Update: I’ve done a trace of the power pins of the USB connnector on the OpenSprinkler PCB and from what I’m able to assess is that both the GND and Vin pins on the USB are connected to the same GND and Vin pads (above the LCD header) without any resistors, capacitors affecting the current or voltage (as far as I can see).
So, now I’m thinking of cutting off the one end of a USB B-Micro cable and connecting the RED and BLACK inner cables (after confirming that they are the GND and Vin leads) and connecting the cable to the pads on the OpenSprinkler.
Is this a bad idea for me-the-ignorant-electronics-tinkerer to be doing some reason?