Another issue raised in the meeting with the project’s supervisor was the energy supply for the left hand, right hand setups and central hardware unit. The central hardware unit must supply power for the splitter and mixer and, additionally, for the ESP32-CU (central unit). The solution the splitter and the mixer was quite straightforward: both need a 9 volt power supply to function and the typical guitar effect pedal is also supplied by a 9 Volt battery. Accordingly, a 9 Volt battery or power input will be used to power most of the central hardware unit.
The greater challenge was to find a suitable power supply for the ESP32s. Using a 9 Volt battery would have been a practicable and comparatively small solution but as an ESP32 only needs 3.3 Volts a 9 Volt battery would also be a very inefficient solution. An Internet research session yielded two better possibilities: using 3 AA batteries or using one 18650 battery. The latter are lithium-ion rechargeable batteries with a voltage of 3.7 Volts. They are actually an ideal solution but are a bit dangerous when carelessly used. That is why the solution using 3 AA batteries was looked at first. At Neuhold a battery case for 4 AA batteries was obtained. Consequently, one battery slot had to be connected by a cable soldered to the contacts. To start off, the battery pack was used to power a LED, and this worked out fine. Next, an ESP32 connected to a ToF sensor was tested. The ESP32 sent the sensor data to the computer using MIDI-BLE so the USB-cable could be disconnected to see if it was really the battery pack that was powering the microcontroller. This worked as well. This made the 3 AA battery pack a suitable option to power the ESP32s in this project. The big disadvantage of using the AA battery pack is its size and unwieldiness. While this is not an issue for the ESP32-CU, it is a great problem for the right hand setup for example where the whole setup should basically fit into the back of the hand and the forearm without interfering too much with the actual guitar playing. This is why using a 18650 battery is re-considered, at least for the right hand setup.
When using the ESP32s on battery supply, it was decided that it would be a good feature to have some kind of battery monitoring, indicating a low battery status to the user. Luckily, there was a good tutorial online explaining how to get an ESP32 to monitor its own battery level. It appeared to be fairly straightforward, involving just two 10k resistors and a few lines of code. The idea was to flash a LED if the battery voltage dropped to a certain level, just like the principle of the power supply of electro-acoustic guitars where a low battery is also indicated by a red LED. Unfortunately, all (or just my) ESP32s seem to have a problem with the analogRead() function and the monitoring did not work as expected. This issue is something to be solved in the future.
Sources:
Everything You Need to Know About the 18650 Battery (commonsensehome.com)
Getting the ESP32 to monitor its own battery level – XTronical
