Audio signal indicator, simple circuit

Level indicators

It is no secret that the sound of a system largely depends on the signal level at its sections. By monitoring the signal at the transient sections of the circuit, we can judge about the work of various functional blocks: the gain, insertion of distortion, etc. Also there are cases when the resulting signal is simply impossible to hear. In those cases where it is not possible to monitor the signal by ear, various kinds of level indicators are used. For observation can be used as pointer devices, and special devices, providing the work of “column” indicators. So, let’s consider their work in more detail.

1 Scale indicators 1.1 The simplest scale indicator.

This type of indicators is the simplest of all existing indicators. The scale indicator consists of a pointer device and a divider. A simplified indicator scheme is shown in Fig. 1.

Bar Level Indicator


The most commonly used meters are microammeters with a total current deflection of 100 – 500mA. Such devices are designed for direct current, so for their work the sound signal must be rectified by a diode. The resistor is designed to convert voltage to current. Actually, the device measures the current passing through the resistor. Calculated elementary, according to Ohm’s law (there was one. Georgy Semenych Ohm) for a section of the circuit. It should be taken into account that the voltage after the diode will be 2 times less. The diode brand is not important, so any diode operating at a frequency greater than 20kHz will do. So, the calculation: R = 0.5U/I where: R – resistance of the resistor (Ohm) U – Maximum measured voltage (V) I – current of full indicator deflection (A)

It is much more convenient to estimate the signal level, setting it some inertia. I.e. the indicator shows the average value of the level. It is easy to achieve by connecting an electrolytic capacitor in parallel with the device, but it should be taken into account that this will increase the voltage on the device by (root of 2) times. Such an indicator can be used to measure the output power of an amplifier. What to do, if the level of the measured signal is not enough to “shake up” the device? In this case such guys as transistors and operational amplifiers (further Op-Amps) come to the rescue.

1.2 Transistor stage indicator.

If you can measure the current through the resistor, you can also measure the collector current of the transistor. For this we need the transistor and the collector load (the same resistor). Schematic diagram of the transistor scale indicator is shown in Fig.2

Transistor based bar gauge


The circuit is very simple. The transistor boosts the signal with current and everything else works in the same way. The collector current of transistor should exceed the current of full deviation of the device at least 2 times (so it is quieter for transistor and for you), that is, if the full deviation current is 100 mA, then the collector current should be at least 200 mA. Actually, this is relevant for milliamperes, because 50mA flies through the weakest transistor “with a whistle”. Now look in the reference book and find in it the current transfer coefficient h21э. Let’s calculate the input current: Ib = Ik/h21Э where: Ib – input current Ik – total deviation current = collector current h21Э – current transfer ratio

Handle illumination with their own hands

R1 is calculated using Ohm’s law for a circuit section: R=Ue/Ik where: R – resistance R1 Ue – supply voltage Ik – total current = collector current

R2 is designed to suppress the voltage at the base. By selecting it it is necessary to achieve maximum sensitivity with a minimum deflection of the arrow in the absence of signal. R3 adjusts the sensitivity and its resistance is not critical.

There are cases when it is necessary to amplify the signal not only with current but also with voltage. In this case, the indicator circuit is supplemented by a cascade with a ОE. Such an indicator is used, for example, in the “Comet 212” tape recorder. Its scheme is shown on fig.3

Track indicator on transistor with amplifier


1.3 A stepped on-die indicator

Such indicators have high sensitivity and input impedance, and therefore make minimal changes in the measured signal. One way of using DTs – the converter “voltage – current” is shown in Fig. 4.

A bargraph indicator on op-amp


Such an indicator has a smaller input resistance, but is very easy to calculate and manufacture. Let’s calculate the resistance R1: R=Us /Imax where: R – resistance of the input resistor Us – maximum signal level Imax – total aberration current

The diodes are selected according to the same criterion as in the other circuits. If the signal level is low and/or high input impedance is required, a repeater can be used. It is shown on Fig.5.

A track indicator on op-amp with a repeater


It is recommended to raise the output voltage to 2-3V to make the diodes work properly. So in the calculations we start from the output voltage of Op-Amp. The first thing we do is to find out the gain we need: K= Uout/Uвх . Now let us calculate the resistors R1 and R2: K=1+(R2/R1) There seem to be no limitations in the choice of ratings, but it is not recommended to set R1 less than 1kOhm. Now let’s calculate R3: R=Uo/I where: R – resistance R3 Uo – output voltage of Op-Amp I – total deflection current

