I am familiar with the debounce of a switch connected to GND and swinging between the same voltage levels of the logics:
simulate this circuit – Schematic created using CircuitLab
But I need a circuit to debunce and reduce the voltage from a 24V switch. The output signal must be 3.3V. The switch has one side connected to the 24V rail, and can't be changed.
What about the following circuit? Are the calculations correct?
Consider what happens in the circuit at different moments in time.
Let's say the line is at 0, and you apply 24V input. You're charging the capacitor only through R1, and there is also part of the current that goes via R2 that would otherwise charge C1. It's not constant since voltage at C1 rises exponentially and current via R2 is proportional to it. In theory, 3.3V equilibrium is where charging current via 100k equals discharging current via 16k. Since they all depend on each other, you're gonna have to (probably) compute for charge, and it may involve integrals (or maybe I'm overcomplicating).
When input goes to 0, you have 100k and 16k in parallel to GND, that is pretty simple.
Circuit like this is easy to simulate.
Here, take a look, I simulated with 24V supply and 0V initial C1 voltage and 0V supply and 3.3V initial C1 voltage (link in the first picture is left for you on purpose):
