LED output power indicator. Level meters Logarithmic power output indicator

You can, of course, use a microcircuit instead of transistors as the main component, but in my opinion, a device made on a chip has a smaller range of creative thought, that is, you cannot make such fine adjustments that can be set in a transistor version. Transistor topology makes it possible to flexibly adjust various parameters with the required indication range, soft signal response to LEDs and the same smooth decay. An indicator chain can be assembled with almost any number of LEDs, as long as there is a desire and a need for it. p>

Although in fairness it should be noted that transistor circuits with a large number of installed LEDs require a lot of time to debug and adjust them. But on the other hand, it is pleasant to work with such a design later, it is very difficult to disable it. But even in the event of an emergency situation with any of the cells, you can fix everything without any problems. Clip-on power output indicator does not require large financial costs for its manufacture, the most popular silicon transistors of the KT315 type are used. Any radio amateur is well acquainted with such semiconductors; many began their journey in electronics precisely with the use of such transistors.

The amplifier output power indicator circuit shown here has a logarithmic scale, assuming that the output power will be more than 110 watts. If, for simplicity, a scale of a linear type were made, then, for example, at 4-6 W, the LEDs would not be able to open, or a line of about 120 cells would have to be made. Therefore, an indication device intended for high-power amplifiers must be assembled in such a way that there is a logarithmic relationship with respect to the output power of the amplifier and the number of LEDs installed.

Schematic diagram of a peak indicator

Peak power output indicator and his presented circuit is absolutely simple, and made with identical cells displaying a visual indication, each of which indicates a different level of output voltage of the amplifier. Here is a diagram for 5 points of indication:

The scheme of the peak indicator of the output power of the amplifier on KT315 transistors

According to the principle of the diagram shown above, it is easy to make an indication for ten points.

Hello, friends!

In continuation of articles about amplifiers, I think the logarithmic signal level indicator circuit will also come in handy. This device is based on the LM3915 chip in the amount of two pieces (each chip works on its own channel), you can see detailed information about the chip, the recommended supply voltage is 12V. The LM358 microcircuit acts as a pre-amplifier. Detailed information about the microchip.

In place of the LM3915, you can use the following similar microcircuits: LM3914 and LM3916. It is worth considering that the 3914 chip has a linear jackal, the LEDs light up in 3 dB steps, and the 3915 and 3916 step is logarithmic.

Instead of LM358, you can use the following similar chips: NE532, OP04, OP221, OP290, OP295, OPA2237, TA75358P, UPC358C.

Advantages of this device

• Ease of manufacture
• Reliability

Flaws

• The high cost of the microcircuit. This disadvantage is eliminated by purchasing radio components in China.

Schematic diagram of the stereo signal level indicator

PCB signal strength indicator

List of radio components

Microcircuits. To install chips on the board, I recommend purchasing a DIP18 socket and installing chips in the socket last. In order to reduce the likelihood of failure of the microcircuit by being hit by static electricity when it is installed on the board.

• LM358 - 1pc
• LM3915 - 2 pcs.

Resistors

• trimmer resistor RV1 and RV2 - 100 kOhm - 2 pcs.
• R1, R2 - 22kOhm -2pcs
• R5, R6 - 220kOhm -2pcs
• R3, R4 - 1kΩ - 2pcs
• R7, R8 - 47kOhm -2pcs
• R9, R11 - 1.3 kOhm -2pcs
• R10, R12 -3.6kOhm - 2 pcs

Capacitors

• 1.0 mF - 4 pcs
• electrolytic capacitor 100mF x 32V -1 pc

• 1N4148 - 4 pcs.
• LEDs -10pcs. Selected to taste with a supply voltage of 3V. We recommend choosing the last two LEDs in a different color.

If you have any questions about this article, please write to the site administrator.

It's no secret that the sound of the system largely depends on the signal level in its sections. By monitoring the signal at the transition sections of the circuit, we can judge the operation of various functional blocks: gain, introduced distortion, etc. There are also cases when the resulting signal is simply not possible to hear. In cases where it is not possible to control the signal by ear, various kinds of level indicators are used.
For observation, both pointer instruments and special devices that ensure the operation of "bar" indicators can be used. So, let's look at their work in more detail.

