LEDs indicate sound level

Stephen Kamichik, Ile Bizard, PQ, Canada; Edited by Paul Rako and Fran Granville -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.

LEDs indicate sound level figure 1

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).

Talkback buttonIn this circuit, the input resistance is 10.8 kΩ. Capacitor C3 limits the high-frequency response of the amplifier. Capacitor C5 is a filter capacitor. Capacitor C4 couples the output of the amplifier to the load, P1.

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.

Read more design ideasIn the sound-level-meter circuit, IC2 provides a logarithmic 30-dB display. IC2 also contains an adjustable voltage reference and an accurate 10-step voltage divider.

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.

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