One GPIO pin for LED, Pushbutton and Buzzer via high-pass filter?

Question

There's Pin as both input (for a switch) and output (for LED) with ATmega32U4? but I also need one of six such pins to drive a piezo buzzer. Due to its capacitive nature, I figured it could be connected to the GPIO as well (has no effect on DC signals):

^{simulate this circuit – Schematic created using CircuitLab}

The idea was to suppress the noise, that would likely originate from switching the GPIO from output-low (to drive the LED) to input-pull-up (for sampling the pushbutton), using a high pass filter with an "odd" 1.5 kHz or so, so sampling at 100 Hz or so does not affect the 4 kHz piezo as much.

Drawbacks (both are acceptable):

• pushing the button lights the LED (and dumps C1's charge to ground; maybe add a series resistor?)
• driving the piezo with a 4 kHz square wave lights the LED at 50 % (maybe this can be a lower duty-cycle due to the filter?)

Does this work and can it be improved?

Answer 1

You can of course, instead of directly driving the LED, add a high-impedance low pass filter, driving a thresholding element (e.g. a transistor, a comparator with a reference, or an elegantly connected diode), which in turn drives the LED.

                                     +3.3V
                                       |
                                       R
                                       |
                                      LED
     +----High Pass----Piezo---GND     |
GPIO-+----------------Switch---GND   |/
     +----R_large---+----------------| 
                    |                |\
                    C                  v
                    |                  |
                   GND                GND