Circuit diagrams of low-frequency amplifier input switches. Radio circuits - electronic input switch. Digital signal switching

A switch is a device that allows you to switch (turn on or switch) electrical signals. An analog switch is designed for switching analog, i.e., signals that vary in amplitude over time.

I will note; that analog switches can be successfully used for switching digital signals.

Typically, the on/off state of an analog switch is controlled by applying a control signal to the control input. To simplify the switching process, digital signals are used for these purposes:

♦ logical one - the key is turned on;

♦ logical zero - disabled.

Most often, the level of a logical unit corresponds to a range of control voltages ranging from 2/3 to 1 of the supply voltage of the switch microcircuit; the level of a logical zero corresponds to a zone of control voltages ranging from 0 to 1/3 of the supply voltage. The entire intermediate region of the control voltage range (from 1/3 to 2/3 of the supply voltage) corresponds to the uncertainty zone. Since the switching process is, albeit implicitly, threshold in nature, an analog switch can be considered in relation to the control input as the simplest.

The main characteristics of analog switches are:

The disadvantages of the switch include the fact that the limit

When the generator is turned on, both key elements of the microcircuit are open. C2 is charged through R5 to the voltage at which the DA1.1 switch turns on. The resistive divider R1-R3 is supplied with supply voltage; C1 is charged through R4, R3 and part of the potentiometer R2. When the voltage on its positive plate reaches the switch-on voltage of switch DA1.2, both capacitors will be discharged, and the process of their charge-discharge will be repeated periodically.

To check the serviceability of the light indication elements, you must briefly press the SA1 “Test” button.

When working on inductive load(electromagnets, windings, etc.) to protect the output transistors of the microcircuit, pin 9 of the microcircuit should be connected to the power bus, as shown in Fig. 23.26.

Rice. 23.24. Structural Fig. 23.26. turning on the microcircuit

ULN2003A (ILN2003A) microcircuits (JLN2003A when operating on an inductive load

The UDN2580A contains 8 keys (Figure 23.27). It is capable of operating resistive and inductive loads with a supply voltage of 50 V and a maximum load current of up to 500 mA.

Rice. 23.27. Pinout and equivalent chip UDN2580A

UDN6118A (Fig. 23.28) is designed for 8-channel switched active load control at a maximum voltage of up to 70(85) V and a current of up to 25(40) mA. One of the areas of application of this chip is matching low-voltage logic levels with high-voltage loads, in particular, vacuum fluorescent displays. The input voltage sufficient to turn on the load is from 2.4 to 15 V.

They coincide with the UDN2580A microcircuits in pinout, and in their internal structure with the UDN6118A microcircuits, other microcircuits in this series are UDN2981 - UDN2984.

Rice. 23.29. Structure and pinout of the ADG408 analog multiplexer chip

Rice. 23.28. Pinout and equivalent chip UDN6118A

Analog multiplexers ADG408!ADG409 from Analog Device can be classified as multi-channel electronic switches controlled by a digital code. The first of the multiplexers (ADG408) is capable of switching a single input (output) to 8 outputs (inputs), Fig. 23.29. The second (ADG409) - switches 2 inputs (outputs) to 4 outputs (inputs), fig. 23.30.

The maximum closed switch does not exceed 100 Ohms and the supply voltage of the microcircuit.

The microcircuits can be powered from a bi- or unipolar power source with a voltage of up to ±25 V; accordingly, the switched signals must be within these ranges in sign and amplitude. Multiplexers are characterized by low current consumption - up to 75 μA. The maximum frequency of switched signals is 1 MHz.

The load resistance is at least 4.7 kOhm with its capacity up to 100 ηF.

Shustov M. A., Circuitry. 500 devices per analog microcircuits. - St. Petersburg: Science and Technology, 2013. -352 p.

Switcher switches up to four different stereo sources audio frequency. It is designed for installation at the entrance preamp audio frequency of the audio center. Switching is quasi-touch, using four switching buttons without fixation. Indication of the number of the enabled input using a single-digit LED seven-segment indicator(readings from "0" to "3").

