Analog Sound Level Indicator Circuit

Overview

Developed an analog sound level indicator circuit that senses ambient sound amplitude and produces a proportional PWM-driven LED output. The design was fully simulated in LTspice and validated on a breadboard, demonstrating four discrete duty-cycle levels corresponding to increasing sound intensities.

Components & Technologies

  • Sensor & Pre-amp: Electret condenser microphone with a custom driver (6 V bias and coupling capacitor)
  • Amplification: Two‐stage op-amp chain using LM741 (non-inverting + inverting) for ~20–60 mV → multi-volt swing
  • Signal Conditioning: Half-wave rectifier with smoothing capacitor and potentiometer-tuned diode network
  • Classification: 3-level voltage classifier (LM741 comparators + diode-OR) mapping 0–2 V, 2–4 V, 4–6 V, >6 V to discrete DC levels
  • PWM Generation:
    • Square Wave Oscillator (≈1 Hz) → Integrator → Triangular Wave
    • Offset Generator to shift the triangle baseline
    • Comparator to compare DC level vs. triangle → PWM
  • Output: LED driven by PWM; duty cycles of 0%, 25%, 55%, 90% for 10 mV, 30 mV, 40 mV, 50 mV inputs

Key Results

  • Duty Cycle Mapping:

    Input (mV)Simulated Duty Cycle
    100%
    3025%
    4055%
    5090%

  • Validation: Hardware measurements closely matched simulation, with minor deviations due to component tolerances.

Project Workflow

  1. Design & Simulation (LTspice)
  2. Equipment Selection & Cost Analysis (~$20 total)
  3. Breadboard Implementation & Power Analysis
  4. Functional Testing (DSO & multimeter)
  5. Comparison of Simulation vs. Physical Results