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The principle is very simple turn potentiometer is replaced with a digital volume control circuit. But there are a few things to note. To realize the reader needs to understand the scheme of the circuit shown in the picture. For the images shown are able to lead you to assemble and connect the equipment operational.
Working Arrangement: Digital volume control circuit uses IC 9. Required to operate the voltage regulator circuit for 12 Volt. IC1 (555) is very good to function as a flip-flop. Frequency or period can be determined by memilh resistor values R44, R45 is combined with the capacitor C6. In this series has a 0.3 second period.
IC2 is used to raise or lower the calculation. In this circuit mode is used to raise up and down mode is used to decrease the volume. Meanwhile, IC3, and IC4 has a 16-channel, while the analogue multiplexers serves as an analog switch. IC3 in the circuit is used as an indicator level while IC4 is used as a potentiometer.
Then, as the power in on her, then pressed the switch S1 to reset. When switch S2 is pressed, IC2 raise pulses and output response in the form of leg B, C, and D in CMOS IC2. Output B, C and D control input line IC2 and IC3, and the selected one, 16-channel output, by turning on analogue.
In this circuit, IC4 is used as a potentiometer 15 connected to the resistor (R9 to R23) respectively among the 16 input pins and resistor / capacitor combination with C2, C3 and R7 to the output. Switch S2 is used to raise and switch S3 is used to decrease the volume.
Electrolytic capacitors of 1uF (C4) be used to prevent the occurrence of noise. While resistors R8 and R6 is used to inhibit the voltage at half the supply voltage in order to avoid any distortion audio signal originating from the Preamplifier. While capacitors C2, C3 and resistor R7 is provided to filter the audio. Good work, good luck.

Components list :

R1, 2, 3, 4	: 560R	R16	: 1,5K	R45	: 150K
R5	: 680R	R17	: 3,3K	C1	: 0,01uF
R6, 8	: 100K	R18	: 5,6K	C2, 3	: 100nF
R7	: 1M	R19	: 8,2K	C4	: 1uF/50V
R9	: 10R	R20	: 15K	C5	: 100nF
R10	: 12R	R21	: 33K	IC1	: 555
R11	: 42R	R22	: 56K	IC2	: 74193
R12	: 100R	R23	: 1,5M	IC3	: CD4067
R13	: 220R	R24…R39	: 560R	IC4	: CD4067
R14	: 470R	R40..R43	: 4,7K	IC5	: 7805
R15	: 820R	R44	: 330K	IC6 (N1-N4)	: 7407