The scenario
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.
What is in this drawing
Read the decisions behind it.
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.
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