Schemes of level indicators for umzch. Radio constructor - LED indicator of low-frequency signal level. Sound indicator circuit and principle of its operation
LM3915 is an integrated circuit (IC) manufactured by Texas Instruments that responds to changes in the input signal and outputs a signal to one or several of its outputs. Due to its design feature, the IC is widely used in LED indicator circuits. Since the LED indicator based on LM3915 operates on a logarithmic scale, it has found practical application in displaying and monitoring the signal level in audio amplifiers.
The LM3915 should not be confused with its relatives LM3914 and LM3916, which have a similar layout and pin assignment. The 3914 series IC has a linear characteristic and is ideal for measuring linear quantities (current, voltage), while the 3916 series IC is more universal and is capable of driving different types of loads.
Brief description of LM3915
The LM3915 block diagram consists of ten identical operational amplifiers operating on the comparator principle. The direct inputs of the op-amp are connected through a chain of resistive dividers with different resistance values. Thanks to this, the LEDs in the load light up according to a logarithmic dependence. The inverse inputs receive an input signal, which is processed by a buffer op-amp (pin 5).
The internal structure 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 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.
Sound indicator circuit and 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, the load for which is ten LEDs. For easy connection of power and audio signals, it can be supplemented with two solder connectors. Anyone, even a beginner radio amateur, can assemble such a simple device.
A typical connection provides power from a 12V source, which is supplied to the third pin of the LM3915. It also goes to the LEDs through the current-limiting resistor R2 and two filter capacitors C1 and C2. Resistors R1 and R8 serve to reduce the brightness of the last two red LEDs and are optional. 12V also comes to the jumper, which controls the operating mode of the IC through pin 9. In the open state, the circuit operates in the “point” mode, i.e. one LED corresponding to the input signal lights up. Closing the jumper switches the circuit to the “column” mode, when the input signal level is proportional to the height of the illuminated column.
A resistive divider assembled at R3, R4 and R7 limits the input signal level. More precise adjustment is carried out by multi-turn trimming resistor R4. Resistor R9 sets the bias for the upper level (pin 6), the exact value of which is determined by resistance R6. The lower level (pin 4) is connected to the common wire. Resistor R5 (pin 7.8) increases the reference voltage and affects the brightness of the LEDs. It is R5 that sets the current through the LEDs and is calculated using the formula:
R5=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 lower level threshold plus the resistance at the direct input of the first comparator, the first LED (pin 1) will light up. A further increase in the sound signal will lead to the comparators being activated one by one, which will be indicated by 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.
Printed circuit board and assembly parts
The printed circuit board of the sound level indicator in lay format can be downloaded. It has dimensions 65x28 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 trimmer;
- R6 – 560 Ohm;
- R7 – 10 Ohm;
- R9 – 20 kOhm.
Capacitors C1, C2 – 0.1 µF. It is recommended to solder the LM3915 IC not directly, but through a special socket for the chip. The load can use ultra-bright LEDs of any color, even purple. But these are personal aesthetic preferences. 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 MasterKits.
Read also
Today, entire electronic devices are used as an indicator of the output signal level for various sound reproduction equipment, which display not only the signal level, but also other useful information. But previously, dial indicators were used for this, which were a type microammeter M476 or M4762. Although I will make a reservation: today some developers also use dial indicators, although they look much more interesting and differ not only in backlighting, but also in design. Getting hold of an old dial indicator might be a problem now. But I had a couple of M4762 from an old Soviet amplifier, and I decided to use them.
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
If you find broken links, you can leave a comment, and the links will be restored as soon as possible.
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 settings that can be set in the transistor version. The transistor topology makes it possible to flexibly configure various parameters with the required indication range, soft response of the signal to the LEDs and the same smooth attenuation. An indicator chain can be assembled with almost any number of LEDs, as long as there is a desire and 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; it is very difficult to disable it. But even in the event of an emergency with any of the cells, everything can be repaired without any problems. Clip-on power output indicator does not require large financial costs for its production; the most common silicon transistors of the KT315 type are used. Any radio amateur is well acquainted with such semiconductors; many began their journey in electronics precisely by using such transistors.
The amplifier output power indicator circuit shown here has a logarithmic scale, taking into account that the output power will be more than 110 W. If, for simplicity, we made a scale of a linear type, then, for example, at 4-6 W the LEDs would not be able to open, or we would have to make a ruler of about 120 cells. Therefore, an indication device intended for powerful amplifiers must be assembled in such a way that there is a logarithmic relationship between the output power of the amplifier and the number of installed LEDs.
Schematic diagram of a peak indicator
Peak output power indicator and its presented circuit is absolutely simple, and is made with identical cells displaying a visual indication, each of which shows its own level of the amplifier's output voltage. Here is a diagram for 5 indication points:
Circuit of the peak indicator of the output power of the amplifier using KT315 transistors
Using the principle of the diagram shown above, you can easily make an indication for ten points.
