The simplest LED sound indicator. The simplest LED sound indicator Making a segment sound level indicator
Signal level indicators are increasingly being replaced by light indicators. They can be found in modern high-quality radios, tape recorders, and sound reproducing devices.
A simple indicator light can be assembled using several LEDs and transistors. Compared to a dial indicator, such an indicator will have a higher input resistance and high sensitivity, which will allow it to be connected directly to a radio receiver detector or a high-impedance load of an audio frequency signal source.
The LED indicator diagram is shown on the 4th page. tabs (Fig. 3). It consists of an amplifier with transistors VT1, VT2 and a “light” scale formed by seven adjacent LEDs (HL1 - HL7).
While there is no input signal, the field-effect transistor VTt is almost closed - this state is determined by the voltage at the source of the transistor, which, in turn, is set by the adjusted resistor R4. Insignificant current flows in the drain circuit, and the voltage drop across resistor R2 is not enough to open transistor VT2. The LEDs are off.
When a positive (relative to the source) voltage is applied to the gate of a field-effect transistor, this transistor opens more strongly, the higher the voltage. The drain tone changes accordingly, and hence the voltage drop across resistor R2.
A similar phenomenon is observed in the cascade on transistor VT2: the greater the voltage drop across resistor R2, the more the transistor opens, the greater the current flows in its collector circuit. As this current* increases, LEDs HL1 - HL7 light up one by one, starting from the lowest one in the circuit. Here's how it happens.
At the moment the collector current of transistor VT2 appears, it flows almost completely through resistor R12 and the HL7 diode, creating a voltage drop in this section (at point A relative to the common wire) * At a certain current, the santdiode flashes, the voltage on it becomes equal to 1.8... 1.9 V and does not change with further increase in current. In other words, the LED becomes a zener diode.
But as the current increases, the voltage at point A will increase. As soon as it reaches the sum of the voltage drops on the “working” LED and the open diode VD6 (0.7 V), i.e. approximately 2.5...2.6 V, the HL6 LED will flash.
The next LED (HL5) will light up with a further increase in the collector current of transistor VT2, when the voltage at the anode of this diode (at point B) exceeds the sum of the voltage drops on the burning LED and open diodes VD4, VDS. Subsequent LEDs will flash only after the voltage at their anodes (relative to the common wire) increases by approximately 0.7 V compared to the voltage at the anode of the previous one (lower in the circuit) with a veto diode.
When the collector current of transistor VT2 decreases, the LEDs go out one by one from the top, one by one, to the bottom.
The LED indicator has good linearity - this is evidenced by its “amplitude” characteristic shown in Fig. 2 tabs - the dependence of the switching on (ignition) of one or another diode on the input signal level. Linearity is determined both by the accuracy of the selection of resistors R7 - RI2, and by the same parameters of LEDs and diodes.
The indicator is capable of operating not only from a constant voltage at the input, but also from an audio frequency signal. In this case, it is controlled only by positive half-waves of alternating voltage.
In addition to those indicated in the diagram, transistors KP302A, KP303D KP307B, KP307Zh can be used in the indicator
(VT1), KT208K. KT209A - KT20$K, KT501A - KT501K, KT502A, KT502B (VT2), LEDs AL102A - AL102G, AL307A, AL307B, any diodes of the KD102, KDYUZ, D220 series. D223, D226, KD521. The tuning resistor can be SPZ-1, SP5-2, SP5-16, the remaining resistors can be MLT or BC with a power of 0.125 or 0.25 W.
The indicator parts are mounted on a printed circuit board (Fig. 4 on the inset) made of single-sided foil
fiberglass. The LEDs are arranged in a row (Fig. I tab) to form a kind of light scale when the board is mounted on the front panel of a device, say, a tuner.
Setting up the indicator comes down to setting the tuning resistor R4 to such a collector current of the transistor VT2 that the HL7 LED barely glows or is on the verge of ignition.