The role of the switching device is performed by a two-channel four-position multiplexer. Schematic diagram shown in the figure. The quasi-sensory device is made on the basis of a four-phase trigger D1 - K561TM3. Four buttons S1 - S4 are connected to its inputs. Initially, when the power is turned on, all the triggers of the microcircuit are set to the zero position, since the contacts of the buttons S1-S4 in the initial unpressed state supply logical zeros to all inputs “D”.

In this case, zeros are also set at the outputs of the triggers, and the first input is turned on, because zeros are supplied to the control inputs (pins 10 and 9) of the multiplexer D2 through resistors R6 and R7 and the first channels of the multiplexer are opened. At the same time, these same zeros are supplied to the inputs of the decoder D3 and the H1 indicator indicates “0”.

When pressing the S1 button, the position does not change. When you press the S2 button, a one is sent to pin 7 of D1 through R3, and at the same time, a zero is sent to the common inputs of C1 (pin 5) through S2. As a result, the state from input D of the second flip-flop is transferred to its output and the second flip-flop of microcircuit D1 is set to the single state. In this case, a unit is set at pin 10 D1, which is supplied through diode VD2 to pin 10 D2 and pin 5 D3. As a result, the multiplexer closes its first channels and opens the second, connecting input 2 (X2) to output (X5). The number “1” appears on the indicator.

When you press the S3 button, one goes through R4 to input D of the third trigger (pin 13), and zero goes to the general input C1 (pin 5). As a result, the second trigger, previously set to one, returns to zero, and the third goes to one. In this case, one is set at pin 11 of D1, which is supplied through diode VD3 to control input 2 (pin 9) of D2 and to pin 3 of D3. As a result, connector X5 switches through the internal channels of multiplexer D2 to the third input (connector X3), and the number “2” is displayed on indicator H1.

When you press the S4 button, the fourth trigger goes into the single state, and the third, or some other one that was turned on before, is set to the zero position. As a result, a unit appears at pin 1 of D1 and through diodes VD1 and VD4 it is supplied simultaneously to both control inputs D2 and to both inputs D3. As a result, the fourth input (X4) is turned on, and the number “3” is displayed on the indicator.

Thus, pressing any button leads to setting one trigger, to the input D of which this button is connected, to a single state. In this case, any other trigger that was previously set to a single state is forcibly transferred to zero. Therefore, the S1 button is used to transfer all the other three triggers to zero states, and thus the code “00” is obtained at input D2 and the first input is turned on.

Multiplexer D2 is powered by bipolar voltage, the negative voltage supplied to pin 7 should be no more than 5V and no less than 1 V, it serves to transfer the input signal to the linear section of the transfer characteristic of the open channel of the multiplexer, in which the coefficient nonlinear distortion signal pe exceeds 0.01%. In the absence of negative voltage, the SOI can increase to several percent. It must be taken into account that the potential difference applied between pins 16 and 7 of D2 should not exceed 15V (9+5=14V).

In the absence of a K176ID2 decoder or a seven-segment indicator, indications can be made using four LEDs, with which the buttons are illuminated. The LEDs need to be connected via transistor switches to the outputs of all four D1 triggers (the output of the first is pin 2, it is not shown in the diagram).

The K561KP1 multiplexer can be replaced with two K561KP2 multiplexers, using only half of each (K561KP1 switches eight single-channel inputs). The K561TM3 chip can be replaced with a K176TM3. K176ID2 can be replaced with K176IDZ or KR514ID2, but in this case the power will have to be reduced to +5V. KD522 diodes can be replaced with KD521, KD503, or even D9 or D220-D223.

If the H1 indicator with common cathodes is used, you need to connect its common pin to the common wire and apply a logical zero to pin 6 of D3.

The conclusion suggests itself: we need to turn our single-beam oscilloscope into a two-beam one, then we can observe its own signal on each beam. Devices that allow such a desire to be fulfilled are called an electronic switch. We will get acquainted with some options for an electronic switch.

So, an electronic switch. It is connected to the input probe of the oscilloscope, and the signals under study are sent to the inputs (there are two) of the switch. Using switch electronics, signals from each input are fed in turn to an oscilloscope. But the oscilloscope scan line for each signal shifts: for one signal, say, the first channel, upward; for the other (second channel) - down. In other words, the switch “draws” two scan lines on the screen, each of which shows its own signal. As a result, it becomes possible to visually compare signals by shape and amplitude, which makes it possible to conduct a wide variety of equipment tests and identify cascades that introduce distortion.