1 Dial indicators
1.1 The simplest scale indicator.

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

As meters, microammeters with a total deflection current of 100 - 500 μA are most often used. Such devices are designed for direct current, therefore, for their operation, the sound signal must be rectified by a diode. The resistor is designed to convert voltage to current. Strictly speaking, the device measures the current passing through the resistor. It is calculated elementarily, according to Ohm's law (there was such. Georgy Semenych Om) for a section of the circuit. In this case, it should be taken into account that the voltage after the diode will be 2 times less. The brand of the diode is not important, so any that operates at a frequency greater than 20 kHz will do. So, calculation: R = 0.5U/I
where: R is the resistance of the resistor (Ohm)
U - Maximum measured voltage (V)
I - indicator total deflection current (A)

It is much more convenient to evaluate the signal level by giving it some inertia. Those. the indicator shows the average value of the level. This can be easily achieved by connecting an electrolytic capacitor in parallel with the device, however, it should be noted that in this case the voltage on the device will increase 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 “stir up” the device? In this case, guys like the transistor and the operational amplifier (hereinafter referred to as the op-amp) come to the rescue.

If you can measure the current through the resistor, then you can measure the collector current of the transistor. To do this, we need the transistor itself and the collector load (the same resistor). The diagram of a bar graph indicator on a transistor is shown in fig.2

Fig.2

Here, too, everything is simple. The transistor amplifies the current signal, but otherwise everything works the same. The collector current of the transistor must exceed the total deflection current of the device by at least 2 times (this way it is calmer both for the transistor and for you), i.e. if the total deflection current is 100 µA, then the collector current must be at least 200 µA. As a matter of fact, this is true for milliammeters, because. 50 mA flies through the weakest transistor "with a whistle". Now we look at the reference book and find in it the current transfer coefficient h 21e. We calculate the input current: I b \u003d I k / h 21E where:
I b - input current

R1 is calculated according to Ohm's law for the chain section: R=U e /I k where:
R - resistance R1
U e - supply voltage
I k - total deflection current = collector current

R2 is designed to suppress voltage at the base. Choosing it, you need to achieve maximum sensitivity with a minimum deviation of the arrow in the absence of a signal. R3 adjusts the sensitivity and its resistance is practically not critical.

There are times when the signal needs to be amplified not only in current, but also in voltage. In this case, the indicator circuit is supplemented with a cascade with OE. Such an indicator is used, for example, in the Comet 212 tape recorder. Its diagram is shown in fig.3

Fig.3

Such indicators have high sensitivity and input resistance, therefore, they make a minimum of changes in the measured signal. One of the ways to use an op-amp - a voltage-to-current converter is shown on fig.4.

Fig.4

Such an indicator has a lower input resistance, but it is very simple in calculations and manufacturing. Calculate the resistance R1: R=U s /I max where:
R is the resistance of the input resistor
U s - Maximum signal level
I max - total deflection current

Diodes are selected according to the same criteria as in other circuits.
If the signal level is low and/or high input impedance is required, a repeater can be used. Its diagram is shown in fig.5.

Fig.5

For confident operation of the diodes, it is recommended to raise the output voltage to 2-3 V. So, in the calculations, we start from the output voltage of the op-amp. First of all, let's find out the gain we need: K \u003d U out / U in. Now let's calculate the resistors R1 and R2: K=1+(R2/R1)
It would seem that there are no restrictions in the choice of ratings, but it is not recommended to set R1 less than 1 kOhm. Now we calculate R3: R=U o /I where:
R - resistance R3
U o - output voltage of the OU
I - total deflection current

2 Peak (LED) indicators

2.1 Analog indicator

Perhaps the most popular type of indicators at present. Let's start with the simplest ones. On fig.6 the diagram of the signal/peak indicator based on the comparator is shown. Consider the principle of action. The response threshold is set by the reference voltage, which is set at the inverting input of the op-amp by the divider R1R2. When the signal at the direct input exceeds the reference voltage, + U p appears at the output of the op-amp, VT1 opens and VD2 lights up. When the signal is below the reference voltage, -U p acts at the output of the op-amp. In this case, VT2 is open and VD2 is lit. Now let's calculate this miracle. Let's start with the comparator. To begin with, we select the operation voltage (reference voltage) and the resistor R2 in the range of 3 - 68 kOhm. Calculate the current in the reference voltage source I att \u003d U op / R b where:
I att - current through R2 (the current of the inverting input can be neglected)
U op - reference voltage
R b - resistance R2

