상황
A battery-powered pen detects live AC wires without metallic contact. The antenna picks up the electric field; C1 couples the signal to the base of Q1, which switches on and lights the lamp L1 when AC voltage is present.
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01
Use a single NPN transistor Q1 as a high-gain switch driven by the antenna signal.
02
Add high-value resistors R1 and R4 to provide a DC path and set sensitivity; R1 can be made a 1M potentiometer for adjustable threshold.
03
Power the circuit from a 9V battery and include C2 for supply decoupling.
Use for DIY non-contact voltage testers, educational transistor circuits, or low-cost mains detection pens.