The level indicator turns out to be very easy to assemble. Even a person with trembling and inexperienced hands can assemble it :) Put a resistor from about 1 to 22 kilo-ohms - this will be enough. The diode put KD226. This rectifier diode is any that can withstand the entire load, of course with some margin. Diodes VD3-VD6 are silicon, with a direct voltage drop of 0.7 ... 1 V and a permissible current of at least 300 mA.
A slightly more complicated circuit is able to show five different signal levels, but they can be reduced, for example, to two, or increased.
However, when increasing, it should be remembered that by increasing their number, the power consumption of the entire indicator also increases, and the more it goes to the indication, the less it will reach the speaker, therefore, if you go too far with the number of levels, dips in sound may appear.
In general, it turned out to be a very simple and interesting design of the LED sound indicator. Instead of dim darkness, lighting effects appeared in the room.
The radio designer came in a bag:
Details: 
The board is one-sided, without metallization, made with high quality, soldering is easy, part designations and denominations are marked: 
The photo shows that the board is different from the board displayed on the seller's lot - there is a J3 connector
Instruction and diagram: 
Scheme in high resolution
Soldered. Here's what happened:
Do not scold for soldering - I have not soldered anything on seals for 27 years. First experience.
There are no extra parts included.
When I soldered, three misunderstandings emerged.
1. It is not clear why there is a jumper connector J3? There is no connector or jumper included in the kit. When turned on, it is somehow incomprehensible that only half of the LEDs (red and below) work. Soldered (shorted) contacts J3
2. Resistor R9. The printout says 560 ohms. In the set - 2.2 kOhm. I installed an MLT resistor from old stocks, as indicated in the diagram - 560 ohms. I thought that the Chinese mixed up something. When turned on, the two lower yellow LEDs - D1, D2 - were constantly on. I soldered the resistor - I took a 2.2 kOhm resistor from the kit - it began to work as it should.
Change in circuit - correct resistor
3. If the extreme red LED lights up and burns constantly, then the resistor R5 starts to heat up to 60 degrees. Strange.
The power supply of the circuit is 9-12 Volts. Apply 12V to power. Everything works fine. The trimmer resistor can be used to set the maximum displayed signal level. The minimum level, if a 1.9 Volt signal is applied to the device: 
Hence the conclusion - at a standard supply voltage of 9-12 Volts, it is better to connect the indicator to the ULF outputs, and not after the pre-amplifier or to the ULF input after the volume control.
LED glow scale is logarithmic. Can't be used as a low battery indicator. If you connect the headphone output cell phone at maximum input volume, then a maximum of 6 yellow LEDs are lit.
Then I decided to experiment with reducing the supply voltage. Conclusion - the lower the supply voltage - the more sensitive the device. It worked fine from 5 V - the red LEDs in this case were also on from the cell phone. If the voltage is reduced to 3 volts, the LEDs are dim but do not blink. Apparently this is the limit. So I would not power from a voltage less than 5 volts.
Conclusion: a simple, interesting radio constructor. You can equip them with some homemade ULF. Cons - inconvenient board mounting - only one mounting hole. The board (due to the socket and microcircuit) is quite high. If you put two boards in parallel, then the distance between the LEDs of both channels will be quite large.
This circuit is a simple level indicator based on the popular and inexpensive LM3916 chip. The device is perfect for a mixer, amplifier or. It allows you to visually control the level sound signal, thanks to which we can avoid overloads and the distortions associated with them.
circuit diagram
Wiring diagram for the LM3916 chip A linear AC voltage signal rectifier operates at the input; it is built on the basis of operational amplifier TL081, which allows you to maintain high accuracy even with input signals of the order of several tens of millivolts. The design of the board allows you to cut it into 2 parts and solder at an angle of 90 degrees. This will make it easy to make an indicator for mounting on the front panel, and for two channels at once - stereo.
About the functions of radio elements
Resistor R4 (2.2 k) limits the current of the LED, and R5 (4.7 k) acts as an "artificial mass" for the operational amplifier U2 (TL081). The input impedance of the system is determined by the value of R1 (470k). Elements R1 (470k), R2 (470k), R3 (10k), C4, D11 (1N4007) and D12 (1N4007) are the piping of the op-amp amplifier U2 (TL081), together they form a rectifier. The circuit must be powered with 9-25 V. The average current consumption is 10 mA at 12 V.