Fig.6

Now let's calculate R1. R1=(U e -U op)/ I att where:
U e - power supply voltage
U op - reference voltage (trip voltage)
I att - current through R2

Limiting resistor R6 is selected according to the formula R1=U e/I LED where:
R - resistance R6
U e - supply voltage
I LED - direct current of the LED (it is recommended to choose within 5 - 15 mA)
Compensating resistors R4, R5 are selected from the reference book and correspond to the minimum load resistance for the selected op-amp.

Let's start with a limit indicator with one LED ( fig.7). This indicator is based on the Schmitt trigger. As you know, the Schmitt trigger has some hysteresis those. the trigger threshold is different from the release threshold. The difference between these thresholds (width of the hysteresis loop) is determined by the ratio of R2 to R1 since The Schmitt trigger is a positive feedback amplifier. The limiting resistor R4 is calculated according to the same principle 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 approximately 1.5 mA. First, let's calculate the input current of the transistor stage: I b \u003d I LED / h 21E where:

Fig.7

I b - input current of the transistor stage
I LED - forward current of the LED (it is recommended to set 5 - 15 mA)
h 21E - current transfer coefficient

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

To measure the “bar” signal, you can assemble a multilevel indicator ( fig.8). Such an indicator is simple, but its sensitivity is low and is only suitable for measuring signals from 3 volts and above. LE operation thresholds are set by tuning resistors. The indicator uses TTL elements, in the case of CMOS, an amplifier stage should be installed at the output of each LE.

Fig.8

The easiest way to make them. Some diagrams are shown in fig.9

Fig.9

You can also use other display amplifiers. You can ask for connection schemes for them in the store or from Yandex.

3. Peak (luminescent) indicators

At one time they were used in domestic technology, now they are widely used in music centers. Such indicators are very difficult to manufacture (include specialized microcircuits and microcontrollers) and to connect (require several power supplies). I do not recommend using them in amateur technology.

List of radio elements

LM3915 - an integrated circuit (IC) manufactured by Texas Instruments, responds to a change in the input signal and outputs a signal to one or more of its outputs. Due to its design features, the IC has become widespread in LED indicator circuits. Since the LM3915 based LED indicator works on a logarithmic scale, it has found practical application in displaying and monitoring the signal level in audio amplifiers.

Do not confuse the LM3915 with its relatives LM3914 and LM3916, which have a similar layout and pinout. The 3914 series IC has a linear characteristic and is ideal for measuring linear quantities (current, voltage), while the 3916 series IC is more versatile and is able to control various types of loads.

Brief description of LM3915

The block diagram of the LM3915 consists of ten of the same type of operational amplifiers operating on the principle of a comparator. The direct inputs of the op-amp are connected through a chain of resistive dividers with different resistance values. Due to this, the LEDs in the load light up according to a logarithmic dependence. The input signal comes to the inverse inputs, which is processed by the buffer op-amp (pin 5).

The internal device of the IC includes a low-power integrated stabilizer connected to pins 3, 7, 8 and a device for setting the glow mode (pin 9). The supply voltage range is 3–25V. The value of the reference voltage can be set in the range from 1.2 to 12V using external resistors. The entire scale corresponds to a signal level of 30 dB in 3 dB steps. The output current can be set from 1 to 30 mA.

Scheme of the sound indicator and the principle of its operation

As can be seen from the figure, the circuit diagram of the sound level indicator consists of two capacitors, nine resistors and a microcircuit, for which ten LEDs serve as a load. For the convenience of connecting power and audio signal, it can be supplemented with two solder connectors. To assemble such a simple device is within the power of any, even a beginner, radio amateur.

A typical inclusion provides power from a 12V source, which is supplied to the third output of the LM3915. It, through the current-limiting resistor R2 and two filter capacitors C1 and C2, goes to the LEDs. Resistors R1 and R8 serve to reduce the brightness of the last two red LEDs and are optional. 12V also comes to the jumper that controls the operating mode of the IC through pin 9. In the open state, the circuit operates in the "point" mode, i.e. there is a glow of one LED corresponding to the input signal. Closing the jumper puts the circuit into "column" mode, when the input signal level is proportional to the height of the luminous column.

