This device uses a simple 555 timer and some passive components to measure the resistance of cloth. No programming is needed. All components can be ordered from the major distributors.
If the mask is dry, it lights up a green LED. If the mask is moist it blinks a red LED and also beeps with a piezo buzzer. The faster is blinks and the higher the beepers pitch the more moisture is in your mask.
Replace and/or wash moist masks immediately as wearing a wet mask is dangerous to your health.
The electronics was designed in KiCad (design files included) and has all information (LCSC part numbers) to order it assembled with JLCPCBs free SMT assembly service.