Assembling and setting up the LED indicator
PCB 3916 The indicator is collected on printed circuit board. Installation should begin with the installation of one jumper. In the future, you should install the elements R2 and R3, lying under U1 and R1, located under U2. The order of soldering the remaining elements is arbitrary, but it is better to solder the panels for the microcircuits first, since it will be harder later due to the very large compaction of the radio elements. If you want to make a stereo version of the indicator, you can cut the board in the place between U1 and LED, soldering both parts at a right angle. This will allow you to place 2 level indicator boards close to each other (as in the photo).
Homemade LED indicator of audio signal PCB files
The drawing of the board and the location of the parts on it can be downloaded in this
An LED signal level indicator that imitates a pointer indicator is not a new idea, and it would seem that what can be done here is new? Well, in this regard, I did not invent anything .. I even find it difficult to indicate the source. The goal is different: to make a simple circuit, on the available elements. The circuit does not even have the ubiquitous microcontrollers. Moreover, it’s not just to solder the board, but to make a complete design that can be installed in the amplifier without damage appearance. And also, based on this scheme, make your own version of the indicator, taking into account skills in electronics, or, for example, color music. For this purpose, the indicator is made on two boards: the LED control board and the indication board. Within the framework of this article, I propose 3 options for the indicator, we will conditionally call it “arrow”, “lamp 6E1P” and “arc”. There are also 2 scale illumination options (A and B) to choose from. And all this can be done on 5mm, 3mm or SMD 0805 LEDs. Like any other, this scheme has its advantages and disadvantages. Advantage: cheap element base, with great interchangeability, tolerances, relatively simple circuit. Display options, as they say, for every taste. Disadvantages: the selection of many elements, otherwise you would have to be tied to one type of LED. Small dynamic range, i.e. on a powerful amplifier at low volume, the indicator will be “silent”. Visual bifurcation of the “arrow”, which is caused by smooth switching of the LM3915 comparators in the “point” mode. Elimination of this phenomenon is possible, but requires the complication of the scheme. High density and small thickness of traces on the board. It is solved by buying ready-made boards, but I did it myself using a photoresist.
The circuit works as follows. The input signal is applied to VT1. Level input signal regulated by R1. After amplification and rectification, the input signal is fed to the input of the LM3915. LEDs are directly connected to the MS outputs (1 line). Through transistor switches on VT2-VT11, an additional 6 lines of LEDs. Transistor keys are used, because. The thermal resistance of the MS case is 55°C/W, which allows a maximum power of 1365 mW at an ambient temperature of 25°C. However, we will not delve into the boring world of numbers, I will only say that no more than 2 LEDs can be connected to each output of the LM3915. Otherwise, the MS will overheat. The S1 button switches the display modes "column" and "dot". The S2 button turns on additional lines of LEDs, which makes it possible to implement 2 more modes of operation of the indicator. As can be seen from the diagram, many elements (R and C) must be selected. This can be attributed to the scheme's disadvantage and advantage. Selection allows you to use any LEDs, not tied to Vpit. 12V and adjust the brightness of the indicator and backlight LEDs to your taste. R6 ensures the glow of the "arrow" at "zero" in the absence of an input signal. As a rule, the selection of R6 is not required when the circuit is powered by 12V. If the “arrow” on “zero” is not needed, then R6 is not installed. By selecting R7, we set the required brightness of the LEDs, connected directly to the LM3915, this is according to the scheme HL7, 14, 21, 26, 35, 42, 49, 56, 63, 70. The smaller R7, the greater the current through the LEDs, the minimum allowable value of R7 is 20 kOhm . Resistor R8 adjust the brightness of the backlight LEDs. Power R8 is not less than 1W. Resistors R9-R18 adjust the brightness of the remaining LEDs. Approximately 10 kΩ for 1000 mcd LEDs, 1 kΩ for 200-300 mcd LEDs. Capacitor C3 can adjust the inertia of the "arrow". The device is powered by a source of stabilized voltage 12V with a current of 0.2-0.3A for the mono version. The supply voltage can be increased up to 18V.
External design and differences between indicator options. Externally, the design is set out in the video report. I will add that by selecting the current of the LEDs, you need to achieve a balanced glow of the indicator and backlight. Then the indicator will look nice. Illumination option "A" looks more beautiful than "B", but more difficult to manufacture. Find the stencil for the indicator in the LAY file with the board. There is no need to “mirror” boards and stencils when printing. Mount the indicator in the amplifier in any convenient way, outside the front panel window. Do not place near hot elements. You can slightly tint the glass of the front panel to hide possible minor defects in the external design. The indicator input is connected in parallel with the output of the volume control or the input of the final amplifier. The setting consists in setting the "arrows" of the indicator to + 3db with a tuning resistor R1 at the rated power of the amplifier.
