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ELECTRONICS

2-Transistor Non-Contact AC Voltage Detector with Piezo Buzzer

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UPDATED 2026-09-12
TYPECircuit
USE-CASENon Contact Ac Voltage Detector
EXAMPLE2-Transistor Non-Contact AC Voltage Detector with Piezo Buzzer
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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.

← See more non contact ac voltage detector examples
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