Audio Equalizer Using Combinational Logic Circuit

In previous articles, we discussed some audio amplifier circuits. To equalise the audio signal, today we are going to build an Audio Equalizer circuit. This circuit contains four ICs and two potentiometers and some passive and active components.

This circuit can change the tune or melody of an input audio signal with different pitch levels. The characteristics of the signal may vary at different moments in time. The melody which is produced by melody generator IC can be postponed and the pitch of the tune is mottled at a different instance of time. This circuit can generate different sounds from a single melody.

Circuit Diagram

Audio Equalizer Circuit
Audio Equalizer Circuit

Components Required

  • CD4046 IC (x2)
  • CD4051 IC
  • 555 Timer IC
  • UM66 IC
  • Resistor (2.2KΩ, 10KΩ, 220KΩ, 56KΩ)
  • Variable Resistance (5KΩ, 10KΩ)
  • 470uF/25V Electrolyte Capacitor
  • 0.1uF Ceramic Capacitor
  • 1N4148 Zener Diode
  • SL100 Transistor
  • 4Ω Speaker

Circuit Connection for Audio Equalizer

Phase-Locked Loop

CD4046 IC for the phase-locked loop is connected with the 220KΩ resistor (R2 and R4) using pin 12. The offsets are mainly provided by these resistors. A large value of R2 or R4 resistor implies that there is merely a minute offset. To eliminate total silence at the time input is 0V, the R2 resistor offsets the range of frequency.

The frequency at the output from VCO (pin 4) at the time when the voltage at pin 9 is half of the voltage is given.

f=1/ (R1×C1) = 1/(2.2KΩ×4.7uF) = 96.71Hz.

The frequency alters with the variation of the voltage at VCO’s input altered by the potentiometer.

555 Timer IC

The 1N4148 Zener diode is connected to block the path during OFF time while discharging the capacitor. The main concept is to have similar ON and OFF times (50% duty cycle). To cancel out any unwanted noise present in the circuit, a 0.1uF ceramic capacitor is used (not mandatory).

555 Timer output is act as a selection line for the multiplexer, both the outputs of VCO are multiplexed by it.

2:1 Multiplexer

IC CD4051 is working here as a 2:1 multiplexer. Both the output pins of VCO work as inputs for pins 13 and 14 of MUX and a selection line is used to select output from each of these from PLL having the same time.

By adding some additional VCO’s variety of effects can be inserted into the audio and multiplexed them and the speaker get it as an output. Lots of varieties of UM66 are available in the market that produces tones of various music. For example ‘Jingle Bells’.

PCB Design

For removing messy wiring and giving a clean look, I designed a PCB prototype for this project. It is also helpful for troubleshooting that runs great without any errors. To design this PCB board, I used EasyEDA as it is too easy to use. For ordering PCB for this, I prefer PCBWay.

Gerber file for Audio Equalizer Circuit Gerber.

PCB View

Audio Equalizer 2D View
2D View
Audio Equalizer 3D View
3D View

Order PCB From PCBWay

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Working Principle of Audio Equalizer

UM66 melody generator IC is used in this circuit to change the tone that is produced by the melody generator. Phase-locked loop ICs that are IC1 and IC2 in the circuit are used to comprise VCO as well as a Phase comparator.

With the help of the potentiometer, the voltage which is given to VCOs can be changed. The potentiometers that are 5KΩ and 10KΩ are used to modify the VCO input voltage in this circuit. This voltage is also given to IC1 and IC2. The output of the VCOs at pin 4 of the CD4046 is time-multiplexed by a CD4051 multiplexer.

The selected signal for the 2:1 multiplexer is produced by the 555 timer IC that is working as an astable multivibrator. The time which is 4.7 mili seconds is taken by each high as well as low pulse coming from the 555 timer IC. The duty cycle is produced sure to be 50% by bringing a 1N4148 Zener diode. The output is then applied to the base of the SL100 NPN transistor using a 56KΩ resistor.

The input from the collector terminal is given to the tone generator IC UM66. The output terminal of the tone generator is connected to one terminal of a 4Ω speaker and another one is connected to a 5V power supply.

With the varying of the potentiometer, the same tune can listen to in many distinct ways, height as well as pitch. In the same manner, variety of melodies can be changed by just replacing that particular melody IC at the place of UM66.

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