Most ATmega32U4 boards work, but you need to verify that they support keyboard emulation; ATmega328P boards like Arduino Uno don't work.
ESP32 is strongly recommended because it's significantly more powerful than ATmega32U4. This project uses an ESP32-WROOM-32 board.
2. 4 piezoelectric sensors.
3. 8 100kΩ resistors.
4. (Optional) 4 bridge rectifier chips such as [DB107](https://www.diodes.com/assets/Datasheets/products_inactive_data/ds21211_R5.pdf) (see the Additional Notes section for details).
7. Wood planks and cutting tools (only if you need to make your physical taiko drum from scratch). If you have an aftermarket taiko or a Big Power Lv. 5 drum, you can use them directly.
You may need to fine-tune some parameters like `SAMPLE_CACHE_LENGTH`, `HIT_THRES`, `RESET_THRES`, and `sensitivity`. See the following section for details.
Set `DEBUG 1` (this disables the keyboard output and sends signal values from the serial port), flash the firmware, roll on one of the 4 areas of the drum, and visualize the graph from the output of the serial monitor. The hit threshold should be lower than your heaviest hit on the drum, and the reset threshold should be greater than the trough between roll hits. The reset value should also be below the hit value.
For maximum runtime speed, the `cache.h` library has been optimized to work with `SAMPLE_CACHE_LENGTH` window sizes of powers of 2. That means 2, 8, 16, and 32, etc. Practically 16 is the best value for Arduino, but if you have a powerful microcontroller that samples the input at the speed of at least 4000Hz or more, you can change the value to 32 for a smoother (in other words, less noisy) curve.
Not all piezoelectric sensors are the same, and due to installation errors, the captured signals from the 4 sensors may vary significantly. The sensitivity values are multipliers to normalize the differences. In the following example, the right-don area generates a much higher value than the rest 3, so you can adjust `sensitivity` to `{1.0, 1.0, 0.5, 1.0}` to eliminate the issue.
Note that the installation of the sensors is very critical. You should make sure that the sensors are firmly attached on the wood and located properly.
Without biasing the voltage of the piezoelectric sensors, their output voltage range is about -5V to +5V. However, the ADCs of the analog inputs only accepts positive voltage values (0-3.3V for ESP32 and 0-5V for ATmega32U4). When they receive a negative voltage, it's simply truncated to 0.
It's usually okay for normal electronic drums because we're just losing half of the input energy and it doesn't influence how we calculate the hitting time. But it can cause problems for *taiko* drums, especially with slow processors like ATmega32U4.
In a taiko drum, all the 4 vibrating pieces are connected together, meaning that if you hit left-don, the processor also receives signals from left-kat, right-don, and right-kat. If the left-don piezoelectric sensor generates a negative voltage at the beginning and is truncated by the ADC, it will cause a minor "delay" of about 3 to 4 milliseconds, and the processor could incorrectly treat this hit as a right-don, a left-kat, or even a right-kat, whichever sends a highest positive value.
Using a bridge rectifier, all negative values are converted to positive. In other words, it's like the `abs()` function, ensuring that we don't lose any negative voltages.
