Our measurement device got a small hardware upgrade: Two status LEDs and a button:
With R1 = U(R1) / I = (U(GPIO) – U(LED)) / 0.010A = (3.3V – 2.0V) / 0.010A = 130 Ohm
We connected the button to GPIO 5 with a pull-up resistor (10kOhm). This way the Raspberry PI can be booted with the button (no software code needed).
Currently only our RPIs are powered by battieres – our switch is connected to the car on-board-socket (through a converter). If the whole messbox needs to run with battery packs only there are adapters available to plug your low-power switch to a regular USB port (battery pack).
In our case (D-Link DGS-1008D) we need 7.5V, 1A to USB adapter – available e.g. here(1).
(1) This was the only one we found online but the seller confirmed that it works for our device: “Yes, I just checked and our adaptor B00HM59B4C will suit the D-Link DGS-1008D. The current draw will be fine and the tip is the correct size.”