2 Peak (LED) indicators

2.1 Analog indicator

Probably the most popular type of indicators nowadays. Let’s start with the simplest ones. Figure 6 shows a schematic diagram of an indicator “signal/peak” based on the comparator. Let’s see the principle of operation. The threshold of operation is given by the reference voltage, which is set at the inverting input of Op-Amp by the divider R1R2. When the signal at the direct input exceeds the reference voltage, the output of Op-Amp appears +Uп, VT1 opens and VD2 lights up. When the signal is below the reference voltage, there is -Uп. In this case VT2 is open and VD2 lights up. Now let’s calculate this miracle. Let’s start with the comparator. First choose the trigger voltage (reference voltage) and resistor R2 in the range 3 – 68 kOhm. Let’s calculate the current in the reference voltage source Iatt=Uоп/Rб where: Iatt – current through R2 (inverting input current can be neglected) Uоп – reference voltage Rб – resistance R2

Circuit for protection against polarity reversal

Peak Indicator LED

Fig. 6

Now let’s calculate R1. R1=(Ue-Uоп)/ Iatt where: Ue – power supply voltage Uоп – reference voltage (pickup voltage) Iatt – current through R2

The limiting resistor R6 is chosen according to the formula R1=U e / I LED where: R – resistance R6 Ue – supply voltage ILED – direct LED current (it is recommended to choose between 5 – 15 mA) Compensation resistors R4, R5 are chosen from the reference book and correspond to the minimum load resistance for the selected Op-Amp.

2.2 Indicators on logical elements

Let’s start with the level limit indicator with one LED (Fig. 7). This indicator is based on a Schmitt trigger. As we know, the Schmitt trigger has some hysteresis, i.e. the triggering threshold differs from the releasing threshold. The difference of these thresholds (hysteresis loop width) is determined by the ratio of R2 to R1, since the Schmitt trigger is an amplifier with positive feedback. The limiting resistor R4 is calculated in the same way as in the previous circuit. The limiting resistor in the base circuit is calculated based on the load capacity of the LE. For CMOS (CMOS logic is recommended) the output current is about 1.5 mA. To begin with we calculate the input current of the transistor stage: Ib=ILED/h21Э Where:

Indicators on logic elements


Ib – input current of the transistor stage ILED – is the forward current of the LED (it is recommended to set 5 – 15 mA) h21Э – current transfer ratio

Now we can roughly calculate the input resistance: Z=E/Ib where: Z is input resistance E is supply voltage Ib – input current of the transistor stage

If the input current does not exceed the load capacity of the LE we can do without R3, otherwise it can be calculated by the formula: R=(E/Ib)-Z where: R – R3 E – supply voltage Ib – input current Z – input resistance of the cascade

To measure the signal “in columns” you can assemble a multi-level indicator (Fig. 8). Such an indicator is simple, but its sensitivity is low and suitable only for measuring signals from 3 volts and above. The thresholds are set by trim resistors. It uses TTL elements, in case you use CMOS, you must install an amplifier stage at the output of each LPU.

Multi-level indicator on LEDs


2.3 Peak indicators on specialized microcircuits

Dashboard re-lighting and insertion of rings on Priora, Kalina

The easiest way to make them. Some circuits are shown in Fig.9

Peak indicators on specialized chips


You can also use other indication amplifiers. You can ask for their schemes in a store or at Yandex.

3. Peak (fluorescent) indicators

At one time they were used in domestic machinery, now they are widely used in music centers. These indicators are very complicated to build (they include specialized microcircuits and microcontrollers) and to connect (they require several power supplies). I do not recommend to use them in amateur technique.

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LED audio level indicator as an ornament to the amateur design. An overview of two-channel ready to use indicator

Audio level indicators (more precisely, the level of electrical signal in the sound path) can be very responsible devices, and can serve just to decorate the equipment. They are often referred to as VU-meters for short.

In professional equipment VU-meter is an essential device, which must accurately display the desired parameter to prevent over or underloading of the sound path.

But in household equipment it is not very responsible element that can serve either for approximate estimation of signal level, or just for beauty – for lights to run or arrow to move in time with the music.

In this review we will analyze the off-the-shelf LED stereo signal indicator, designed to be built into the amateur radio equipment.

(image from the seller’s page on Aliexpress)

Tactical specifications, appearance, configuration and design of the LED sound level indicator

A small set of tactical and technical specifications from the manufacturer is presented in the following table :

Number of LEDs per channel. 12 pcs. (7 green + 2 orange + 3 red)
Number of signal scale modes 2 (Logarithmic + AGC)
Number of signal display modes 6
Power supply voltage 7. 12 В
Current consumption 100mA
Indicator board size 80*14 mm
Indicator board size 58*14 mm

The real consumption depends heavily on the brightness and the number of working LEDs.

With all LEDs on at maximum brightness consumption was 54 mA, with only two LEDs on consumption was 17 mA.

The scope of delivery of the indicator is very simple, it consists of the indicator board and a cable for external connections:

(photos in the review are clickable)

The instructions for setting up the indicator you can find on the pages of some sellers, in principle they are correct but not very useful:

I had to make up my own instruction, it will be presented later in the review.

