DIY simple non-contact electric pen, judging neutral and live wire
The electric pen we usually used is very inconvenient and dangerous as must contact live wires, but it can be easily solved with such a triode amplifier circuit. Just need to close to or touch the rubber surface of the wire to judge whether it is charged (AC only).
H-shaped inductor instead of contacts.
This size can be stuffed into the shell of some electric pens.