The post explains a very handy little decibel meter circuit which may be built by any new hobbyist at home.
Description
Decibel (dB) is a unit of sound pressure. In this particular circuit three LEDs or lamps are employed to indicate the pressure of a available audio input. When the three lights are ON, the audio level would be 4x. When two lights are ON it would be 2x and when one light ON it would be x. the circuit is built around a quad OP-AMP LM324.
One section of the IC is being used as an amplifier whereas the other three OP-AMPs have been used as voltage comparators.
Here in this case the audio sensor is a standard 8ohm speaker. One side of the speaker is grounded and the other pin of the speaker is connected to the negative pin of a 10μF electrolytic capacitor. The positive pin of the electrolytic capacitor is connected to the base of a switching transistor of high gain. In this case 2N2222 or 2N3904 is recommended.
The first OP-AMP which is used as an amplifier as a trim pot of value 500K connected between output and inverting input. By adjusting this potentiometer the value of the output is adjusted. In the comparator circuit a potential divider circuit is employed in all the inverting input as shown in the circuit. The output of the amplifier is connected to the non-inverting pins of all the three comparators as shown in the circuit. As soon as the audio signal is available, the 10μF capacitor starts getting charged. As the audio level increases the DC voltage at non-inverting comparator input start increasing and the lamp sequentially get turned ON depending on the audio input available and the adjustment of the potentio divider chain.
Description
Decibel (dB) is a unit of sound pressure. In this particular circuit three LEDs or lamps are employed to indicate the pressure of a available audio input. When the three lights are ON, the audio level would be 4x. When two lights are ON it would be 2x and when one light ON it would be x. the circuit is built around a quad OP-AMP LM324.
One section of the IC is being used as an amplifier whereas the other three OP-AMPs have been used as voltage comparators.
Here in this case the audio sensor is a standard 8ohm speaker. One side of the speaker is grounded and the other pin of the speaker is connected to the negative pin of a 10μF electrolytic capacitor. The positive pin of the electrolytic capacitor is connected to the base of a switching transistor of high gain. In this case 2N2222 or 2N3904 is recommended.
The first OP-AMP which is used as an amplifier as a trim pot of value 500K connected between output and inverting input. By adjusting this potentiometer the value of the output is adjusted. In the comparator circuit a potential divider circuit is employed in all the inverting input as shown in the circuit. The output of the amplifier is connected to the non-inverting pins of all the three comparators as shown in the circuit. As soon as the audio signal is available, the 10μF capacitor starts getting charged. As the audio level increases the DC voltage at non-inverting comparator input start increasing and the lamp sequentially get turned ON depending on the audio input available and the adjustment of the potentio divider chain.
A dynamic microphone can be employed instead of a speaker but it has been observed in practice that the performance of the speakers as a decibel transducer is often better than that of microphone. However, it is to be noted that the speaker needs, a rigid mounting and the power supply applied to this particular circuit demands noise less filter DC. In fact decibel is a ratio metric term and a logarithmic expression so if a circuit is to be built to discretize the audio level more steps of light sources need to be employed which demands more stages of comparators and ratio metric tuning of the potential dividers employed in the inverter input of the comparators. One number LM324 can offer further four stages and it cost very nominal. However, a calibrated decibel meter should be used while tuning the resistance change of the potential divider in order to design and accurate sub standard decibel meter having bar graph representation.
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