Could you do similar to diagram 2, but instead of an N-FET use a P-FET between the battery and first resistor in the potential divider?
That's a great idea! Unfortunately I don't have a P-FET lying around, so cannot try it right now.
Or use a zener/TVS diode instead of second resistor to clamp the voltage instead of dividing (more robust).
Not sure I understand this point. Which resistor would you replace with a diode?
You could probably increase the 82K and 10K resistors to be much bigger
That's what I thought initially, but this stackoverflow post dissuaded me. The argument there is that the measurement will be wrong, if the input current is not enough to charge the internal cap within the measurement period. But I've done some testing now, and measurements done with 820k and 100k agree well with what my voltmeter measures, so I'll go with this solution!
a fresh alkaline 9V battery is actually 9.6V or more, not 9V.
Indeed! 9.6V * 10k/92k = 1.04V
is still below 1.1V, so I should be fine in this case :)
9V battery voltages droop noticeably when under load because of their high internal resistance. Make sure to measure under the same conditions.
This is a good point!
My firmware will be pretty monotonic though, basically:
So, the load should be always the same at step (2).
Call me dumb, but... to me that just sounds like an insecure network, labeled "secure" as long as someone physically guards all the ethernet ports
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