If it is necessary to reduce the sensitivity of the indicator, you should connect a resistor between its input and the signal source and select its resistance. If the indicator is used to monitor an audio frequency signal, instead of an additional resistor at the input, a capacitor (KLS, KM-1) with a capacity of approximately 0.033 μF is included, and resistors R7 - R12 are taken with values half as large as those indicated in the diagram. If the indicator is connected directly to the output of a powerful amplifier, the cascades on transistors can be removed altogether by connecting any of the above diodes between the left terminal of resistor R6 in the circuit and the output of the amplifier. The cathode of the diode must be connected to a resistor. 
While sorting through the trash in the closet, I accidentally found my last year (autumn 2013) craft - a dial indicator of the sound level on the K157UD2 microcircuit. For some reason, she didn’t want to work for me then, and I threw her far away. And now I’ve decided to finally figure out what’s the matter? After all, the first copy of the device, made that same summer, still works properly.
The article that describes the amplifier circuit on a microcircuit is located, option 2, “Single-supply circuit.” There you can also see the pinout of the K157UD2 microcircuit. I am attaching a diagram with my denominations, the main part of which is the M68501 indicator and its wiring.
I’ll note right away that it can be connected either to exit sound amplifier and entrance. In the first case, the dial indicator will show the power of the output signal (and, accordingly, when the volume is reduced by the control, the arrow will “fall”), and in the second case, the power of the input signal, which is sometimes more useful (for example, visually monitoring the power of the input signal, since if it too much comes in, the signal may begin to distort). In the diagram, some numbers of the pins of the microcircuit are indicated in brackets - this means that you can assemble two identical amplifiers on one chip, and, accordingly, connect two indicators: to the right and left channels (or to the input and output of the amplifier).
It turned out that the cannons did not fire for twenty reasons, and the first of them was that there were no shells. And if we talk about the microcircuit, there were serious problems with its power supply. I also had to replace both electrolytic capacitors (at that time I didn’t yet buy them in buckets, so I installed them pulled out from somewhere), deal with the falling leg of the 22 nF capacitor and connect it correctly. After this, the circuit worked, although I still don’t know where it can be adapted.
Diodes - D311. D18 will be a little worse.
Resistor R5 is trimmer and has an asterisk - this means that not only will it have to be adjusted to the signal level (so that, for example, at normal amplifier volume the needle dangles around 75% of the scale), it is also not a fact that 47 kOhm suitable for all occasions.
If you increase the value of resistor R4 (470 - 910k), you can raise the gain of the microcircuit and make it “feel” weaker signals (this is just useful if the indicator is connected to entrance sound amplifier). For example, to observe the sound output from the player I had to install a 1 MOhm resistor.
Some photos of my circuit: 
And a demonstration of the work when the output of “VEF 216” is monitored:
A special feature of the circuit is its low sensitivity to high-frequency signals (the needle moves with greater pleasure from drums and bass guitars than from voices and guitar solos).
And for the night, I built two blue five-millimeter LEDs into the indicator housing. Normally they light from five volts, if less, then only one works, the second one turned out to be burnt. For compatibility with other supply voltages, the backlight is turned on through a 500 Ohm trimming resistor - you can easily power the entire circuit from 5 to 9 volts, you just need to adjust the voltage.
On Fig.1 A diagram for one channel is presented. For stereo we will need to assemble two such devices. The signal level indicator is assembled on one transistor T1, any of the series KT315. To increase sensitivity, a voltage doubling circuit was used on diodes D1 and D2 from the D9 series. The device does not contain scarce radio components, so you can use any with similar parameters.
The indicator reading corresponding to the nominal level is set using trimming resistor R2. The integration time of the indicator is 150-350 ms, and the return time of the needle, determined by the discharge time of capacitor C5, is 0.5-1.5 s. Capacitor C4 is one for two devices. It is used to smooth out ripples when turned on. In principle, this capacitor can be abandoned.