The cascades are switched on alternately by a multivibrator made on transistors VT3 and VT4, to the collectors of which the emitter circuits of the transistors of the amplifier stages are connected.
As you know, during operation of a multivibrator, its transistors alternately open and close. Therefore, when transistor VT3 is open, resistor R4 is connected through its collector-emitter section to the common wire (plus the power supply), which means power is supplied to transistor VT1 of the first channel. When transistor VT4 opens, power is supplied to transistor VT2 of the second channel. Channels are switched at a fairly high frequency - about 80 kHz. It depends on the ratings of the parts of the multivibrator timing circuits -C3R12 and C4R13.

It is enough now to apply an AF signal to the input of the first channel and the upper scan line will reflect its shape (Fig. 25, c). And when the same signal (multiple in frequency) is supplied to the input of the second channel, the “calmness” of the second line will be disrupted (Fig. 25, d). The scope of the image of a particular signal can be adjusted with the appropriate variable resistor (R1 for the first channel and R10 for the second).

Some of the switch parts are placed on a board (Fig. 26) made of foil fiberglass, and some are located on the walls and front panel of the case (Fig. 27).

Now release the oscilloscope’s “MS-MKS” button, thereby setting the sweep duration to about a thousand times longer. Two lines will appear on the screen (Fig. 28, d) - two rays. The upper beam should “belong” to the first channel, the lower one to the second. This position is corrected with variable resistor R5.

Next, set the slider of the variable resistor R1 to the upper position according to the diagram and apply a signal from the AF generator (say, with a frequency of 1000 Hz) to the terminals XT1, XT2. The signal amplitude must be at least 0.5 V. The upper beam will immediately “blur” (Fig. 29, a). If the lower beam turns out to be “blurred”, swap the beams with variable resistor R5. By moving the slider of resistor R1, select the span of the “track” equal to 2... 3 divisions. Using the oscilloscope sweep duration switches and the sweep length knob, try to achieve a stable image of several sinusoidal oscillations on the screen (Fig. 29.6). This is not so easy to do, since there is practically no synchronization and it is difficult to implement - after all, several signals (pulse and sinusoidal) are received at the oscilloscope input and the sweep is not able to select any of them.

The second method is that the scan is synchronized external signal with an amplitude of at least 1 V from the AF generator with which the equipment is supposed to be tested. We have already talked about a similar synchronization method, we hope that you will be able to press the necessary buttons correctly and send a signal to the “INPUT X” jack.

And one more piece of advice. To be able to examine signals of small amplitude, you need to use variable resistor R5 to bring the beams as close as possible and switch to a more sensitive range of -0.05 V/div. or even 0.02 V/div. True, in this case, the scanning lines may become somewhat “blurred” due to the noise of the transistors and various interferences.

What is the idea behind this option? There is a socket on the back wall of the oscilloscope to which the sawtooth voltage of the sweep generator is output. Here it will control the switch: during one stroke of the “saw” the transistor of the amplifier stage of the first channel will open, during another stroke the transistor of the second channel will open, etc. The convenience of this method of switching, first of all, is that it allows you to consider oscillations much more wide band frequencies compared to the previous version. It is not difficult to verify what has been said by assembling, testing and comparing both switches in operation.

The diagram of the variable part of the switch is shown in Fig. 30. A trigger is assembled on transistors VT3 and VT4, which has two stable states. Depending on the state in which the trigger is currently located, either resistor R4 or R7 is connected to the common wire of the switch, which means that the input transistor of either the first or second channel is open - as in the previous version of the switch.

To transfer the trigger from one state to another, a short pulse of positive polarity must be received at its input (connection point of capacitors SZ, C4). Such a pulse is removed from the Schmitt trigger, made on transistors VT6 and VT7. In turn, the Schmitt trigger is connected to a limiting amplifier assembled on transistor VT5 - to its input (terminal XT7) and a sawtooth voltage is supplied from the oscilloscope. Moreover, for normal operation of the entire pulse shaper, a signal with an amplitude of 0.5 to 20 V can be supplied to the XT7 terminal. The “excess” signal is limited by resistor R17, so the emitter current
transition of transistor VT5 does not exceed the permissible value over the entire range of specified signal amplitudes.
All transistors of the additional device can be the same as in the previous switch, diodes - any of the D9 series, capacitors - KLS (SZ, S4), KM, MBM (C6), resistors - MLT-0.25 or MLT-0.125.

