub. Lib for the Arduino can be downloaded from GitHub:  https://github.com/coryjfowler/MCP_CAN_lib
More PIDs can be added to the Arduino code. The code is a revised code from example in the GitHub MCP CAN LIB library that needs to be added to the Arduino IDE,
OBD2 plug https://www.ebay.co.uk/

This project is suitable for cars with OBD2 protocol ISO 15765 11 bits 500 Kb/s, most cars have this protocol. The application reads data of the ECU using CAN. The MCP2515 module drives the CAN BUS. The circuit supply is from the car 12V , the Arduino generates the 5V for the LCD and MCP2515.
For compiling the code you need to install a lib from GitHub. The code requests the data twice every second. The request is an address 7DF, number of bytes, service #, PID and padding 5 bytes of 0x55. The reply from the ECU is address 7E8, number of bytes, 0x41, PID and 2 bytes of data. The data i calculatedRPM = (byte1*256+byte2)/4.
Lib for the Arduino can be downloaded from GitHub:  https://github.com/coryjfowler/MCP_CAN_lib
CAR SIMULATOR
OBD2 car simulator Using Arduino Nano and MCP2515 CAN driver. It simulate ISO 15765-4 CAN BUS 11 bit, 500 Kb/s. obd_sim.ino is for Bluetooth with ELM v2.1, obd_usb_sim.ino is for USB OBD plug with ELM v1.5.
Only most common PIDs are simulated. The simulator listen to messages from an Diagnostic tool and reply as an ECU. It can be used to test Diagnostic tool or to be used for development.
More PIDs can be added to the Arduino code. The code is a revised code from example in the GitHub MCP CAN LIB library that needs to be added to the Arduino IDE, https://github.com/coryjfowler/MCP_CAN_lib .
The ODB socket I used is: https://www.ebay.co.uk/

Android app uses OBD2 to USB, it has ELM327 that connects to all OBDII protocols. The app reads common PIDs, DTC (faults) and VID. Code is in B4A (basic for android) https://www.b4x.com/

Windows application reads current data. The bluetooth obd2 plug has ELM327 that connects to all OBDII protocols. The computer's bluetooth pairs with the OBD-II plug and connects to the app via COM Port. On windows computer you can find the COM port number in 'device manager' and select the port in the app.

In here  https://en.wikipedia.org/wiki/OBD-II_PIDs#Bitwise_encoded_PIDs  you can find the parameter IDs of data request. A request is 2 hexadecimal numbers and the ELM response is several bytes that contain the data. Every car responds to different request, the app has only 12 popular requests, others can be added.

The bluetooth plug used: https://www.ebay.co.uk/

The USB diagnostic plug with ELM327 v1.5, connected to Android device via OTG adaptor.
https://www.ebay.co.uk/itm/195165625444

You are free to use the circuit diagram and the VB software with no limitations.

The Android app was made using MIT App Inventor which is an easy way to make Android apps. Measure common parameters, DTC (faults) and VID (vehicle ID). The Android device, phone or tablet, have to be paired with the BT plug.
The app APK file, has to be transferred to the device and when opened it instructs how to be installed. .aia file is the app and can be modified in the MIT App Inventor https://appinventor.mit.edu/.
The Bluetooth plug I used, make sure it's ELM327 V2.1: https://www.ebay.
The USB diagnostic plug with ELM327 v1.5, connected to Android device via OTG adaptor.
https://www.ebay.co.uk/itm/195165625444

A rev counter connected to OBD2 diagnostic plug with ELM327. This RPM meter works with all OBD protocols. The Android program is written using B4A (basic for Android). The display is digital and traditional analogue. The app request data from the car ECU every 0.2 sec. The app APK file, has to be transferred to the device and when opened it instructs how to be installed. B4A code is included, if you want to edit the code free IDE can be found at    https://www.b4x.com/.

The Android device, phone or tablet, has to be paired with the Bluetooth plug.
The OBD2 Bluetooth diagnostic plug with ELM327 v2.1: https://www.ebay.

The USB diagnostic plug with ELM327 v1.5, connected to Android device via OTG adaptor.
https://www.ebay.co.uk/itm/195165625444

A rev counter connected to OBD2 Bluetooth diagnostic plug with ELM327. This RPM meter works with all OBD protocols. The Android program is written using MIT App Inventor https://appinventor.mit.edu/. The Android device, phone or tablet, have to be paired with the BT plug. The display is digital and traditional analogue. The app request data from the car ECU every 0.5 sec. The app APK file, has to be transferred to the device and when opened it instructs how to be installed. .aia file is the app and can be modified in the MIT App Inventor.
The Bluetooth plug I used: https://www.ebay.co.uk/itm/Mini-ELM327
If you buy it locally, make sure it's ELM327 V2.1