This is how the VU-meter looks from the side, in which you can clearly see the size ratio between its different parts:

This is what the indicator looks like from the line side of the LEDs:

The pin assignment of the connector is signed on the board in a very clear way, no additional explanations are needed.

Tester to check the ignition coils with my own hands

Now let’s look at the board from the element side:

The electronic “stuffing” of the indicator seems to be simple. In fact there is even a real microprocessor with its own firmware!

But we are going to find out about it, but for now let’s pay attention to a small round button at the bottom right part of the board.

With this one button all the settings are done. It is better to set the desired mode before you connect the indicator to the board because the button might be hard to reach after the installation.

A view of the part of the board near the connector:

Here you will find the extremely popular LM358 dual op-amp and a small three-legged 5V line stabilizer chip.

The op-amp takes the analog signal from the input lines and sends it to the other side of the board, where the processor awaits:

Here you have a pair of transistors, another 5V stabilizer, a mode control button, and the “heart” of the display – the STM8S003F3P6 analog-to-digital processor.

This processor supports up to 5 channels of 10-bit analog-to-digital conversion.

Its processing part runs at 16 MHz, has 8K bytes of firmware memory and 1K bytes of RAM. These are all small values, but enough to accomplish the task at hand.

Now let’s move on to the analytical part of the review.

Technical tests of the LED level indicator

First let’s talk a little about the theory of signal analysis and displaying (as applied to the indicator under test).

Indicators can respond to different values: the peak value of the signal, its average value or rms (effective).

The scale of indication can be linear, logarithmic (“decibel”) or with automatic gain control (AGC). There are also more exotic methods, we do not consider them.

The first two types of scale show the user the real value of the signal, and the last one (with AGC) serves only for a beautiful dynamic indication.

The methods of visual representation of the measured signal value on the LED indicators can also be different.

The signal level can be represented as a “classic” bar (sometimes as a two-sided bar growing from the middle of the indicator), or as one or more segments moving up or down, depending on the signal level. These methods can have additional options, for example, in the form of fixation for some time by a single segment of the maximum signal level.

The hero of this review has two modes of signal scale: logarithmic and with automatic gain control (AGC).

The mode of Automatic Gain Control is named so, of course, conditionally. There are no gain control circuits in the indicator; the automatic adjustment of the signal display is performed in a purely computational way.

To find out what exactly the VU-meter under test reacts to (peak or average value), a rectangular signal with a variable fill from 10% to 30% (frequency of 1 kHz) was applied to the indicator.

Panel rebuild from VAZ 2110

In the case of the indicator’s response to the signal’s peak, the “bar” on the indicator in the decibel mode should not change; and in the response to the average value should increase as the filling increases.

The tests showed that the bar increases, i.e. the average level is used for indication. We reject the possibility of using the RMS level and other “exotics” in the indicator, as they create an excessive computing load.

Now here is the table with the results of measuring the input voltage, necessary for stable switching on the segments of indicator in the decibel mode at 1 kHz (sine); it’s displayed in the classic bar. The signal was fed from the FY6800 signal generator; the voltage in the table is the signal peak-to-peak, i.e. double amplitude (because it is what the indicator of the FY6800 generator shows).

The increase to the previous value in dB is shown in brackets.

Segment Voltage
1 is always lit
2 65 mV
3 195 mV (+9.5 dB)
4 350 mV (+5.1 dB)
5 530 mV (+3.6 dB)
6 750 mV (+3.0 dB)
7 1.04 V (+2.84 dB)
8 1.47 V (+3.0 dB)
9 2.07 V (+2.9 dB)
10 3.00 V (+3.2 dB)
11 4.2 V (+2.9 dB)
12 6.1 V (+3.2 dB)

Thus, taking into account the error of the measurement method, we can say that the manufacturer has taken a logarithmic scale with a division value of 3 dB in the main part; but with a loaded division value at small signals.

On the one hand, it allows you to expand the dynamic range of the indicator (it was 39.5 dB); but, on the other hand, it will make less accurate and dynamic readings at low signal.

In other words, in decibel mode at a low signal the lower segments will move slowly and lazily (which was confirmed when tested with a real music signal).

But in AGC mode, everything works quite differently. In this mode, the processor automatically moves the average signal level to the middle of the scale and the picture comes out very dynamic with any signal (except for the signal reaching outside the dynamic range).

A few words about the frequency bandwidth of the sound level indicator.

There is a noticeable dip in the lower frequencies, the bandwidth by minus 3 dB starts from 170 Hz.

In the middle and high frequencies the response is quite flat, with a gentle 20% increase to 20 kHz.

In general, the characteristic is far from ideal, and the real signal level of the indicator does not display very accurately.

Now let’s see how the indicator works with the real music signal.

Examples of signal display in AGC mode and in three different visualization modes (out of 6 possible) are shown in the following videos.

1. Classic bar display:

2: Bar display with level fixing at the maximum and then dropping down:

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