comparator
I am reading this website and don't understand why this circuit doesn't blow up. If Vin = Vcc then seems to me top opamp A1 would drive low (Vcc > 2/3Vcc and inverting comparator) and the bottom one A2 would drive high (Vcc > 1/3Vcc and non-inverting comparator). So now you have two opamps driving against each other and whichever one overheats first looses.
https://www.electronics-tutorials.ws/opamp/op-amp-comparator.html
So to close my own question, this was solved by the insights of noise on a trace given here
The oscillation happens because of parasitic capacitance between the output of the comparator and it's negative pin. Since I've had the input resistors of this pin close to the source, instead of close to the input, the signal source had a bad time trying to reject the induced noise.
This was solved with better layout and normal hysteresis.
Others solutions that would solve the issue would be lower value for those input resistors (lowering input impedance), or a better comparator IC with much higher input impedance - like the TLC372-EP (bigger output impedance)
If your Arduino is reading both states, 0 and 1, your comparator may be OK. The problem is your inputs. They should not be "~0.5 volts". Instead, try a circuit like this
simulate this circuit – Schematic created using CircuitLab
Best Answer
There is no issue if the output is an open collector type.
I found the text in the article you where referring to: