Mini Spectrum Analyzer for Audio/Sound by LM3915

Figure 1 

The circuit has been designed to create a spectrum analyzer that will provide an analysis of a sound to determine at various frequencies, the volume of sounds that make up the overall sound spectrum.

• Sound Spectrum – a short sample representation of a sound with the distribution of energy in terms of the amount of vibration at each individual frequency for a particular sound source
• LM3915 – a monolithic integrated circuit that detects analog voltage levels and powers ten LEDs, LCDs or vacuum fluorescent displays, supplying a logarithmic 3 dB/step analog display which can be calibrated logarithmically and is suitable for log measurements as logarithmic bar graph driver
• 4011 – a quad 2-input NAND gate integrated circuit, generally characterized by small fluctuation in voltage supply, very high impedance, outputs that can sink and source, one output can drive up to 50 inputs, high speed gate propagation time, high frequency, and low power consumption
• 4017 – a decade counter where the count advances as the clock input becomes high that may be combined with diodes for some functions such as flash sequences

The device is sensitive enough to determine the sound wave components of frequency and amplitude with the changing of frequency and the width of an acoustic signal. The proportionality of signal width is indicated by the brightness of LED as it turns ON while the color indicates the proportionality of frequency. In order for the red LED to turn ON in strong signal, the sensitivity of the input circuit is adjusted by resistor R2. The middle signal is represented by a yellow LED while the low signal is indicated the green LED.

The 10 LEDs in 3 lines comprise the display unit which is ensured the IC2 as it functions as a counter decoder represented by the two gates ICa-b.the frequency of the counter is being regulated by R6. No LED will turn ON in the absence of any signal in the input. The LEDs will begin to flicker or blink depending on the intensity and tempo of the signal, once a signal has been applied in the input. The values of the resistors R4 & R5 can be varied that will be suitable for the desired requirements. Alternatively, this can be done by placing a 1K ohm trimmer in place of R4 & R5 during the initial regulation and adjustment of the values. It can be eventually removed and replaced with permanent resistors as soon as the desired values are achieved. Additional LEDs can be added in connection to IC2 although this circuit does not precisely measure the input signal.

R1= 1K8Kohm
R2= 100Kohm trimmer
R3= 1Kohm
R4= 100 ohm…..1Kohm
R5= 100 ohm…..1Kohm R6= 100Kohm trimmer
C1= 100nF 100V
D1….10= RED LED
D11….20= YELLOW LED
D21….30= GREEN LED IC1= LM3915
IC2= 4017
IC3= 4011

This audio spectrum analyzer is a user interface component capable of making visible the sound pressure for a range of frequencies over time by taking a sample from an audio data stream and an animated visualization during the play is created in real time. It is ideal for any purpose which includes analysis and identification of human speech, ham radio audio reception tuning, analysis of vocal and instrumental music, evaluation and tuning of musical instruments, analysis of bat echolocation sounds, evaluation and calibration of home audio systems, and analysis and identification of biological sounds. Other uses of the audio spectrum analyzer are in distortion analysis, transfer functions, and digital filtering.

Source:users.otenet.gr/~athsam/mini_audio_analyzer.htm