The ZMPT101B produces an isolated, scaled analog waveform related to AC voltage, but a small transformer on an exposed breakout does not make a mains installation safe.
Correct measurement
Sample across complete cycles, estimate and remove the midpoint, then calculate the square root of the mean of squared samples. Peak-to-RMS conversion is valid only for a clean sine wave and fails for distorted waveforms.
Calibration
The potentiometer adjusts gain, not the real voltage. Calibrate against a trusted true-RMS meter at several points, confirm the waveform is not clipped, and keep the module output within ADC limits.
Module limits
Usable range and safety depend on the complete board, components, and enclosure, not the ZMPT101B name alone. Do not use a hobby prototype for permanent mains monitoring without an engineered, compliant design.
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