I had the feeling that I know what I do but the reality is proving me wrong.
I try to use a signal from my car (LED-status of my automatic gearshift, neutral vs. drive position) with an ATTINY85 (powered with 3.3V).
My schematic is based on using an NPN-transistor BC548 with a resistor beforehand. The signal going into the resistor comes from the car and is based on status-switching between 0.25V and 4.75V.
The transistor then is grounding (or not) PB2/INT0 (waking up from Idle based on rising edge). Internal Pull-Up is used.
This works in general but the input signal which is 'switching' the BC548 is low with 0.25V and high with 4.75V.
The measured high/low-signal at PB2 seems wildly jumping between the states.
I guess the resistor is maybe miscalculated by me. I tried then with 1MOhm, with 3MOhm, with 10kOhm. But always the same issue.
I have the feeling that I do something wrong and hope that you experts here immediately see whats wrong with the setup.
Any hint is welcome.