The printed circuit board drawing for this switch option is shown in Fig. 31, The design of the switch remains the same, with the exception that an additional XT7 clamp is installed on the rear panel of the case, which is connected by a conductor to the socket on the rear wall of the oscilloscope.

Testing this switch begins by monitoring the sawtooth voltage at the XT7 terminal. To do this, the “ground” probe of the oscilloscope is connected, as before, to the XT4 terminal, and the input probe is connected to the XT7 terminal (the oscilloscope operates in automatic mode with the input open, the start of the scan is set at the beginning of the lower left scale division). At a sensitivity of 1 V/div. in the extreme right position of the sweep length adjustment knob, an image of one sawtooth oscillation will appear on the screen in the form of an inclined straight line (Fig. 32, a). This image will be saved when setting any sweep duration.

When you move the sweep length adjustment knob to another extreme position, the length of the inclined line will begin to decrease and reach a minimum value (Fig. 32.6).
Using the scale grid, you can determine the amplitude of the sawtooth voltage at the extreme positions of the specified adjustment knob - 3.5 V and 1 V.

Then switch the input probe of the oscilloscope to the collector output of transistor VT7 (or to the connection point of capacitors S3 and C4), and switch the oscilloscope itself to the closed input mode and move the scan line to the middle of the scale grid. A positive pulse should appear on the screen (Fig. 32, c), the image of which in the divisions of the scale grid will remain stable when the duration changes within a wide range, as well as the length of its line. If, when changing the sweep length, and therefore the amplitude of the input signal at the XT7 terminal, the pulse disappears, resistor R18 should be selected more precisely.

At long sweep durations (10, 20 and 50 ms/div), signal distortion will be observed (Fig. 32, d), indicating differentiation of the pulse in the input circuits of the oscilloscope due to insufficient capacitance of the isolation capacitor. The solution here is simple - switch the oscilloscope to open input mode, and connect the input probe to the circuit under test through a paper capacitor with a capacity of 1...2 μF,

After this, the probe with a capacitor is connected in exactly the same way to the output terminal of the HTZ and two scan lines are observed on the screen, as with the previous switch. The sensitivity of the oscilloscope is set to 0.1 V/div. Further work with the switch does not differ from that previously described.

You may want to make sure to alternate scan lines. Then use the oscilloscope buttons to set the longest duration - 50 ms/div. and turn the reamer length knob to the far right position. You will see a point slowly moving either along the trajectory of the upper scan line or along the trajectory of the lower line.

Switches on microcircuits are of no less interest. Figure 33, for example, shows a diagram of the simplest switch on a single chip, developed by Kursk radio amateur I. Nechaev. True, the switch has a relatively low input impedance, which limits the possibilities of its use. However, it deserves attention for its simplicity and interesting principle of operation.

The elements DD1.1 and DD1.2 of the microcircuit are used to assemble a generator of rectangular pulses with a frequency of about 200 kHz. Elements DD1.3 and DD1.4 operate as inverters and make it possible to match the output resistance of the generator with the resistance of the electronic keys that control the passage of signals through the switch channels, as well as to provide appropriate isolation between the channels.

From the outputs of the inverters, the pulses (they are antiphase) of the generator are supplied through resistors R4-R7 to switches made on diodes VD1-VD4 for the first channel and on bottoms YD5-VD8 for the second. If, for example, the output of element DD1.3 is logical level 1, and at this time the output of element DD1.4 is logical level 0, current will flow through resistors R5, R7 and nodes VD5-VD8. The key on these diodes will be open, the signal from the XS2 connector sockets will go to the XS3 connector sockets, to which the X input probes of the oscilloscope are connected. At the same time, the switch on the VDl-VD4 diodes will be closed; the signal from the input jacks of the XS1 connector will not reach the oscilloscope.
When the logical levels at the outputs of elements DD1.3 and DD1.4 change, the signal arriving at the XS1 connector will reach the oscilloscope. The amplitude of the signal coming from the input connectors XS1 and XS2 to the oscilloscope can be adjusted with variable resistors R1 and R2. The distance between the “scan lines” created by the commutator is adjusted by variable resistor R9. When the resistor slider moves up the circuit, these lines diverge, and vice versa.