The main part of the power consumption in sound reproducing equipment falls on the output stage, that is, on the UMZCH. Despite the fact that in the absence of an input signal, the UMZCH practically does not manifest itself in any way (with the exception of a barely noticeable hiss in the speakers, which also does not always occur).
But all remote control is usually concentrated precisely at the signal source (DVD player, TV, etc.). The UMZCH is often turned off only by a mechanical switch. Because of this, an unpleasant situation arises when the UMZCH almost always remains on.
Of course, you can somehow connect the switch-off circuit to a relay or standby switch-off (blocking, energy-saving mode) of the UMZCH with the signal source control system, but this requires intervention in the signal source circuit and ties the UMZCH to one specific signal source.
Which is not always convenient. It is easier to make a sensor for the presence of an input signal, which will turn on the UMZCH automatically when a signal arrives at its input and also automatically turn off if there is no signal for some time.
The circuit shown in the figure differs in that it uses LED level indicators as input signal detectors, showing the input signal levels separately for each stereo channel.
Signals arriving at the input of the UMZCH simultaneously arrive at the inputs of the meters on microcircuits A1 and A2. These are BA6125 microcircuits, multicomparator five-stage LED indicators of the LF signal level.
Microcircuits are included according to standard circuits. The sensitivity, depending on the nominal signal level in a particular audio system, is set by adjusted resistors R3 and R7. At the lowest signal level, the lower LED in the circuit lights up, that is, for the right NPO channel, and HL5 for the left.
Further, these LEDs light up even at a higher signal level (indication type - “pillar”). Therefore, the signal to turn on the UMZCH is the moment HL5 or HL10 lights up. The LED ignition sensors are made using transistors VT1 and VT2.
When the LED lights up, the voltage across it reaches the standard forward voltage value for the LED used. For indicator LEDs of the AL307 type, this value ranges from 1.6 to 2.2V depending on the color (the voltage is higher on green ones).
This voltage is enough to turn on the transistor. Accordingly, VT1 or VT2 (or both) opens and the voltage across resistor R9 rises to a high logic level. Schmitt trigger D1.1 switches to a logical zero state at the output.
If capacitor C5 was previously charged, then it discharges through diode VD1 and resistor R11 quite quickly. As a result, the second Schmitt trigger D1.2 switches to the logical one state at the output. Transistor VT3 opens and relay K1 turns on the UMZCH.
The connection diagram for the UMZCH may be different. It is not necessary to use a relay at all. If the UMZCH has an energy-saving mode or a blocking mode, then the logical level from output D1.2 can be applied directly to the corresponding input of the UMZCH microcircuit or its control unit.
Either through a switch on transistor VT3 or through an additional inverter using one of the two free inverters of the D1 chip. It all depends on the control circuit of the UMZCH, on what level the specific UMZCH is turned on and off. So we can say that the circuit on VT3 and K1 is shown conditionally.
If the input signal in both stereo channels is lost, the HL5 and NPO LEDs go out. Transistors VT1 and VT2 close and the voltage at the D1.1 inputs connected together drops to a low logic level. Schmitt trigger D1.1 switches to the logical one state at the output.
Capacitor C1 begins to slowly charge through the reverse resistance of diode VD1 and resistor R10. This takes about 20-30 minutes. As soon as the voltage on the capacitor reaches the switching threshold of the Schmitt trigger D1.2. it will switch and transistor VT3 will close, then a relay or some other circuit will turn off the UMZCH or switch it to “stand-by”.
If, before the moment of charging C5 to the voltage of a logical unit, the signal is resumed, then HL5 or HL10 (or both) lights up, the voltage at the inputs D1.1 rises to a logical unit and capacitor C5 is rapidly discharged through diode VD1 and resistor R11.
Thus, the UMZCH turns off only if the pause in the input signal in both channels exceeds the charging time of C5 to the logical one threshold. It turns on almost immediately when a signal arrives in any of the channels.
BA6125 indicator microcircuits can be replaced with other complete or incomplete analogues - many such microcircuits are produced. Of the complete analogues, you can use the BA6884, although its input sensitivity is slightly lower.
However, if this audio system uses a sensitive UMZCH, and accordingly the level of the nominal input signal is low, then, of course, additional amplification stages will be required in front of the A1 and A2 microcircuits. LEDs - almost any indicator LEDs, AL307 or similar imported ones (except for flashing ones). The K561TL1 chip can be replaced with an imported analog CD4093.
The choice of capacitor C5 is very important; it must be a high-quality capacitor with low leakage current. If there is a large leakage, the circuit may not work due to the fact that the shunt resistance of the leakage current of the capacitor will be less than or close to the resistance of resistor R10.
