ELECTRONICS
Single-Transistor Non-Contact AC Voltage Detector
A worked electronics example, rendered live. Open it in the AI editor and adapt it to your own case.
Make this diagram your own.
Open it in the AI editor with a prompt pre-filled — keep what works, change what doesn't.
CASE ANALYSIS
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.
Key decisions
- Use a single NPN transistor Q1 as a high-gain switch driven by the antenna signal.
- 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.
- Power the circuit from a 9V battery and include C2 for supply decoupling.
When to reuse this
Use for DIY non-contact voltage testers, educational transistor circuits, or low-cost mains detection pens.
Open this example in the editor →
Tweak it with chat, export PNG/SVG, or fork it for your own use case.
circuit-single-transistor-non-contact-ac-voltage-detector-pen-with-a