In order to maximally suppress interference from the pulse generator penetrating the input and output circuits of the switch, a chain of oxide capacitors C2, SZ and trimming resistor R10 is connected in parallel to the power source (of course, with the contacts of the SBI switch closed) - it creates an artificial midpoint.

All diodes, except those indicated in the diagram, can be D2B-D2Zh. D9B-D9Zh, D310, D311, D312. Resistors Rl, R2, R9, R10 are SPO type, the rest are MLT-0.125 or MLT-0.25. Capacitor C1 - BM, PM, KLS or KT, oxide capacitors C2, SZ-K50-3, K50-6, K50-12. Push-button switch - P2K with position fixation. Connectors - any design, for example, used in televisions as antennas. The power source is a battery 3336 or three series-connected elements 316, 332, 343.

Some parts are mounted on printed circuit board(Fig. 34), attached to the cover of a plastic case (Fig. 35) with dimensions of approximately 40X70X95 mm, the power supply is located on the bottom of the case, and the connectors are on the side walls.

Set up the switch like this. The resistor sliders Rl, R2 and R9 are first installed in the lowest position according to the diagram and connected to connector XS3 input probes oscilloscope. By turning on the switch, moving the slider of resistor R10 achieves the minimum level of noise on the oscilloscope screen (it is advisable to set its sensitivity as high as possible). After this, you can apply controlled signals to connectors XS1 and XS2, adjust their range on the oscilloscope screen with variable resistors Rl, R2, and “move them apart” relative to each other with variable resistor R9.

When working with this switch, you should remember that the input resistance of the channels at the upper positions of the resistor sliders Rl, R2 in the diagram can drop to 1 kOhm. Therefore, it is advisable to work at such a sensitivity of the oscilloscope that the sliders of these resistors can be installed as close as possible to the lower terminals in the circuit. Then the input impedance of the channels will be 5 ... 10 kOhm.

Another development of I. Nechaev is a three-channel switch that allows you to study three signals simultaneously. This switch is especially convenient for testing and adjustment various devices with digital chips.

The diagram of a three-channel switch is shown in Fig. 36. It contains three microcircuits and four transistors. A pulse generator is made on transistor VT1 and elements DD1.3, DD1.4. The pulse repetition frequency depends on the ratings of parts C1, C7 and in this case is 100... 200 kHz.

A frequency divider on trigger DD3 is connected to the generator. From the outputs of the generator and divider, pulses are supplied to the decoder, in which elements DD1.1, DD1.2 and DD2.1 operate. The decoder controls the amplification stages assembled on transistors VT2-VT4. The input of each stage receives its own signal under study, which will be visible later on one or another scan line of the oscilloscope. In the collector circuits of the transistors there are inverters (DD2.2-DD2.4), the outputs of which are connected through resistors (R8-R10) to socket XS4 - it is connected to the input signal of an oscilloscope operating in open input mode.

This is how a switch works. At the initial moment, at one of the inputs of the decoder elements there will be a logical level of 0, which means that at their outputs, i.e. at the emitters of the transistors of the amplifier stages, there will be a logical level of I. signal (i.e., there will be a logical level of 0 at the inputs of the switch), the transistors will be closed. Since the absence of input current is perceived by the TTL logic elements as the presence of a logical level of 1 at the input pins, the outputs of all inverters will have a logical level of 0.
If, when checking the operating modes of a digital device, logical 1 levels are applied to the switch inputs (3...4 V for TTL and 6...15 V for CMOS logic), the transistors will open, but the inverter inputs will still logic 1 levels arrive and their signal at the outputs does not change.
This is only possible at the initial moment, before the generator starts working. When the generator starts working, “various combinations of logical levels will appear at the inputs of the decoders. As soon as, say, a logical level of 1 appears at the inputs of element DD1.1, which controls the amplifier stage of the first channel, a logical level of 0 is established at its output and the emitter of transistor VT2 is practically connected to the common wire of the switch (minus the power source). In addition, the logical 1 level from the output of element DD2.1 will flow through the divider R12R13 to the input of the oscilloscope and will form a scan line corresponding to the “zero” level (about 1 V) of the first channel of the switch.