In this case, the leakage current with resistor R10 forms a voltage divider and the voltage on the capacitor will never reach the logical one level.
You can use a capacitor of smaller capacity, respectively increasing the resistance R10. For example, you can use a high-quality non-electrolytic capacitor of 2.2 μF, increasing the resistance of R10 to 15 M.
When setting up, the pause time in the signal after which it turns off is selected with resistance R10 (or capacitance C5).
Article by V.V. Poluvertov sent by D. Lebedev from Moscow.
It is no secret that the sound of a system largely depends on the signal level in its sections. By monitoring the signal in 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 simply cannot be heard. In cases where it is not possible to control the signal by ear, various types of level indicators are used.
For observation, both pointer instruments and special devices that ensure the operation of “column” indicators can be used. So, let's look at their work in more detail.
1 Scale indicators
1.1 The simplest scale indicator.
This type of indicator 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.
Microammeters with a total deviation current of 100 - 500 μA are most often used as meters. Such devices are designed for direct current, so for them to work, the audio signal must be rectified with a diode. A resistor is designed to convert voltage into current. Strictly speaking, the device measures the current passing through the resistor. It is calculated simply, according to Ohm’s law (there was such a thing. Georgy Semenych Ohm) for a section of the chain. It should be taken into account that the voltage after the diode will be 2 times less. The brand of diode is not important, so any one operating at a frequency greater than 20 kHz will do. So, the calculation: R = 0.5U/I
where: R – resistor resistance (Ohm)
U - Maximum measured voltage (V)
I – total deflection current of the indicator (A)
It is much more convenient to evaluate the signal level by giving it some inertia. Those. the indicator shows the average level value. This can be easily achieved by connecting an electrolytic capacitor in parallel with the device, but it should be taken into account that in this case the voltage on the device will increase (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 transistor and operational amplifier (hereinafter referred to as op-amp) come to the rescue.
If you can measure the current through a resistor, then you can also measure the collector current of the transistor. To do this, we need the transistor itself and a collector load (the same resistor). The diagram of a scale indicator on a transistor is shown in Fig.2
Fig.2
Everything is simple here too. 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 is calmer for both the transistor and you), i.e. if the total deviation current is 100 μA, then the collector current must be at least 200 μA. As a matter of fact, this is relevant for milliammeters, because 50 mA “whistles” through the weakest transistor. Now we look at the reference book and find in it the current transfer coefficient h 21e. We calculate the input current: I b = I k /h 21E where:
I b – input current
R1 is calculated according to Ohm's law for a section of the circuit: R=U e /I k where:
R – resistance R1
U e – supply voltage
I k – total deviation current = collector current
R2 is designed to suppress voltage at the base. When selecting it, you need to achieve maximum sensitivity with minimal needle deviation in the absence of a signal. R3 regulates sensitivity and its resistance is practically not critical.
There are cases when the signal needs to be amplified not only by current, but also by 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 on Fig.3
Fig.3
Such indicators have high sensitivity and input resistance, therefore, they make minimal changes to the measured signal. One way to use an op-amp - a voltage-current converter - is shown in Fig.4.
Fig.4
Such an indicator has a lower input resistance, but is very simple to calculate and manufacture. Let's calculate the resistance R1: R=U s /I max where:
R – input resistor resistance
U s – Maximum signal level
I max – total deviation 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 on Fig.5.
Fig.5
For reliable 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 = U out / U in. Now let's calculate resistors R1 and R2: K=1+(R2/R1)
There seem to be no restrictions in the choice of denominations, but it is not recommended to set R1 to less than 1 kOhm. Now let's calculate R3: R=U o /I where:
R – resistance R3
U o – op-amp output voltage
I – total deviation 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 a signal/peak indicator based on a comparator is shown. Let's consider the principle of operation. 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 op-amp output, VT1 opens and VD2 lights up. When the signal is below the reference voltage, –U p operates at the op-amp output. In this case, VT2 is open and VD2 lights up. Now let's calculate this miracle. Let's start with the comparator. First, let's select the response voltage (reference voltage) and resistor R2 within the range of 3 - 68 kOhm. Let's calculate the current in the reference voltage source I att =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 (operation 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 LED current (recommended to be selected 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 level indicator with one LED ( Fig.7). This indicator is based on a Schmitt trigger. As is known, the Schmitt trigger has some hysteresis those. The actuation threshold is different from the release threshold. The difference between these thresholds (the width of the hysteresis loop) is determined by the ratio of R2 to R1 since The Schmitt trigger is a positive feedback amplifier. 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 =I LED /h 21E where:
Fig.7
I b – input current of the transistor stage
I LED – direct 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 using the formula: R=(E/I b)-Z where:
R–R3
E – supply voltage
I b – input current
Z – cascade input impedance
To measure the signal in a “column”, you can assemble a multi-level indicator ( Fig.8). This indicator is simple, but its sensitivity is low and is only suitable for measuring signals from 3 volts and above. The LE response thresholds are set by trimming resistors. The indicator uses TTL elements; if CMOS is used, an amplification stage should be installed at the output of each LE.
