Electronic volume control. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Tone, volume controls

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When developing the device offered to readers, the author sought to create an electronic volume control that would correspond in terms of technical characteristics to similar controls using a DAC, but containing a minimum number of scarce radio elements. It took only eight microcircuits to implement it, but the range of functions it performs is quite wide. This is an increase and decrease in volume while pressing the corresponding "+" and "-" buttons, an automatic smooth decrease in volume to zero by briefly pressing the "- Auto" button, the ability to interrupt this decrease by briefly pressing the "+" button, setting the desired a preset volume level at power-up and finally an LED indication of the highest and lowest gain of the volume control.

The regulator has the following technical characteristics: number of regulation channels - 2; volume control range - not less than 60 dB; number of discretization steps - 256; the current consumed from the +15 (-15) V voltage source is not more than 10 (6) mA. The regulation characteristic is linear.

The regulator can be used in simple stereo AF amplifiers and sound reproducing stereo complexes. All volume changes occur in both channels synchronously. Separate volume control by channels requires unjustified complication of the circuit and, moreover, reduces the usability of the volume control. For the same reason, it was decided to abandon step-by-step volume control: with 256 control increments, each individual volume step is hardly distinguishable, and the implementation of such a mode requires additional complication of the circuit. Therefore, it is advisable to entrust the function of equalizing the volume in stereo channels to the stereo balance control, and the step-by-step volume control mode is somehow able to provide this device with a short press of the "+" or "-" button.

Schematic diagram of the electronic volume control is shown in fig. 1. It consists of a control unit and a "Code - volume" converter. The control unit includes: a device to suppress the bounce of the SB1-SB3 buttons on the elements DD1.1, DD1.3 of the DD1 microcircuit, a signal generator for the direction of the account on the element DD1.2 of the DD1 microcircuit and diodes VD3, VD4, an automatic volume reduction trigger on the elements DD2.1 , DD2.2 microcircuit DD2, decoder of the states of the buttons SB1-SB3 and the volume down trigger on the DD1.4 element of the DD1 microcircuit, the pulse generator on the elements DD2.3, DD2.4 of the DD2 microcircuit and a reversible eight-digit binary counter with a preset on the DD3, DD4 microcircuits . Transistor VT1 and LED HL1 form a device for indicating the extreme states of the counter, or, equivalently, the maximum and minimum gain of the volume control. The signals of code combinations from a binary counter are fed to the "Code - volume" converter, made according to the standard DAC circuit on DA1, DA2 and DA3, DA4 microcircuits.

The volume control works as follows. When the power is turned on, a positive voltage pulse appears on the resistor R20, caused by the charging current flowing through it in the capacitor C6. Under the influence of this pulse, information from the inputs of the binary up/down counter preset is rewritten to its outputs and, thus, a code corresponding to any desired initial volume set using the microswitch SA1-SA8 is set on the digital inputs of the DAC. In this state, the transfer output 7 of the counter DD4 has a logic one level, so the transistor VT1 is closed and the HL1 LED is off. If none of the SB1-SB3 buttons is pressed, the output of the counting direction signal generator (connection point of the diodes VD3 and VD4) has a logic zero level corresponding to the counting command to decrease. However, the pulse generator on the elements DD2.3 and DD2.4 does not generate, since its operation is prohibited by a signal corresponding to the level of logical zero coming to it from the decoder of the button states (output 11 of the element DD1.4) through the VD5 diode of the "diode OR" circuit (VD5, VD6).

Rice. 1 (click to enlarge)

When you press the "+" button, the level of logical zero at the output of the counting direction shaper will change to the level of a logical unit (count command to increase) and at the same time the output state of the decoder of the button states will change, instead of the level of logical zero, the level of logical one will appear. As a result, the pulse generator will start to work and the reversible counter will count up until the "+" button is released or until it overflows. In the first case, the input 8 of the element DD2.3 of the pulse generator through the "diode OR" circuit (VD5, VD6) will receive a logical zero signal prohibiting its operation from the output of the decoder of the button states (output 11 of the element DD1.4), and in the second - from the output transfer of a reversible counter (output 7 of the DD4 chip). When the counter overflows, the logic zero level coming from the transfer output enables the operation of the display device and the HL1 LED that lights up will signal that the upper volume limit has been reached.