The resistive divider assembled on R3, R4 and R7 limits the input signal level. More precise tuning is carried out by a multi-turn tuning resistor R4. Resistor R9 sets the offset for the high level (pin 6), the exact value of which is determined by the resistance R6. The lower level (pin 4) is connected to the common wire. Resistor R5 (pin 7.8) increases the value of the reference voltage and affects the brightness of the LEDs. It is R5 that sets the current through the LEDs and is calculated by the formula:

R5 \u003d 12.5 / I LED, where I LED is the current of one LED, A.

The sound level indicator works as follows. At the moment when the input signal overcomes the low level threshold plus the resistance at the direct input of the first comparator, the first LED will light up (pin 1). A further increase in the sound signal will lead to the sequential operation of the comparators, which will let you know the corresponding LED. To avoid overheating of the IC case, the LED current should not exceed 20 mA. Still, this is an indicator, not a New Year's garland.

PCB and Assembly Parts

The printed circuit board of the sound level indicator in lay format can be downloaded. It has dimensions of 65×28 mm. Assembly requires precision parts. Resistors type MLT-0.125W:

• R1, R5 R8 - 1 kOhm;
• R2 - 100 Ohm;
• R3 - 10 kOhm;
• R4 - 50 kOhm, any tuning;
• R6 - 560 Ohm;
• R7 - 10 Ohm;
• R9 - 20 kOhm.

Capacitors C1, C2 - 0.1 uF. It is recommended to solder the LM3915 IC not directly, but through a special socket for the microcircuit. In the load, you can use ultra-bright LEDs of any glow color, up to purple. But this is a personal aesthetic preference. To display a stereo signal, you will need two identical boards with independent inputs. More details about the LM3915 can be found in the datasheet here.

The performance of this indicator has been proven in practice by many amateur radio clubs and is still available in the form of MasterKit sets.

Read also

This article continues a series of publications on amateur radio designers MasterKit. It describes the stereo signal level indicator module for the “Low Frequency Amplifier” kit (“PX” N? 6.2000, N? 1 and N? 2.2001).

The proposed indicator will "revive" the appearance of the amateur radio power amplifier and make its use more comfortable and attractive. The stereo indicator consists of three independent blocks - two universal LED linear indicators and a two-channel logarithmic rectifier. This construction made it possible to obtain a very flexible device both in terms of functionality and external design. The following is a description of the individual components that make up the indicator, and also shows a variant of the design of the stereo indicator.

Schematic diagram. The LED linear indicator is a universal constant voltage linear indicator. The signal is indicated by a LED scale of 12 LEDs. Two options have been developed: with LEDs lighting up sequentially in the form of a continuous column (“luminous column” NM5201) and with one lighting up LED moving along a ruler (“running dot” NM5301). Schematic diagram of the indicator "glowing pillar" (NM 5201) is shown in Fig.1. Such indicators, made on a compact board, can be used not only in a power amplifier, but also in automotive electronics, instrumentation and household appliances. The UAA180 chip (domestic analogue of KR1003PP1) is used as the basis of the indicator. The choice was due to the fact that on the basis of this microcircuit it is possible to create indicators of both the “luminous column” and “running dot” types, while ensuring their high efficiency. In addition, the presence of a domestic analogue significantly reduces the cost of the device, which is important in our conditions. The lower limit of the input voltage is determined by the level at pin 16 of the microcircuit (in this case it is 0). The upper limit of the input voltage is set by the potentiometer R2 and can be varied within +1...+5 V. Pin 2 is designed to adjust the brightness of the LEDs. When this output is connected to a common wire, all LEDs go out, and when connected to a power source through a 100 kΩ limiting resistor, the brightness of the glow increases by about two times, which makes it possible to use this mode as an additional indication, such as overload.

