“Draw a 9 V non-contact AC voltage detector with a power switch, pick-up antenna, two BC547 NPN transistors, bias resistors, and a piezo buzzer.”
Read the decisions behind it.
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.
Use this topology for non-contact voltage detectors, live-wire testers, and educational demonstrations of Darlington pair high-gain sensing.