The device for two audio channels is assembled on a printed circuit board measuring 100X43 mm (see Fig.2). Indicators are also mounted here. For easy access to the construction resistors, holes are drilled in the board (not shown in the figure) so that a small screwdriver can pass through to adjust the nominal signal level. However, that’s all the setup of this device comes down to. You may need to select resistor R1 depending on the output signal strength of your device. Because On the other side of the board there are dial indicators; elements Cl, R1 had to be mounted on the side of the printed circuit conductors. It is better to take these parts as miniature as possible, for example, unframed.
Download: Dial indicator of output signal level
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Hi all. I used to assemble such circuits using light bulbs, and when LEDs became more readily available, . When the Internet appeared, such an abundance of circuits poured in, but a big problem appeared - you solder the circuit, and it either does not work at all or works but not as it should, and then you begin to conduct experiments with it, to achieve the desired result. But while tinkering with the circuit, you learn a lot of interesting things, you understand what detail affects what, and in general you develop to the fullest. Here are several really tested and 100% working schemes that you can safely do.
Collection of AF LED indicator circuits
Here are a few more level indicator schemes adjusted to good blinking from music
Here’s another strobe controlled by a sound signal that I once made, maybe it will be useful for someone else:
I made these two strobe lights, one like a policeman, the other just like a disco.
This indicator was still soldering.
And this indicator was amplified under a powerful load.
As for this indicator, the LEDs must all be the same color; this is a prerequisite, since the scale itself is passive.
Now here’s an interesting circuit, somehow I got a two-color LED, so I decided to make it blink beautifully to the music - this is the circuit that came out.
But even such a specialized indicator circuit as 3915 requires its own control circuit, the most suitable one is the one in the circuit, the parts are also selected for the best performance. Since it has a very sensitive input, a divider is added at the signal input. A resistor R7 was added to prevent the first LED from lighting up. But the circuit converts perfectly into a simple active frequency filter. Let’s take this figure as an example, it all depends on the capacity of the input capacitor C1 and the additional capacitor C5, which is placed between the collector and the common wire.
In this way, you can make three frequency channels and already apply this whole thing to the DMU. To begin with, you can solder such a pre-drive amplifier with regulators for each channel, and load an LM with control circuits configured at the outputs of the regulators (variable resistors). its frequency range.
Also, if anyone needs the indicator to work purely on drums, or in other words, an instrument that sets the beat of a melody, this version of the control circuit is very suitable for these purposes.
And lastly, in the wiring of the microcircuit there is such a resistor R6, through it the common plus is supplied to the LEDs, it can be disconnected from the main plus and connected to this breaker circuit, then the LEDs in the column will not only glow but also flicker, the effect is cool, I did this too.
Discuss the article SOUND LEVEL INDICATORS ON LED
UMZCHs look beautiful and stylish, just where to find them... There is a way out - we will make a meter in which the role of the arrow will be played by light-emitting diodes controlled by a microcircuit. LM3916- This is a special chip for LED level indicators.
Diagram of an LED indicator
The LEDs are connected via connectors J3 - J12 (only one row of LEDs is shown in the diagram). The indicator circuit will require a bipolar power supply to operate properly. The positive supply potential of the LED strips should be below +25 V and, in combination with the negative voltage, should not exceed 36 V. The minimum voltage level depends on the operating voltage of the LEDs. For example, if the LED is 1.9V, and we have 7 LEDs per pin, then the minimum positive voltage will be 7 x 1.9V + 1.5V (voltage drop across the LM3916) = 14.8 volts. Green LEDs tend to have a slightly higher voltage of 2.2-2.4V, so +18V will be sufficient in most cases.
The LED current is determined by resistor R1_REF, and with a resistance of 2.2 kOhm there will be 5 mA.
Formula for calculation: Iled = 10 x (1.2 V / R1_REF)
You can use TL072, TL082, LM358 as a dual operational amplifier at the input. The output mode can be set by 3-pin jumper JP1. The maximum input voltage for LM3916 is 1.2V, and R8-R7 can be used to adjust the input level.
Video of the indicator
LED color of your choice. Used here green LEDs for negative levels, yellow- 0dB and red for positive audio signal level. For this you need rectangular LEDs. An archive with drawings of printed circuit boards is available.