Technical characteristics of the indicator.
Supply voltage...................................................9 -18V
Consumption current, no more than ............................... 30 mA
Rated input voltage range ..... 0 - 4 V
Current through the LEDs (pin 5 is free) .................... 5 - 6 mA
PCB size..............................................75x25mm

Design. The appearance of the assembled module is shown in Fig. 2, and the printed circuit board and the arrangement of elements in Fig. 3 and Fig. 4. The installation is made on a board made of foil fiberglass. There is an additional hole under the adjusting resistor, which allows it to be adjusted from any side of the board. The design of the board provides for the possibility of assembling a shortened version of the indicator for 8 LEDs: it is enough to cut the board along the dotted line, and use an additional mounting hole for fastening. You can use LEDs of any desired color, depending on the functional and style design. The design provides that the LEDs during installation lie on the straight outer edge of the board, this ensures their smooth installation without the use of additional fasteners and leveling elements. If necessary, you can additionally fix them on the board with some kind of glue. There are no high components on the indicator board, which allows indicators to be mounted on top of each other with a minimum clearance, for example, to create display panels for spectrum analyzers.

Logarithmic rectifier.
Schematic diagram. The logarithmic rectifier is made (Fig. 5) based on the KR157DA1 microcircuit, which is a two-channel full-wave rectifier. The microcircuit converts the alternating voltage supplied to its input pin 2(6) into the direct current of the current source flowing from pin 13(9), with a value proportional to the average value of the alternating voltage. If a voltage output is required, then pin 13(9) is grounded, and the signal is taken from pin 12(10) - the output of the emitter follower installed after the internal load resistor of the current source. Capacitor C5(C6) is connected to terminal 12(10), which, together with the internal limiting resistor and resistors R15, R16 (R17, R18), provides the dynamic characteristics (rise and fall time constants) required for a standard VU meter. The output divider on resistors R15, R16 (R17, R18) is necessary to match the levels of the rectifier and the linear indicator. In a standard switching circuit, a linear rectifier provides an indication of signal levels in the range of just over 20 dB, which is clearly not enough for a high-quality amplifier. For this reason, a logarithm circuit was introduced into the circuit on the elements R8, R9, (R7, RIO), Rll, R12, R13 and VT1, VT2 (VT3, VT4). It provides a non-linear load to the internal rectified current sources, boosting up to +20dB of gain on small signals and leaving it unchanged on large signals. The divider on resistors R11-R13 sets the inflection points of the logarithm curve. The use of a common divider guarantees the identity of the characteristics of the channels, and the use of transistors instead of diodes ensures that they do not interfere. As a result of using the logarithm circuit, it was possible to expand the indication range to more than 40 dB. In this scheme, radio amateurs can easily experiment with the logarithm scheme and evaluate its effectiveness. In order to turn off the logarithm circuit and switch the detector to linear mode, it is enough to bridge the resistor R8 (R7). Resistors R1 and R2 adjust the sensitivity of the rectifier, which allows the device to be used with various sources of audio signals. To use a rectifier at the linear output of the amplifier (250 mV), resistors with a nominal value of 10 kΩ are required, and to connect to the powerful output of the amplifier, their value will need to be increased to several hundred kΩ. The exact value is best chosen experimentally.

Tech. characteristics of a logarithmic rectifier.
Supply voltage...................................6...20V
Consumption current................................................ .....5 mA
Nominal input level*..........250 mV
Output level ..............................0...4 V
Range of displayed signals, not less than ....... 40 dB
PCB size ..............................................75x25 mm.
*When Rl, R2 = YukOhm.

Design. The appearance of the module installed above the linear indicators is shown on the cover of the magazine and fig. The installation is made on a board made of foil fiberglass (Fig. 6, 7). The board dimensions, mounting holes and pin arrangement are consistent with the NM 5201 and NM 5301 linear indicator modules. The fixed resistors on the board are mounted vertically to keep the module compact.

In order to assemble a stereo indicator of an amateur radio amplifier on the basis of the described modules, it is enough to connect two linear indicators and a rectifier using screws with bushings, as shown in Fig. 8. Then it is necessary to connect their power outputs, and the rectifier outputs to the inputs of the corresponding indicators. The shown design is not the only one. Thanks to the division of the stereo indicator into modules, you can choose the option of installing indicators, for example, in a line one after another or opposite.
Setting up a stereo indicator. After assembly, only a calibration operation is required: by applying a signal with a nominal level to the inputs from the sound generator, the resistor R2 achieves the “lighting up” of the tenth LED.