Fig.8
The simplest option for making them. Some diagrams are shown on Fig.9
Fig.9
You can also use other display amplifiers. You can ask the store or Yandex for connection diagrams for them.
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 complex to manufacture (they include specialized microcircuits and microcontrollers) and to connect (they require several power supplies). I do not recommend using them in amateur equipment.
List of radioelements
| Designation | Type | Denomination | Quantity | Note | Shop | My notepad | |
|---|---|---|---|---|---|---|---|
| 1.1 The simplest scale indicator | |||||||
| VD1 | Diode | 1 | To notepad | ||||
| R1 | Resistor | 1 | To notepad | ||||
| PA1 | Microammeter | 1 | To notepad | ||||
| Fig.2 | |||||||
| VT1 | Transistor | 1 | To notepad | ||||
| VD1 | Diode | 1 | To notepad | ||||
| R1 | Resistor | 1 | To notepad | ||||
| R2 | Resistor | 1 | To notepad | ||||
| R3 | Variable resistor | 10 kOhm | 1 | To notepad | |||
| PA1 | Microammeter | 1 | To notepad | ||||
| Fig.3 | |||||||
| VT1, VT2 | Bipolar transistor | KT315A | 2 | To notepad | |||
| VD1 | Diode | D9E | 1 | To notepad | |||
| C1 | 10 µF | 1 | To notepad | ||||
| C2 | Electrolytic capacitor | 1 µF | 1 | To notepad | |||
| R1 | Resistor | 750 Ohm | 1 | To notepad | |||
| R2 | Resistor | 6.8 kOhm | 1 | To notepad | |||
| R3, R5 | Resistor | 100 kOhm | 2 | To notepad | |||
| R4 | Trimmer resistor | 47 kOhm | 1 | To notepad | |||
| R6 | Resistor | 22 kOhm | 1 | To notepad | |||
| PA1 | Microammeter | 1 | To notepad | ||||
| Fig.4 | |||||||
| Op-amp | 1 | To notepad | |||||
| Diode bridge | 1 | To notepad | |||||
| R1 | Resistor | 1 | To notepad | ||||
| PA1 | Microammeter | 1 | To notepad | ||||
| Fig.5 | |||||||
| Op-amp | 1 | To notepad | |||||
| Diode bridge | 1 | To notepad | |||||
| R1 | Resistor | 1 | To notepad | ||||
| R2 | Resistor | 1 | To notepad | ||||
| R3 | Resistor | 1 | To notepad | ||||
| PA1 | Microammeter | 1 | To notepad | ||||
| 2.1 Analog indicator | |||||||
| Fig.6 | |||||||
| Op-amp | 1 | To notepad | |||||
| VT1 | Transistor | N-P-N | 1 | To notepad | |||
| VT2 | Transistor | P-N-P | 1 | To notepad | |||
| VD1 | Diode | 1 | To notepad | ||||
| R1, R2 | Resistor | 2 | To notepad | ||||
| R3 | Trimmer resistor | 1 | To notepad | ||||
| R4, R5 | Resistor | 2 | To notepad | ||||
| R6 | Resistor | 1 | To notepad | ||||
| HL1, VD2 | LED | 2 | To notepad | ||||
| Fig.7 | |||||||
| DD1 | Logic IC | 1 | To notepad | ||||
| VT1 | Transistor | N-P-N | 1 | To notepad | |||
| R1 | Resistor | 1 | To notepad | ||||
| R2 | Resistor | 1 | To notepad | ||||
| R3 | Resistor | 1 | To notepad | ||||
| R4 | Resistor | 1 | To notepad | ||||
| HL1 | LED | 1 | To notepad | ||||
| Fig.8 | |||||||
| DD1 | Logic IC | 1 | To notepad | ||||
| R1-R4 | Resistor | 4 | To notepad | ||||
| R5-R8 | Trimmer resistor | 4 | To notepad | ||||
| HL1-HL4 | LED | 4 | To notepad | ||||
| Fig.9 | |||||||
| Chip | A277D | 1 | To notepad | ||||
| Electrolytic capacitor | 100 µF | 1 | To notepad | ||||
| Variable resistor | 10 kOhm | 1 | To notepad | ||||
| Resistor | 1 kOhm | 1 | To notepad | ||||
| Resistor | 56 kOhm | 1 | To notepad | ||||
| Resistor | 13 kOhm | 1 | To notepad | ||||
| Resistor | 12 kOhm | 1 | To notepad | ||||
| LED | 12 | ||||||