LM3915 is an integrated circuit (IC) manufactured by Texas Instruments, which reacts to changes in the input signal and outputs a signal to one or more of its outputs. Due to its design features, the IC is widely used in LED indicator circuits. Since the LED indicator based on the 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 cousins LM3914 and LM3916, which have the same pinout and layout. 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 capable of controlling various types of loads.
Short description LM3915
The block diagram of the LM3915 consists of ten operational amplifiers of the same type, operating according to the comparator principle. The direct inputs of the op-amp are connected through a chain of resistive dividers with different resistance ratings. Due to this, the LEDs in the load are lit in a logarithmic manner. An 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 integral 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 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 its principle of 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. Anyone, even a beginner, radio amateur can assemble such a simple device.
A typical turn-on provides power from a 12V source, which goes 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 dim the last two red LEDs and are optional. 12V also comes to a jumper that controls the mode of operation of the IC through pin 9. In the open state, the circuit operates in the "point" mode, i.e. one LED is lit, corresponding to the input signal. Closing the jumper switches the circuit to the “column” mode, when the input signal level is proportional to the height of the luminous column.
A resistive divider at R3, R4 and R7 limits the input signal level. A more precise setting is carried out with a multiturn trimmer R4. Resistor R9 sets the offset for the high 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 by the formula:
R5 = 12.5 / I LED, where I LED is the current of one LED, A.
The audio 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 alternate operation of the comparators, which will be indicated by the corresponding LED. To avoid overheating of the IC case, do not exceed the LED current of more than 20 mA. After all, this is an indicator, not a Christmas garland.
PCB and assembly parts
The printed circuit board of the audio level indicator in lay format can be downloaded. It measures 65 × 28 mm. Precision parts required for assembly. Resistors MLT-0.125W:
- R1, R5 R8 - 1 kΩ;
- 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 microcircuit. In the load, you can use ultra-bright LEDs of any glow color, up to violet. But these are personal aesthetic preferences. To display a stereo signal, you need two identical boards with independent inputs. More details on the LM3915 can be found in the datasheet here.
The efficiency of this indicator has been proven in practice by many radio amateur circles and is still available in the form of MasterKit sets.
Read the same
Self-made block of peak indication of a stereo signal with your own hands, a circuit of a simple peak indicator. The audio peaks indicate that the AF signal exceeds a certain preset value.
Here is a description of the peak LED indicator based on the CD4093 chip. The domestic analogue of which is K561TL1. The microcircuit contains four logical elements "2I-Not" with the effect of Schmitt triggers. In this circuit, the inputs of each of the elements are interconnected, so the elements work as inverters - Schmitt triggers.
Schematic diagram
The output signals of the stereo channels from the ULF output are fed through capacitors C1 and C2 to the inputs of elements D1.1 and D1.2, respectively. A constant bias voltage is supplied to the inputs of these elements through resistors R2 and R3 from the trimmer resistor R1.
At the inputs of the logic gates, the DC bias voltage is added to the variable component of the audio signal. The task of the resistor R1 is to set the optimal bias voltage at which the required sensitivity of the indicator will be, that is, this resistor sets the same peak threshold.
Rice. 1. Schematic diagram of a homemade peak indicator.
The state at the outputs of the elements D1.1 and D1.2 will change only when this threshold is exceeded, the set of this circuit is converted into logic level pulses, which, through the diodes VD1 and VD2, charge the capacitors C3 and C4. These circuits of diodes VD1, VD2, capacitors C3, C4 and resistors R4, R6 work as detectors.
And the voltage across capacitors C3 and C4 increases. This is especially important, since the peak moment of the input signal may not be long. And voltage is held in the form of a charge by these capacitors because they are quickly charged through diodes and slowly discharged through resistors.
As soon as the voltage on C3 or C4 reaches the switching threshold of the Schmitt trigger (D1.3 or D1.4, respectively), a logical zero appears at the output of D1.3 or D1.4, which leads to the lighting of the LED HL1 or HL2. The corresponding LED, or if the stereo signal is well balanced, both LEDs flash and stay on for at least the time required to discharge C3 or C4 through R4 or R6.
