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