I want to handle 8 and more buttons or switches with only a few MCU pins and receive interrupt whenever some of the buttons is pressed (even if some switches are active).
I'd like to use PISO shift register to read data from buttons and switches and place voltage dividers with different output voltages on each button (shown at image below). Output of all voltage dividers will be connected to the derivative amplifier which should register any voltage change whenever any button is pressed.
Because more then one button can be pressed at the same time and supply voltage for voltage dividers will be 3.3 V, I'll need to detect voltage change less then 0.3 V, but when I make derivative amplifier with bigger gain, I have a large noise (i have negative voltage source for op amp made by inversing charge pump - image below - is possible that the noise is generated here?).
I would like to help with choosing components values for derivative amplifier for the smallest noise and the bigger gain.
Thank you all for your suggestions. You have right about circuit complexity, so continuous checking of buttons state will be the best way.