|
This project is a four-band audio spectrum level indicator designed to provide a visual representation of how the energy of an audio signal is distributed across the audible frequency range. Unlike a conventional VU meter that only measures the overall amplitude of the input signal, this circuit separates the audio spectrum into four distinct frequency bands and independently evaluates the level present within each of them. The result is a dynamic LED display capable of highlighting the spectral composition of music, speech, or any other audio source in real time. The design exclusively employs resistor and capacitor values belonging to the standard E12 series, ensuring easy component availability, straightforward replication, and consistent performance. The signal-processing section is composed of four independent active band-pass filters fed in parallel by the same input signal. Each filter is realized as a cascade of two identical first-order active band-pass stages in order to improve selectivity and increase attenuation outside the desired frequency range. Every stage is based on an operational amplifier in inverting configuration, with a series RC network between the input signal and the inverting terminal and a parallel RC network in the feedback path between the output and the inverting terminal, while the non-inverting terminal is connected to ground. The RC network at the input produces the high-pass behavior, whereas the RC feedback network produces the low-pass behavior, resulting in a band-pass transfer function. Cascading two identical sections allows the overall filter to exhibit a second-order response with improved frequency discrimination. All filtering stages are designed with an approximate voltage gain of 3.3 V/V, providing sufficient amplification while maintaining a simple and robust topology. The overall audio bandwidth from 20 Hz to 20 kHz is divided into four logarithmically spaced regions defined by the frequencies 20 Hz, 112.47 Hz, 632.46 Hz, 3556.56 Hz, and 20000 Hz. Logarithmic spacing is particularly appropriate for audio applications because it distributes the bands more uniformly with respect to human frequency perception. The center frequency of each band is calculated as the geometric mean of the corresponding lower and upper cutoff frequencies, resulting in center frequencies of 48.99 Hz for the first band, 249.81 Hz for the second band, 1376.69 Hz for the third band, and 8615.94 Hz for the fourth band. These frequencies correspond to the points of maximum sensitivity of the respective channels and can be used to verify the correct operation of the filters. By applying a sinusoidal signal at one of these frequencies, the corresponding channel should exhibit the strongest response while the remaining channels show significantly lower activity. The output of each band-pass filter is connected to an independent peak detector whose purpose is to extract the envelope of the filtered waveform and generate a slowly varying DC voltage proportional to the amplitude, and therefore to the energy content, present within the selected frequency band. This stage prevents the subsequent LED display from following the rapid oscillations of the audio waveform itself and instead allows it to indicate the perceived signal level. Since each filter possesses its own dedicated peak detector, the energy measured within one frequency region remains independent from the energy measured in all other regions. The detected voltages are then routed to a multi-threshold display section based on operational amplifiers configured as comparators and referenced to a resistor-ladder network. As the detected voltage increases, progressively higher thresholds are exceeded and additional LEDs illuminate, producing a bar-graph indication proportional to the level present within the corresponding frequency band. An additional feature of the design is the inclusion of relays that allow the comparator and LED display section to be switched between the outputs of the four different band-pass channels. This functionality makes it possible to selectively route each filter output to the display circuitry and therefore test every band-pass filter individually. By activating the relays and applying sinusoidal signals at the corresponding center frequencies of 48.99 Hz, 249.81 Hz, 1376.69 Hz, and 8615.94 Hz, the response of each filter can be verified independently, allowing straightforward validation of the filter tuning, peak detection stage, comparator thresholds, and overall system behavior. The final result is a complete four-band audio spectrum indicator combining logarithmically distributed frequency channels, cascaded active band-pass filtering, independent envelope detection, threshold-based LED bar displays, and exclusive use of standard E12 component values, providing a practical and informative visualization of the spectral distribution of an audio signal while remaining simple to reproduce and experimentally evaluate
|
