Measure AC VOLTAGE through a capacitor

[Ricardoo]

The schematic I'm using is attached:
I intend to acquire the signal from a bedsheet, which functions as a humidity and presence sensor. For that, I'm using the circuit from the schematic, which consists of basically an oscillator 555 and the ESP32. At the terminals of the bedsheet sensor, I'm obtaining an AC voltage of 150mV in the absence of people and 200mV in the presence of people. I want to find a way to detect this variation on the ESP32. What would be the best approach?

[MicroController]

All the cheap&easy ways to measure/estimate capacitance rely on measuring the time it takes to charge or discharge the capacitor to a certain voltage. You can use a 555 in which the sensor is used as the time-defining capacitor; then you can decide to use astable mode, which gives a frequency correlated to the capacitance at the output, or you could use monostable mode, where the ESP would trigger a "measurement" and time the resulting output pulse from the 555 whenever it needs a measurement.
You can also use a comparator or Schmitt-trigger and "emulate" the 555 in software, charging/discharging the capacitor via an IO pin until the comparator triggers.
It may be possible to use the ESP''s ADC to directly measure the capacitor's voltage after charging/discharging for a certain time.
These methods may not be very precise but can be sufficient to discriminate two capacity values or detect a certain change in capacitance.