상황
A handheld non-contact AC voltage detector uses a capacitive antenna to pick up AC field; the signal is amplified by two NPN transistors to trigger an LED and buzzer.
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01
Use capacitive coupling from antenna to first transistor base
02
Two-stage NPN common-emitter amplifier for sensitivity
03
LED and buzzer connected in parallel from VCC to collector for low-side switching
Suitable for battery-powered non-contact voltage testers and educational electronics projects needing a visual and audible alert.