Build Your dB Meter: 3-Level Sound Indicator

Description

The schematic below reacts to sound pressure levels ranging from approximately 60 to 70 dB. The sound is detected by an 8 ohm speaker, boosted by a transistor circuit and one section of the LM324 operational amplifier. Alternatively, a dynamic microphone can be utilized, though the speaker was found to be more responsive. The remaining three sections of the LM324 quad operational amplifier function as voltage comparators and drive three indicator LEDs or incandescent bulbs, spaced roughly 3 dB apart. An additional transistor is necessary for the incandescent bulbs as depicted with the lower lamp. I employed 12 volt, 50mA lamps. Each light corresponds to approximately a 3dB variation in sound intensity, such that when all three lights are illuminated, the sound level is about four times greater than the level required to light one lamp. The circuit’s sensitivity can be adjusted using the 500K potentiometer, ensuring that one lamp activates at a defined reference sound level, while the other two indicate approximately a 2X and 4X increase in volume.

During operation, without an input signal, the DC voltage across pins 1, 2, and 3 of the operational amplifier is around 4 volts. The voltage at the positive inputs to the three comparison circuits (pins 5, 10, 12) is roughly half a volt less due to the 1N914 diode’s voltage drop. The voltage at the negative comparator inputs is approximately 5.1 and 6.5, set by the 560 and 750 ohm resistors respectively.

When an audio signal is present, the 10uF capacitor connected to the diode charges toward the peak audio level generated by the op-amp output at pin 1. As the volume grows, the DC voltage on the capacitor and the positive comparator inputs also rise, and the lamp turns on when the positive input surpasses the negative input. Conversely, as the volume decreases, the capacitor discharges through the parallel 100K resistor, and the lamps extinguish. The response time can be altered by utilizing a larger or smaller capacitor.

This circuit demands a sufficiently filtered power supply; it is susceptible to even slight changes in supply voltage, necessitating a large filter capacitor directly connected to the 330 ohm resistor. I successfully operated this circuit with an unregulated wall transformer power source, but required a 4700uF capacitor. It performed optimally with a regulated supply, utilizing only a 1000uF capacitor.

Circuit diagram