Safety

A microcontroller’s pin is not an oscilloscope probe input. It has no protection against high voltages and no isolation.

• Stay in range. Keep the signal between 0 V and the board’s supply: 5 V on an Arduino Uno, 3.3 V on an ESP32 or Pico.
• Share a ground. Connect the signal’s ground to the board’s ground.
• Never connect mains, anything powered from mains without isolation, or a signal whose size you do not know.
• When unsure, measure first with a multimeter, or reduce the signal with a voltage divider.

Over-voltage can destroy the board and, through the USB cable, harm the computer.