Pypilot with ST4000 autohelm

[CapnKernel]

So I’m not so sure on anything anymore. Seems that I even need at 12V signal and not a 5V as I expected..

I think holding one line at ground, and sending 5V on the other will be fine:

 https://www.electronics-notes.com/articl...torial.php

Hmm, the st4000 service manual says:
NMEA+: Intermittent streams of (nominal) 12V pulses
NMEA-: 0V

Source: http://www.ganssle.com/jack/st4000.pdf

That is gold!  Can't thank you enough!  It shows that their NMEA input just goes through an optoisolator, and doesn't have any special electronics to decouple the ground/power.  And if it's good enough for the famous ST4000, it's good enough for me!

The ST4000 schematic shows a PC357 optocoupler on the NMEA input, with two 2.2kΩ resistors in series with the internal LED.  The datasheet shows Vf for the LED is typically 1.2V, and optimal current for the LED is 10mA (Fig 7).  For an input of 12V, the current will be (12-1.2)/1.1kΩ = 9.8mA, so 12V is optimum for this circuit.  At 5V input, the current is (5-1.2)/1.1kΩ = 3.45mA, which will still drive the optocoupler's LED quite nicely.  For the LED's maximum current of 50mA, that would mean a maximum voltage of 50mA * 1.1kΩ = 55V.  Looks like a lot of room for handling overvoltage scenarios.