LM324 Based LED VU-Meter

LM324 Based LED VU Meter


The circuit was conventionally designed with eight LED audio level meter made out of two low power quad op-amps which utilizes LM324.

Terminology Volume Unit – an entity equal to a decibel for expressing the magnitude of a complex audio; it states the level of complex, nonrecurrent, and non-periodic signals such as music and speechDecibel – a logarithmic unit of measurement that expresses the magnitude of a physical quantity of power or intensity relative to a specified or implied reference levelOp-Amp – a differential amplifier having a large voltage gain, very high input impedance and low output impedanceLM324 – has internal frequency compensated for unity gain, large DC voltage gain, wide bandwidth, wide power supply range, very low supply current drain, low input biasing current, low input offset voltage, large output voltage swing and differential input voltage range equal to the power supply voltage Circuit Explanation

The VU meter is calibrated in volume units that are numerically equal to the number of decibels above the reference level. The circuit is conventional To turn on the LEDs at different audio levels, the 1K ohm resistor is necessary because higher values might cause some LEDs not to function properly even if LED has very low internal resistance. The design is flexible where additional op-amps is allowed and is not limited to LM324 amplifier.

To keep a low level signal input to the circuit, a resistor around 33K ohm may be utilized. The potentiometer can also be employed to adjust the sensitivity of the circuit. The negative audio is used for signal input while the positive input is connected to the main positive rail. The VU meter’s design considers the following such as scale of the instrument, dynamic characteristics, response vs frequency, sensitivity, impedance, harmonic distortion, and overload.


The VU meters were used to monitor audio power levels in recording and broadcast studios, so that the signal can be weakened or amplified to bring it within the optimal range for recording or broadcast. The meter effectively connects in parallel with the input of the receiver or amplifier. In some VCRs, they only included VU meters when it includes a manual level control which is required for recording live music rather than compressed television or radio broadcasts. In earlier times, the VU meter was used in speech research prior to the advent of computers and modern signal processing methods.

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