Digital Volume Control

Most modern TVs and VCRs have a digital volume control for either incrementing or decrementing the volume. In digital control you have to press a switch instead of rotating a knob like in old sets. This versatile digital volume control circuit can be used for tape recorders, amplifiers. TVs etc.

In the circuit IC 555 is used as an a stable multivibrator, which produces a clock pulse of 11 Hz frequency at pin 3. I’his clock pulse is fed to the counter (IC 4510) through a combination of gales. Normally. N1 and N2 will not allow any clock pulse because one of its input is connected to ground through 10k resistor.

When the INCR or DECR switch is pressed, the corresponding gate allows the clock pulses to the counter through the gate N3. N4 gate is used to select the counter IC 4510 in up-down mode. Pin 10 of IC 4510 is normally high but goes low on decrementing the volume.

Each time the INCR/DECR switch is pressed, the count of the counter is incremented or decremented accordingly. This counter has four outputs which give 16 combinations.

The outputs of IC 4510 are fed to the IC4514, which is a 4-16 line decoder. It has 16output lines that go high according to any of the 16 possible combinations of inputs. However, we have used only 12 combinations out of 16.

All the outputs of the decoder are connected to three 4016 ICs, which are SPST switches. Each IC contains four SPST switches, which turn on when the corresponding input is high. Hence, by using three such ICs we have 12 different outputs. One terminal of each switch is connected to a resistive network, and the other terminal is connected to one common output line. When INCR switch is pressed, the resistance between input and output (in case of tape recorder ‘input’ is the output point of preamplifier and the ‘output’ is the input point of power amplifier) decreases and so the volume increases. The reverse is the case when the DECR switch is pressed.

Till now gates a, b. c, d, e, f do not come in the picture. Now we will discuss the advantage of using these gates. If we keep on pressing the I NCR switch, the counter’s count will increment. But after 16 steps it will repeat the count. The same holds true for the DECR switch. It means after getting maximum/minimum volume, you will again, gel minimum/maximum volume, and this is the most irritating aspect of the circuit.

Gales a to f provide solution to this problem. Consider that the volume is maximum. At this time the count is 1,1 1,1 (01 to 04 all high). This is the only case out of the 16 possible cases when Q1 and Q4 bits arc high, and we shall utilise this condition. As soon as Q1 and 04 go simultaneously high, the output at pin 10 of gate C will go low. As a result no high going pulse will occur by pressing the INCR switch.

Hence, there is no affect of pressing the INCR switch in case of maximum volume. This is what we need. In case of minimum volume, the output of counter is 0,0,0,0 (01 to 04 all low). This is the only case (out of 16 possible cases) when 01 to Q4 are low. Now for this condition, outputs of gate e and f will be high, which is the input of NAND gate d. A low output at pin 11 ensures that there is no effect of pressing the DECR switch. So once the volume is minimum, the DECR switch is made ineffective and the volume remains minimum until we press the INCH switch.

Power supply

The circuit works on +5V and -5V DC. The circuit uses a 6V-0-6V, 500 m A transformer with AC input. Bridge rectifier and 1000mF, 25V capacitors are used for rectifying and filtering. Regulators 7805 and 7905 are used to generate +5V DC and -5V DC respectively.

The main advantage of the circuit is that it avoids the scratch noise problem.

Useful hints

Remember that there are 12 steps from minimum volume to the maximum. For each step, certain resistance comes into picture. If you want a particular output at each step, use a 100k preset for each of the resistor network. Adjust the corresponding preset for a particular step. Once you adjust all the presets, your circuit is ready forgiving a predetermined output at each step.

For stereos, you have to use one more circuit for controlling the volume of the second channel.

Readers’ Comments:

The circuit of the ‘Digital Volume Control’ published in EFY Jul. ’92 issue can be greatly simplified by removing the four most expensive ICs (IC6-IC9) and using the following circuit instead.

ICS when we again switch on the circuit?

SAINI G.S. HAPYE

Mohali

Only 12 outputs of IC 4514 have been used and the gates a,b,c,d,e,f are

The main advantage of this circuit is that it gives sixteen steps instead of twelve and the distortion is very low.

T.S. SHANKAR Hyderabad

The author, Mr R. Ravichandran, replies:

Your circuit will work fine for the first two steps. On the third step, i.e. for 0011,2.2 kilo-ohm and 1 kilo-ohm resistors will come in parallel and the resultant will be less than 1 kilo-ohm. The volume output will be lesser in this case. This is repeated for the other steps also. Hence, this circuit does not give a linear output.

‘Digital Volume Control’ project published in July issue is somewhat different from all the DVC circuits published earlier in EFY. The author may please clarify:

1. For stereo channel (where dual pot is used) is one more identical circuit required or can I save some ICs?

2. Do we require one more IC 4016 for remaining 13th to 16th step?

3. How to get visual indication for min-max volume by connecting LEDs at output pins of IC6?

4. Suppose we switched off the power supply when output of CD4510 is 1001, what will be the output of used with reference to the 16th output. What about the counts from 13th to 15th? Won’t the output come suddenly to the minimum as output of IC 4514 changes from 12th to 13th count on pressing INCR switch?

Why two different typos of regulator ICs (7805 and 7905) have been used in power supply instead of two of the same type?

What is the total cost of the project?

RAJEEV DWIVEDI Lucknow

The author, Mr R. Ravi Chandron, replies:

For stereo, connect three more 4016 ICs in parallel to IC7-IC9.

Additional steps are not possible as we are using 10-step counting. After 9th step INCR button is disabled by IC3(a).

One can use the following circuit for visual indication;

Tech Editors Note:

1. Please note that the volume control is meant only for 10-step control (0 through 9)

2. Tactile switches should be used for INCR. and DECR. function for boundless operation.

The changes in volume, if any, can be avoided (after switching ‘off’ and ‘on’) by connecting reset line of 4510 through R-C network.

The volume will not increase above 1001 and will not decrease below 0000, since IC3(a) senses the 1001 and IC4 (e,f) senses the 0000. This will avoid sudden change in volume also.

We are using CD4016 for analogue signal switching. So we should give both +5V and -5V supply, and the signal level should not exceed the IC supply voltage.

The cost of the project is around Rs 150.

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