Details and adjustment
LEDs - any indicator, for example, AL307. Establishment - by adjusting the resistor R1 by the threshold.
Picture 1.
We bring to your attention a two-channel (stereo) level indicator with a peak detector from Ondrej Slovak. This indicator is developed on the PIC16F88 microcontroller, it can also be assembled on the PIC16F1827 microcontroller and on the PIC16F819 microcontroller. Peak indicator firmware for all these types of microcontrollers can be found in the attachment (archived). The schemes are similar, only the firmware is different. We will consider a circuit with a PIC16F88 microcontroller.
The display of levels and peaks in the indicator takes place on two LED scales (bars) of 16 LEDs each, 2 x16.
The modes in which the indicator can operate are shown in the table below, they are the same as in the previous diagram (indicator). They can be combined and combined by installing or removing jumpers (jumpers). Resistor R1 changes the sensitivity of the indicator, changes the voltage at pin 2 of the microcontroller, and the lower the voltage at pin 2, the higher the sensitivity of the indicator. The optimal voltage at the output is within 200-250 mV.
Table 1.Selection of display modes.
The indicator scale operates in two display modes: linear and logarithmic (below in the figure). The linear scale is programmed in the program code, but the values of the logarithmic scale can be changed at your discretion, or even made inversely logarithmic. This data is "hardcoded" into the EEPROM and can be changed.
Figure 2.
How to change the values of the EEPROM data itself, we will consider below.
Figure 3 shows a snapshot of the EEPROM codes of the ISPROG program.
Figure 3.
In the upper part of the table, the lines circled in red are the values (logarithmic) of the "ignition" of each LED (16 values), which correspond to the value of the logarithmic scale in Figure 2. These are the hexadecimal values of the vertical scale (2 to 248). You can build your own scale, for example, inverse logarithmic, and enter your values in these cells.
Further below we will analyze in parts;
03 - The first value is the LED ON time, the default is 12ms (1 = 4.096ms, i.e. 03 = (4.096 * 3) = 12.228ms)
08 - This is the time the last LED is on, the default is 33ms.
08 - This is the decay rate of the peaks, the default is 33ms.
7A - This is the peak persistence time, the default is 500ms. (7A = 122 * 4.096)
64 - This is the correction for the brightness of the LEDs. For LEDs with a luminescence current of 2 ma - value 64, for LEDs with a luminescence current of 20 ma - 08.
Watch a demo video of how the peak indicator works. Here it operates in the display mode with peaks in a falling mode, the scale is logarithmic (jumpers removed).
The diagram of the indicator is shown in Figure 4 below. The LEDs are used for a current of 3 mA, if you put the LEDs more powerful, for a current of 20 mA, then the resistors R1-R8 must be replaced with resistors of 22-33 Ohm, you can put smd resistors on the board. To quickly switch the operating modes of the indicator, the board is equipped with commutated jumpers ("jumpers").
Configuration of the PIC16F88 processor (installation of fuses, "fuses").
CP: OFF, CCPMux: RB0, Debugger: OFF, WRT: Writable, CPD: OFF, LVP: OFF, BOREN: ON, MCLRE: I / O, PWRTE: Disabled, WDTE: ON, OSC: INTRC-I / O, IESO: OFF, FCMEN: OFF
Configuration of the PIC16F1827 processor (installation of fuses, "fuses").
FOSC: INTOSC, WDTE: ON, PWRTE: OFF, MCLRE: OFF, CP: OFF, CPD: OFF, BOREN: ON, CLKOUTEN: OFF, IESO: OFF, FCMEN: OFF, WRT: OFF, PLLEN: OFF, STVREN: OFF, BORV: HI, LVP: ON
In the attachment in the archive, there are also the initial parts of the asm codes for these processors, which indicate the processor configurations.
* When designing and setting up his developments on microcontrollers, the author uses the PRESTO USB programmer and, accordingly, the accompanying ASIX software - the ASIX UP program. Processor configurations are listed for this program.
I repeated this construction using the ExtraPic programmer and the icprog program. I did not install or control the processor configurations. Immediately after the firmware, the circuits started working (I mean the first circuit for 40 LEDs), I repeated it several times - everything started working right after the firmware.
