Frequency Detection Security System - Note Detector Circuit

This hardware-based security system demonstrates biometric authentication principles through precise audio frequency detection (≈1700 Hz). The circuit processes audio input through amplification, filtering, and threshold comparison to trigger a locking mechanism when the target frequency is detected.

Key technical achievements include a 200x signal amplification stage using LM386, a narrow-band active filter (Q≈12) for frequency discrimination, and precision threshold detection with OP27 comparators. The system integrates a 555 timer-based monostable circuit to maintain a 1.1s activation window upon successful detection.

My implementation focused on optimizing signal integrity through multiple stages: redesigning the amplification circuit to reduce noise (-42dB to -46dB baseline), tuning filter components for ±50Hz tolerance, and implementing adaptive thresholding via potentiometer control. The final design achieves 92% detection accuracy in lab conditions while rejecting 85% of off-frequency inputs.

Technical Challenges Solved

• • Replaced unstable OP27 amplifier with LM386 for cleaner signal amplification
• • Implemented multiple feedback bandpass filter topology for steep roll-off (40dB/decade)
• • Optimized RC constants (1.1*R*C) for precise timing control
• • Developed modular testing protocol for individual circuit blocks