If at this time there is a logical level of 0 at connector XS1, the line will remain in place. When the logical I level connector is fed, the line will deviate.

As soon as the logical 1 levels are at the inputs of element DD1.2, the second channel of the switch comes into effect. In this case, the emitter of transistor VT3 will be connected to the common wire, as a result of which resistor R11 will be connected in parallel with resistor R13 and the constant voltage at connector XS4 will drop. A “zero” scan line (about 0.5 V) of the second channel will be formed.
Next, the levels of logical 1 will be at the inputs of element DD2.1, as a result of which only the emitter of transistor VT4 will be connected to the common wire. The “zero” (0 V) line of the third channel of the switch will appear on the oscilloscope screen.

The “distance” between the channel lines is determined by the values ​​of resistors R11 and R13, and the input resistance of the channels is determined by the values ​​of resistors Rl-R3.

Although the maximum channel switching frequency is 200 kHz, and the frequency of the signal under study does not exceed 10 kHz, along with the monitored signal, the moments of channel switching in the form of a light background can also be visible on the oscilloscope screen. To make this background weaker, you need to minimize the length of the connecting wire between the switch and the oscilloscope, and also reduce the brightness of the image. Reducing the frequency of the generator by doubling or tripling the capacitance of capacitor C1 also helps.

The switch can use transistors KT315A-KT315B, KT301D-KT301Zh, KT312A, KT312B, as well as transistors from older versions MP37 and MP38. Diodes - D9B-D9ZH, D2B-D2E. Capacitor O-KT, KD or BM; S2-K50-3 or K50-12 with a capacity of 10...50 µF for a rated voltage of 5...15 V. Resistors - MLT-0.125.

Most parts are mounted on a printed circuit board (Fig. 37, 38), which is then secured inside a suitable housing. On the front wall of the housing, input connectors XS1-XS3 and output jacks XS4, XS5 are installed. Through a hole in the rear wall of the case, a two-wire power supply is output, which is connected during operation of the switch to a rectifier or 5 V battery.

A properly installed switch does not require any setup. If you want to increase the sensitivity of the switch to the level of logical 1 supplied to the input, it is enough to reduce the resistance of resistors R1-R3. True, this will reduce the input impedance of the switch.

A stereo amplifier is rarely used with only one signal source; to quickly switch different signal sources, it is desirable that the stereo amplifier have several switchable inputs.

In the simplest case, the inputs can be switched using a mechanical switch. But the reliability of a mechanical switch is very relative; its contacts corrode and at some point noise arises, often associated with mechanical action.

In the worst case, acoustic feedback may even occur, in which vibrations from operation speaker systems are transmitted to a worn mechanical switch whose contacts rattle.

In this sense, the electronic switch is much more reliable. The figure shows a diagram of a simple electronic switch three stereo amplifier inputs, with quasi-touch control and LED indication of the included input.

Channel selector circuit

The circuit consists of a control device made on the D1 chip and an electronic switch on the D2 chip.

Rice. 1. Schematic diagram of an electronic input switch for a stereo power amplifier.

The circuit on the D1 chip is a well-known three-phase RS trigger circuit implemented on the K561LA7 chip. Changing the state of the trigger is carried out by buttons S1-S3, which supply logical zeros to its three inputs (the active level is logical zero). Accordingly, there are three outputs (the active level is also zero).

A three-phase trigger can take three states, in each of which there is a logical zero at only one of its outputs. Accordingly, the output of the element is D1.1, D1.2 or D1.3. The trigger state is indicated by LEDs HL1-HL3 connected to its outputs via transistor switches VT1-VTZ.

The keys are made on transistors p-p-p structures, therefore they are opened by logical zeros arriving at their bases from the outputs of logic elements through resistors R4-R6.

