- A generic unbranded electronic dartboard (Our specific unit was called "Playfun electronic dartboard")
- LILYGO TTGO T-Display ESP32 module
- An MCP23017 16-Bit I/O Expander w/ I²C interface
- 5 * 7cm double-sided perfboard
- ~2m of Ø1.1mm wire (preferably solid core)
The hardware is based on a generic Chinese no-name dartboard which (like many of them) is based on a 8x8 button matrix to register dart hits (much like a keyboard). Using the MCP23017 16-bit I/O Expander, the ESP32 can use I²C to scan the matrix and register dart hits. The connectors of this matrix looks like this: 
Note about image above
These two cabels are called FPC-cables. These two FPC-cables connect the 8x8 button matrix (through FPC-connectors) to the MCP23017.
Note
Using the MCP23017, we are able to scan the dartboard every 7 milliseconds. This means that if a dart activates a dartboard-button for less than 7 milliseconds, which was sometimes the case in our testing, there's a possibility that the dart hit will not be registered.
The MCP23017 does support faster I2C frequencies than we are using, but this is only supported when powering the MCP23017 with 5 volts, which would require level converters between the ESP32 & the MCP23017 to keep I2C working.
Possible fixes are to use the SPI-version of the MCP23017, the MCP23S17, or to find a different chip for driving the matrix altogether.
Because the original electronic dartboard matrix is made as one giant flexible printed circuit (FPC) and used a custom connector to connect to the original PCB, we decided to salvage the original FPC connectors and use them to connect the I/O expander to the dartboard matrix as well.
A simplified diagram of the matrix scanning using the I/O expander:
A more detailed overview of the circuit we used for our electronical components:
Although we have made a digital schematic of the electronics of this project and we could have ordered a PCB, due to time constraints we ended up (poorly) soldering all of this together on a 5 * 7cm double-sided perfboard, using some Ø1.1mm wire to connect the chips & connectors together.
We then placed the assembly inside the casing of the dartboard, where the original PCB used to be situated. By careful measuring we made sur e that the placement of the FPC connectors should be
Perfboard front & back
- Red: LILYGO TTGO T-Display ESP32 module
- Blue: MCP23017 16-Bit I/O Expander
- Green: Original dartboard Flexible Printed Circuit connectors