Battery-Operated Portable Meter Indicate Sound Level

Published:2011/8/26 3:16:00 Author:Amy From:SeekIC

This Design is about a battery-operated, portable soundlevel meter (Figure 1). It is portable and can detect the source of noise in automotive engines and power trains, factory machinery, and residential HVAC (heating/ventilation/air-conditioning) systems.



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

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

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.

In the circuit, IC2 provides a logarithmic 30-dB display, with an adjustable voltage reference and an accurate 10-step voltage divider contained.

The high-impedance input buffer accepts signals from ground to within 1.5V of the positive supply, then 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 is used as a sensitivity control, while resistor R6 sets the voltage reference for IC2.

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