The electronic switch is made on a D2 chip of type K561KP1. The microcircuit contains two switches with two directions and four positions, controlled by a digital code supplied to the control inputs. The control code is digital and two-digit. That is, there are only four positions “00”, “01”, “10” and “11”.

Accordingly, channels “0”, “1”, “2” and “3” open. To control the switch, logic levels are taken from only two outputs of the three-phase trigger on D1. As a result, in various states of the trigger on D1, codes “01”, “10” and “11” are obtained.

This is enough to control the K561KP1 microcircuit to switch to three positions (“1”, “2” and “3”).

Input signals from three different signal sources are supplied to paired connectors X1, X2 and X3. Each of them is a pair of coaxial tulip jacks, now widely used in various audio and video equipment.

The output is the same X4 connector, but in practice, if the input switch is placed inside a stereo amplifier, this X4 pair may not exist; simply from pins 13 and 3 the signal goes through shielded cables to the input of the preliminary ULF.

Details and connection

The K561KP1 microcircuit can switch both digital and analog signals. But, when switching an analog signal, it is necessary that it be between the power poles, preferably in the middle (this will result in minimal distortion of the audio signal).

Therefore, the second pin of the minus power supply of the keys (pin 7), which is usually connected to the common minus of the power supply, is here connected to a negative power supply (-5V). Thus, the switch's power supply is bipolar.

There are no problems with this, since preliminary ULFs are usually made using op-amp circuits, also powered from a bipolar source. If the source voltage is more than ±7V, you need to supply power to the circuit through step-down stabilizers, for example, make the source +5V on the 7805 integrated stabilizer, and make the negative source on the idle parametric stabilizer from a 4.7-5.6V zener diode and a resistor. LEDs HL1-HL3 - any indicator ones, for example, AL307 or their analogues.

Input selector for relay amplifier (DIY).

To switch multiple input signals to a power amplifier without constantly tugging at the cords, various types of selectors are used. Below is a schematic diagram of such a selector; 12-volt relays are used as switching elements. The circuit is capable of switching 4 stereo sources sound signal. RCA and relay input jacks are located on one small board, this reduces interference and uses fewer shielded cables. The selection of inputs is carried out by a miniature flip switch with 4 positions. The board also contains a rectifier and a filter capacitance for the power supply. The schematic diagram of the selector is shown below:

The power connector is supplied with an alternating voltage of 9...12 Volts from a step-down transformer. In the diagram after the rectifier we see resistor R* marked 0R or more. This resistance is needed to limit the current when using transformers with more high voltage than 9 Volts. When submitting AC voltage 9 Volts just put a jumper. When applying 12 Volts after the rectifier and smoothing capacitance, the result will be 16.92 Volts, and this is already too much for a 12-volt relay; we install a current-limiting resistor. We estimate the nominal value using the formula: 16.92-12 / relay winding current.

The board configuration looks like this:

In the figure, the yellow dot under the resistor R* indicates the location of the droshk cut in the case of using a current-limiting resistor.

Printed circuit board for relay input signal selector in LAY6 format:

Photo view of the LAY6 format selector board:

RCA stereo connector – 4 pcs.
Relay 12 Volt HK19F-DC12V-SHG – 4 pcs.

Link to product page
4-position switch - 1 pc.
5Pin (2.54mm) connector for connecting a biscuit switch – 1 pc.
2Pin connector with bolt clamp (power connection) – 1 pc.
3Pin connector (connecting the selector output to the amplifier input) – 1 pc.
Imported diode assembly type W04, W06 – 1 pc.
You can also install diode assemblies like DB102, DB103 or similar on the board.
Electrolyte capacitor 470...1000mF/25-35V – 1 pc.
Diode 1N4001 (in parallel to the relay windings) – 4 pcs.
LED 5mm – 4 pcs.
Resistors in the LED circuit 1 kOhm – 4 pcs.
Current limiting resistor 200R 0.25W – 1 pc.
Connectors Input1 – Input4 - 3Pin 2.54mm – 4 pcs. This is if you do not use standard RCA input connectors, but external ones, which are installed not on the selector board, but on the amplifier body.
And one more Vcc connector - for supplying a constant supply voltage to the board; in this case, the variable is not connected, and the diode assembly does not need to be soldered.