On/Off switch question

#1

Sometimes it is required to reboot axoloti from my experience (for example when it doesn't recognize midi controller, doesn't load patch etc.)

To make this fast and convenient I wanna include an on/off switch to my boards.
I power externally via the dc input but usually have axoloti also hooked up to the computer via usb.
So if I want te reboot I have to break power both on dc and usb line. I wanna do this with one switch. Is it possible to break the 5V power coming from usb somewhere on the board?

#2

That's a tricky one.

  • Power from USB goes through R26 (a ferrite), so you could desolder R26 and insert your switch there, but that part is rather small and difficult to reach.

  • There is actually an electronic switch already installed in the USB power line (for turning off USB power while there is external DC available), so you could manipulate its ENABLE-input to turn it off, but that ENABLE is low-active, so would have to provide 3.3V from somewhere while you want the Axoloti switched off.

  • You could interrupt the 5V line from the DC/MIDI-half of the Axoloti PCB to the main "half". That would sabotage the supply from the DC input, while USB power is still switched off (because the ENABLE-line is still powered from the DC input), but the opto-coupler for DIN MIDI reception is powered from the DC/MIDI side and would still deliver a high level to the UART input of the CPU, which could power the CPU well enough to prevent a reset. Or not, since the UART-pin must be 5V-tolerant, so the usual diode to VCC should be missing on that pin… I don't know. Anyway, a 10k resistor before the UART input is the usual measure against parasitic supply and should be easy enough to install at the border between the two PCB parts.

  • You could also connect a reset button to the SWD port instead of cutting power to restart the Axoloti, but that would only restart the Axoloti and not any MIDI controller connected via USB.

So, there are several (and probably more) ways, each with its own advantages and disadvatages.

#3

I did something like replacing R26 with a switch. I don't remember what I did exactly but I essentially cut into the incoming 5V from the USB and placed in the switch, take a look at the schematic. It worked well, I soldered on small wires to the tiny pads and put a drop of hot glue around them.

I also put in a 5V USB battery bank inline with the switch.