In the volume down mode, the status decoder is controlled through the debounce node of the SB2 button ("-") or directly from the "-" button, or from the output of the volume down trigger (output 3 of the DD2.1 element) through the diode V02. The logic zero signal to the output of the counting direction signal generator passes through the "diode OR" circuit (VD3, VD4) either from the output of the debounce node of the SB2 button (output 4 of the DD1.3 element) while pressing the SB2 button, or from the output 10 of the DD1.2 element .1 when button SBXNUMX is not pressed. This signal is a command for a binary up/down counter to count down.

When you press the "-" button, the counter counts down until the button is released or until it overflows. When you click on the "-Aut." the up/down counter counts down until it overflows or until the "+" button is pressed, which resets the volume down trigger. A short press on the "+" button in the process of automatic volume reduction stops further volume reduction, and a longer press on the "+" button changes the volume decrease to its increase.

In case of overflow of the counter in the process of lowering the volume, the HL1 LED lights up constantly, regardless of the position of the "-" and "-Avt" buttons, and in the event of a counter overflow in the process of increasing the volume, the HL1 LED lights up only when the "+" button is pressed, so how, when it is released, the logic level at the output of the counting direction signal generator changes and the counter exits the overflow mode.

In this device, the function of the decoder of the button states is performed by the logical element "Exclusive OR", which made it possible to simply and effectively avoid the mode of conflicting commands. So, in particular, when simultaneously pressing the buttons "+" and "-", "+" and "-Aut." or all three buttons together, the same logic levels (logical zeros) are set at the decoder inputs, so it disables the operation of the pulse generator and the volume does not change. By simultaneously pressing the "+" and "-Aut." a forbidden combination is set at the inputs of the volume reduction trigger: at both inputs - logical zeros. Since in this case the trigger loses its trigger properties (a logical unit is set on both of its outputs), then to exclude the mode of conflicting commands, the button "-Aut." connected to the input of the bounce suppression unit of the "-" button through the diode VD1. By simultaneously pressing the "-" and "-Avt." the function of the button "-Avt."

Capacitors C7, C8 are used to increase the noise immunity of a binary reversible counter when changing its operating modes.

In the manufacture of the electronic regulator, resistors MLT-0,125 were used (the values ​​​​of resistors R1, R2, R5-R16, R20 can be in the range of 33 ... 62 kOhm), capacitors - KM-6 and K50-16, buttons SB1-SB3 - homemade random designs, switches SA1-SA8-VDM1-8, and they do not have to have eight groups. You can limit yourself to a switch of four groups by connecting it to the counter preset inputs on the DD4 chip. The inputs of the counter preset on the DD3 chip must in this case be connected to a common wire. Then the minimum resolution of the preset will be equal to 1/16 of the input voltage.

The KR544UD2A microcircuits used in the electronic volume control can be replaced by K574UD1, K544UD1, K140UD6, etc.

The volume control, assembled without errors, practically does not need to be adjusted. If necessary, the regulation rate can be changed by selecting the value of the resistor R17 or capacitor C5.

The regulator is powered by a stabilized bipolar source with a voltage of ±15 V. It remains operational without degrading the parameters when the supply voltage drops to ±5 V. This only reduces the brightness of the HL1 LED.

If necessary, the scheme of the described volume control can be slightly simplified. In this embodiment, the counting direction signal generator is constructed in such a way that the counting command to increase is generated only when the "+" button is pressed when the buttons are not pressed, or when any two or all three buttons are pressed, the counting command to decrease is formed. If, however, diodes VD3, VD4 and resistor R8 are excluded from the circuit and the output 10 of the DD1.2 element is connected directly to the inputs 10 of the microcircuits DD3, DD4, then the counting command to decrease will be generated only if the "+" button is not pressed, and when simultaneously pressing the "+" and "-" or "+" and "-Aut." an increase counting command will be generated, but at the same time, the mode of contradictory commands will be excluded, so the general algorithm of the device operation is preserved.

Author: S. Kolesnichenko, Kursk; Publication: cxem.net

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