LEDs indicate sound level

Build a battery-operated portable meter to measure sound-pressure levels.

Stephen Kamichik, Ile Bizard, PQ, Canada; Edited by Paul Rako and Fran Granville -- EDN, August 25, 2011

This Design Idea describes a battery-operated, portable soundlevel meter (Figure 1). Its portability makes it useful for detecting the source of noise in automotive engines and power trains, factory machinery, and residential HVAC (heating/ventilation/air-conditioning) systems.

Resistor R₁ biases the condenser microphone. C₁ couples R₁ to transistor Q₁. Q₁ and R₂ through R₅ form a self-biasing common-emitter amplifier. Resistor R₅ provides negative feedback. Shunt capacitor C₂, in parallel with R₅, increases the amplifier’s voltage gain. The input resistance of the amplifier is equal to (R₂R₃)/(R₂+R₃).


IC₁, a 5V positive-voltage regulator, regulates the 9V battery supply to 5V for IC₂, an LM3915 dot/bar-display driver. IC₂ senses analog voltage levels and drives LED₁ through LED₁₀.


The high-impedance input buffer accepts signals from ground to within 1.5V of the positive supply. You set the sound-level meter to bar mode by connecting Pin 9 to Pin 3. Pin 3 is the V+ pin of IC₂ and connects to IC₁’s Pin 3, which is the device’s output voltage. Potentiometer P₁ functions as a sensitivity control. Resistor R₆ sets the voltage reference for IC₂.