Figure 4.
The indicator is assembled on a printed circuit board, measuring 84 x 27 mm. Photo of the printed circuit board below in Figure 5. On-board resistors R1-R8 smd.
Figure 5.
Figure 6 below shows the jumpers soldered on the board between the LED strips.
Figure 6.
The appearance of the assembled indicator. The board has flat LEDs, resistors R1 - R8 of the smd type, soldered from the back of the board, from the side of the tracks.
Figure 7.
The printed circuit board of the indicator (in the Sprint-Layout format is available in the archive) with the arrangement of the elements is shown in Figure 8. There are no jumpers indicated on the board between the LED strips, as they are located one above the other. The jumpers are unsoldered to the places designated by the numbers 1 - 7, and first jumper # 1 is installed in place 1-1, then - 2 in place 2-2, etc.
Figure 8.
Below in the archive there is a diagram, drawings of a printed circuit board in Sprint-Layout format, firmware for microcontrollers PIC16F88, PIC16F1827, as well as a printed circuit board in Sprint-Layout format with an increased distance between the rows of LEDs and designed for installing round LEDs, as well as firmware for PIC16F819 microcontroller.
If anyone has any questions about the design of the indicator, ask them.
The radio designer came in a bag:
Details: 
The board is one-sided, without metallization, made with high quality, easy to solder, part designations and ratings are indicated: 
The photo shows that the board differs from the board displayed on the seller's lot - there is a J3 connector
Instruction and diagram: 
High resolution schematic
Soldered. Here's what happened:
Don't scold me for soldering - I haven't soldered anything on seals for 27 years. First experience.
There are no superfluous parts in the kit.
When soldered, three misunderstandings became clear.
1. It's not clear why there is a J3 jumper connector here? The kit does not include a connector or a jumper. When turned on, only half of the LEDs (red and below) work somehow incomprehensibly. Soldered (short-circuited) contacts J3
2. Resistor R9. The printout shows 560 ohms. The set contains 2.2 kOhm. I put an MLT resistor from old stocks, as indicated in the diagram - 560 ohms. I thought that the Chinese had confused something. When turned on, the two lower yellow LEDs were constantly on - D1, D2. I re-soldered the resistor - I took a 2.2 kOhm resistor from the set - it began to work as it should.
Change in circuit - correct resistor
3. If the extreme red LED lights up and is constantly on, then the resistor R5 begins to warm up to 60 degrees. Weird.
The power supply of the circuit is 9-12 volts. Applied 12 V to power supply. Everything works fine. The trimming resistor can be used to set the maximum displayed signal level. The minimum level, if a signal with a voltage of 1.9 volts is applied to the device: 
Hence the conclusion - with 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.
The glow scale of the LEDs is logarithmic. You cannot use it as a battery discharge indicator. If you connect the headphone output of a cell phone at maximum volume to the input, a maximum of 6 yellow LEDs are lit.
Then I decided to experiment with decreasing the supply voltage. Conclusion - the lower the supply voltage, the more sensitive the device. It worked fine from 5V - 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 not blinking. Apparently this is the limit. So I would not be supplying 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 (because of the socket and the microcircuit) turns out to be quite high. If you put two boards in parallel, then the distance between the LEDs of both channels will be quite large.
Indicator on LM3915
The integrated circuit LM3915 is specially designed for building an LED level indicator and allows you to visually assess the level and change of the sound signal in the form of a light "column", "ruler" or a luminous point moving on a conventional scale. The successful design of the LM3915 chip has secured its rightful place in LED indicator circuits. The wizard invites you to assemble a sound indicator based on LM3915 and 10 LEDs. Below are detailed instructions for assembling a sound indicator circuit with photo and video illustrations with your own hands. Even a novice electronics engineer can assemble a sound indicator.