I draw your attention to the fact that the sizes of indicator boards are different and the size of the board is much larger than the working window of the indicator. On the "Arc" indicator, the number of yellow and red LEDs is used 26 each. for the stereo version. This is not reflected in the diagram, but assembly and adjustment are no different. Also in the backlight various options 3 to 10 LEDs are used (see LAY). This is also not reflected in the diagram, so as not to cause confusion.
List of radio elements
| Designation | Type | Denomination | Quantity | Note | Shop | My notepad |
|---|---|---|---|---|---|---|
| U1 | LED driver | LM3915 | 1 | To notepad | ||
| VT1 | bipolar transistor | KT315A | 1 | To notepad | ||
| VT2-VT11 | bipolar transistor | KT361B | 10 | Any PNP | To notepad | |
| VD1, VD2 | Diode | KD522A | 2 | 1N4148, any pulse | To notepad | |
| HL1-HL6 | Light-emitting diode | DFL-3014BD-1 | 6 | blue | To notepad | |
| HL7-HL62 | Light-emitting diode | DFL-3014GD-1 | 56 | green | To notepad | |
| HL63-69 | Light-emitting diode | DFL-3014YD-1 | 7 | yellow | To notepad | |
| HL70-HL76 | Light-emitting diode | DFL-3014RD-1 | 7 | red | To notepad | |
| C1-C3 | Capacitor | 1 uF | 3 | To notepad | ||
| R1 | Trimmer resistor | 50 kOhm | 1 | To notepad | ||
| R2 | Resistor | 220 kOhm | 1 | To notepad | ||
| R3 | Resistor | 3 kOhm | 1 | To notepad | ||
| R4 | Resistor | 10 kOhm | 1 |
About a year ago, I got the idea to assemble a 12-220 volt voltage converter. For implementation, a transformer was needed. The search led to the garage, where the Solntsev amplifier was found, which I assembled about 20 years ago. Simply removing the transformer and thus destroying the amplifier did not raise a hand. The idea was born to revive it. In the process of reviving the amplifier, a lot has changed. Including output power indicator. The scheme of the former indicator was cumbersome, assembled on K155LA3, etc. Even the Internet did not help to find it. But another very simple, but no less effective output power indicator circuit was found.
LED indicator scheme
This scheme is well described on the Internet. Here I will only briefly describe (retell) about her work. The output power indicator is assembled on the LM3915 chip. Ten LEDs are connected to the powerful outputs of the comparators of the microcircuit. The output current of the comparators is stabilized, so there is no need for quenching resistors. The supply voltage of the microcircuit can be in the range of 6 ... 20 V. The indicator reacts to the instantaneous values of the sound voltage. The divider of the microcircuit is designed so that each subsequent LED turns on when the input signal voltage increases by v2 times (by 3 dB), which is convenient for controlling the power of the UMZCH.
The signal is taken directly from the load - speaker system UMZCH - through the divider R * / 10k. The range of powers indicated on the diagram 0.2-0.4-0.8-1.6-3-6-12-25-50-100 W is true if the resistance of the resistor R * = 5.6 kOhm for Rn = 2 Ohm, R*= 10 kOhm for Rn=4 Ohm, R*= 18 kOhm for Rn=8 Ohm and R*=30 kOhm for Rn=16 Ohm. LM3915 makes it easy to change display modes. It is enough just to apply voltage to pin 9 of the LM3915 IC, and it will switch from one display mode to another. Contacts 1 and 2 are used for this. If they are connected, then the IC will switch to the "Luminous column" indication mode, if left free - "Running dot". If the indicator will be operated with UMZCH with a different maximum output power, then you only need to select the resistance of the resistor R * so that the LED connected to pin 10 of the IC glows at the maximum power of the UMZCH.
As you can see, the circuit is simple and does not require complex configuration. Due to the wide range of supply voltages for its operation, I used one shoulder of a pulsed bipolar power supply UMZCH +15 volts. At the signal input, instead of selecting individual resistors, R* set a variable resistance of 20 kOhm, which made the indicator universal for acoustics of different impedance.
To change the display modes, I provided for the installation of a jumper or a button with fixation. In the final, he closed with a jumper.
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