A Roland Dual-trigger drum pad outputs two signals via a TRS jack.
The main pad sensor is on Tip, rim sensor on Ring and both are grounded to Sleeve.
Reading both sensors requires two Analog pins. Most arduino boards have 8 analog pins, meaning i would be able to have 4 pads wired up. This would be good enough for adding a few drumpads to an acoustic kit, for example.
The Arduino Mega has 16 analog pins - ideal for a bigger setup.
I aim to use an Arduino Mega to read 8 pads and send data to an Arduino Pro Micro
The Pro Micro can act as a USB MIDI device without having to flash anything to it, so it will be perfect for taking the Mega data and sending it as a MIDI command to an attached PC running some drum software.
I am going to explore using SPI to pass data between the two boards. I am thinking about using the Pro Micro as the SPI Master, potentially opening the door to adding extra Megas as slaves to add more pads, or even using a bank of Nanos in place of the Megas.
***August 2020 update***
Another avenue I am going to explore is using some 4051 Expander chips to take the pro micro and add enough pins to not require a slave Arduino. This would cut down on any latency introduced by having an extra layer of communication.
Also, having experimented with Pure Data recently for another project I am going to be using it on a Pi to receive data from the microcontroller and trigger sounds based upon it.
While MIDI will be still the planned communication method I also haven't ruled out using standard serial or OSC. I may even add an option to send all three, and use a switch to select the mode.