How to assemble an LED level indicator on the LM3915 with your own hands
The design of the LM3915 microcircuit represents ten comparator operational amplifiers of the same type enclosed in a package. The direct inputs of the amplifiers are connected through a line of resistive dividers selected so that the LEDs in the amplifier load are turned on according to the logarithmic dependence. The reverse inputs of the amplifiers receive the input signal, which is formed by the buffer amplifier (pin 5). The design of the microcircuit also includes an integral stabilizer (pins 3, 7, 8), as well as a key for setting the operating mode of the indicator (pin 9). The microcircuit has a wide supply voltage range from 3 to 25 volts. The value of the reference voltage is set in the range from 1.2 to 12 volts by external resistors. The indicator scale corresponds to a signal level of 30 dB with a step of 3 dB. The output current is adjustable from 1 to 30 mA.
Assembling the indicator is simplified by purchasing a set of parts in the online store using the link https: //ali.pub/2c62ph ... The kit includes a board, a microcircuit, LEDs and all the necessary harness (resistors, capacitors and connectors).
Set of parts "Sound level indicator on the LM3915"
Details of the set "Sound level indicator on the LM3915"
The diagram of the sound indicator on the LM3915 is shown in the photo.
Operating principle. A supply voltage of 12 volts is supplied to the third terminal of the LM3915. It also goes through the limiting resistor R2 to the LEDs. Resistors R1 and R8 equalize the brightness of the red LEDs on the scale. Also, a voltage of 12 Volts is supplied to the jumper for controlling the operating mode of the indicator (pin 9). In the closed state of the jumper, the circuit provides lighting of only one LED corresponding to the signal level. When the jumper is open, the circuit operates in the "column" effect, the input signal level is proportional to the height of the illuminated column or the length of the line. The divider assembled on R3, R4 and R7 limits the input signal level. The divider is fine-tuned with a multiturn trimmer R4. The divider R9 R6 sets the offset for the upper level of the slide rule of the microcircuit resistances (pin 6). The lower level of the slide rule of resistances (pin 4) is connected to the common wire. Resistor R5 (pin 7) increases the reference voltage and affects the brightness of the LEDs. R5 sets the current through the LEDs and is calculated by the formula: R5 = 12.5 / Iled, where Iled 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 alternate operation of the comparators, which will be indicated by the corresponding LED. According to the instructions, in order to avoid damage to the microcircuit, the limitation of 20 mA of current supplied to the LEDs should not be exceeded.
Buzzer indicator assembly
We check the availability and denominations of parts.
Resistances: R1, R5 R8 - 1 kOhm; R2 - 100 Ohm; R3 - 10 kOhm; R4 - 50 kOhm, any trimmer; R6 - 2.2 kOhm (560 Ohm); R7 - 10 Ohm; R9 - 20 kOhm. Capacitors C1, C2 - 0.1 μF. We decipher the resistor values by color code. See photo.
To assemble the circuit, you will need a low-power soldering iron, soldering flux, solder and side cutters. The assembly sequence may be different.
- We install the resistors on the board according to the nominal value and solder them, and also use the key drawn on the board to install and solder the bed for the microcircuit.
- Similarly, we solder the variable resistor, capacitors, connection jacks.
2 option for installing LEDs on the level indicator board on the LM3915
- We check the correctness of assembly and soldering, if necessary, we eliminate errors.
- We insert the microcircuit into the crib using the key drawn on the board.
- We supply a voltage of 12 volts from the power supply.
- We send a signal from the telephone output of any gadget. If all the parts are correctly installed and in good working order, then the circuit will work. Watch the video. The audio signal level at the input is set by the trimmer R4. Watch the video.
Placing the LM3915 chip on the crib comes in handy. The microcircuit has relatives LM3914 and LM3916 with a linear and stretched scale. The microcircuits are absolutely identical in terms of their pins. Therefore, on the basis of this circuit, you can easily assemble an indicator of voltage, power or an indicator of control of any parameter.
A set of parts for assembling an LED audio signal level indicator on the LM3915 can be purchased at the following link http://ali.pub/2z6xyo ... If you want to seriously practice soldering simple structures, the Master recommends purchasing a set of 9 sets, which will greatly save your shipping costs. Here is the link to buy http://ali.pub/2bkb42 ... The master has collected all the sets and they worked.
I wish you success and growth of skills in soldering.
