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Audio Spectrometer (180166)

Status: Finished
January 7, 2019
in Elektor magazine | April 2019 | Find it here
Two versions of an MSGEQ7-based audio spectrum OLED display module:
  1. In the shape of a module to be plugged on top of some other system
  2. As a USB-powered module with 3.5 mm jack connector
Combines an ATmega328P-AU with an MSGEQ07 seven band equaliser chip to display the frequency contents of an audio signal as a colorfull bargraph.

The audio spectrometer displays a 7-band audio spectrum as 7 bars on an OLED display. The heart of the project is the seven band audio equaliser IC that divides the audio spectrum into seven bands: 63 Hz, 160 Hz, 400 Hz, 1 kHz, 2.5 kHz, 6.25 kHz and 16 kHz.
This module is designed in two different PCB versions, one is suitable for internal use which can be fixed inside the audio box or amplifier while the other having easy interface for external connections can be fixed into small enclosure and can be connected anywhere to any audio system with a standard 3.5mm audio cable. Both PCB’s are same in size which is compact for audio system.
180166-1 module is designed to use internally and hence consist of power section and input connections on PCB for easy interfacing. 180166-2 module is designed to use externally and has a USB connector to provide 5V power for PCB and special compact 3.5mm audio jack for connecting audio out or smartphone output to it. The full colour OLED is the main attraction of the modules where multiple patterns can be displayed.
Specification and Features 180166-1:
  • Compact 0.96 inches full colour OLED display
  • 7 audio bands of audio visualization
  • Operates on 8V to 12VDC
  • Small and compact design with low power consumption
  • 4 selective patterns through PCB solder jumpers
Specification and Features 180166-2:
  • Compact 0.96 inches full colour OLED display
  • 7 audio bands of audio visualization
  • Operates on USB 5V
  • Small and compact design with low power consumption
  • Onboard switch for different patterns selection
  • 3.5mm Audio jack for audio input
Hardware 180166-1:
K1: Power supply connector
K2: For connecting audio input
JP1, JP2: Set to view different Display modes
Hardware 180166-2:
K1: Micro USB connector for 5V power
K2: 3.5mm Audio jack for audio input
S1: Switch to select different display modes.
Circuit description:
The heart of both the hardware’s is a seven band audio visualization IC2 (MSGEQ7). IC MSGEQ7 is a CMOS chip that divides the audio spectrum into seven bands, 63Hz, 160Hz, 400Hz, 1 kHz, 2.5 kHz, 6.25 kHz and 16 kHz. The seven frequencies are peaks detected and multiplexed to the output to provide a DC representation of the amplitude of each band. No external components are needed to select the filter responses. Only an off chip resistor and capacitor are needed to select the on chip clock oscillator frequency. The filter center frequencies track this frequency. Other than coupling and decoupling capacitors, no other external components are needed. The multiplexor is controlled by a reset and a strobe, permitting the multiplexor readout with only two pins. The multiplexor readout rate also controls the decay time (10% decay/read), so no external pins are needed for this function.
The microcontroller makes this project more advanced by controlling the MSGEQ7 chip and readout display LCD or LED display. LED bar graphs are more suitable as they have good illumination. OLED can be used for low power consumption. The MCU generates the strobe signal to select all seven bands sequentially using internal 7 bands multiplexer of IC MSGEQ7. The multiplexed output from MSGEQ7 is then given to the ADC pin of MCU which provides current signal level of the selected frequency. The bar graph are multiplexed and driven through respective transistors by MCU.
All this make a compact hardware design which also supports low power consumption with accurate audio spectrum.
Circuit 180166-1:
This circuit consists of IC3 (KF50BD-TR) for generating 5V for board operation and 8V to 12V DC supply can be connected to IC through connector K1. IC1 ATmega328P controls and drives the IC2 and OLED responsible to generate the audio spectrum on OLED. JP1 and JP2 can be set as 00,01,10,11 for four different patterns on OLED by shorting JP1 it can be set to “0” and by not shorting JP1 makes it “1” and same applies for JP2. Audio input can be connected at connector K2. Since board is designed to be used inside the audio system, patterns need to fixed one time as per user requirements. But this can be re-fixed after changing jumper setting when required. 
Circuit 180166-2:
The circuit can be fixed inside small enclosures with proper fitment of connectors and switch.
This circuit is powered through 5V DC from USB connector K1. And the board operates on same 5V supply. IC1 ATmega328P control and drives the IC2 and OLED responsible to generate the audio spectrum on OLED. Different patterns can be selected by pressing the switch S1. Connector K2 3.5mm audio jack provides easy connection of audio input to the circuit.    
The software is written in Atmel studio for ATmega328p MCU which operates at an internal clock frequency of 8MHz. The software controls the multiplexer of IC2 to read different frequencies on ADC pin PC0 by generating required pulses on STROBE and RESET pins of IC2 that are connected to PC1 and PC2 of MCU respectively. These frequencies are then graphically displayed on a 0.96 inch RGB OLED that is connected to PORTB of MCU. The MCU communicates with the OLED using the I2C protocol.
In this project we have provided 4 display modes namely: white bar graph, green-yellow-red bar graph (each bar is made of 3 colors), thin blue bar graph and thin red bar graph.
Building the Prototype:
Solder the components as per the circuit diagram and connect the external peripherals to the indicated connectors. Only after all soldering and assembling is complete connect the RGB OLED to the LCD connector provided on the other side of PCB.
180166-1 Testing:
  • Solder the jumpers JP1 and JP2 to view the different display mode;
  • Connect the audio signal to connector K2
  • Power up the circuit through connector K1 by Appling 8v to 12V DC
  • Check the audio spectrum on display
  • To change the pattern, switch OFF the module and change the jumper settings and start again.
180166-2 Testing:
  • Connect the audio signal to audio jack K2
  • Connect PC USB or 5V USB adapter to K1
  • Check the audio spectrum on display
  • To change the display pattern, press switch S1. The Display mode changes on each switch press
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Version 2 - USB powered with 3.5mm jack connector

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