ELECTRONICS
2-Transistor Non-Contact AC Voltage Detector with Piezo Buzzer
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 portable non-contact AC voltage detector that senses live wires through a pick-up antenna and sounds a piezo buzzer when voltage is present.
Key decisions
- Used a Darlington pair of NPN transistors (Q1, Q2) for high input impedance and current gain from the antenna signal.
- Included R1 (2.2M) from antenna to ground to bleed static charge and keep the detector off when no AC field is present.
- R2 (10k) from Q2 base to ground ensures Q2 remains off until Q1 actively pulls its base high.
- Added a power switch (S1) in series with the 9V battery to prevent battery drain.
- Used a piezo buzzer directly as the collector load, eliminating the need for a separate current-limiting resistor.
When to reuse this
Use this topology for non-contact voltage detectors, live-wire testers, and educational demonstrations of Darlington pair high-gain sensing.
RELATED EXAMPLES
More electronics examples
MAKE YOUR OWN
Try the diagram makers.
Open this example in the editor →
Tweak it with chat, export PNG/SVG, or fork it for your own use case.
circuit-2-transistor-non-contact-ac-voltage-detector-with-piezo-buzz