LED signal strength indicator. Radio constructor - LED indicator of low-frequency signal level

The radio designer came in a bag:

Details:

The board is single-sided, without metallization, made with high quality, easy to solder, part designations and ratings are indicated:

The photo shows that the board is different from the board displayed on the seller's lot - there is a J3 connector

Instructions and diagram:

High resolution diagram

Soldered. Here's what happened:

Don't blame me for the soldering - I haven't soldered anything on a signet for 27 years. First experience.
There are no extra parts included.

When I soldered, three misunderstandings became clear.
1. It’s not clear why there is a jumper connector J3? The kit contains neither a connector nor a jumper. When turned on, somehow incomprehensibly only half of the LEDs work (red and below). Soldered (shorted) contacts J3
2. Resistor R9. The printout shows 560 ohms. The set contains 2.2 kOhm. I installed an MLT resistor from old supplies, as indicated in the diagram - 560 Ohms. I thought that the Chinese had mixed something up. When turned on, the two lower yellow LEDs - D1, D2 - were constantly on. I resoldered the resistor - took a 2.2 kOhm resistor from the kit - and it began to work as it should.

Change in circuit - correct resistor

3. If the outermost red LED lights up and is constantly on, then resistor R5 begins to heat up to 60 degrees. Strange.

The power supply for the circuit is 9-12 Volts. I supplied 12 V for power. Everything is working fine. Using a trimmer resistor, you can set the maximum displayed signal level. Minimum level if a 1.9 Volt signal 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 preamplifier or to the ULF input after the volume control.

The LED luminescence scale is logarithmic. It cannot be used as a battery low indicator. If you connect the headphone output cell phone on maximum volume to the input, then a maximum of 6 yellow LEDs are lit.

Next I decided to experiment with reducing the supply voltage. Conclusion - the lower the supply voltage, the more sensitive the device. It worked normally from 5 V - the red LEDs in this case were also lit from the cell phone. If you reduce the voltage to 3 volts, the LEDs glow dimly but do not blink. Apparently this is the limit. So I would not power it from a voltage less than 5 volts.

Conclusion: a simple, interesting radio design. You can equip it with some homemade ULF. Cons - inconvenient mounting of the board - only one mounting hole. The fee (due to the socket and microcircuit) is quite high. If you place two boards in parallel, the distance between the LEDs of both channels will be quite large.

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Many people remember well how at the dawn of the 80s, tape decks (Japanese) had recording level indicators with peak displays. Having such an indicator at your disposal was the dream of many radio amateurs and music lovers, but assembling it yourself at that time was simply not realistic.
With the advent of microcontrollers, circuitry has changed dramatically, and now the peak indicator circuit does not look more complicated than the circuit a simple transistor radio from the 80s.
We present to your attention peak indicator signal level on the PIC16F88 microcontroller, mono, LEDs are used as indicators or LED matrices. The inputs of the left and right channels are combined in it. Or for the second channel it is necessary to make another similar indicator. The number of LEDs in the indicator (matrix) is 40 pcs. An indicator would look good, for example, on such matrices (10 LEDs each).

4 such matrices are needed per channel. Choose the color of the glow to suit your taste. You can use single-color ones, or you can put the last one, for example, yellow or red) if the first ones are green).
Or, for example, there are also such matrices of 20 LEDs. You need 2 of them per channel.

Watch a demo video of how the peak indicator works. Here it works in indication mode with peaks in a falling mode, the scale is logarithmic (resistors R11-R14 are missing, or the jumpers have been removed).

The indicator can also work in linear mode, with and without peak indication, also in running dot mode with and without peak indication. The peak indication itself operates in two modes - normal and falling. Normal - these peaks burn for 0.5 seconds and go out, falling - these peaks burn for 0.5 seconds and fall down (if the signal level is at at the moment became below the level that was 0.5 sec. back).
The indicator diagram is shown below. The LEDs are used at a current of 3 mA, if you install more powerful LEDs at a current of 20 mA, then resistors R1-R8 must be replaced with resistors of 22-33 Ohms. R11-R14 are set depending on the required operating mode of the indicator. To quickly switch modes, you can install switched jumpers (“jumpers”) at the points of their connection with the common wire.

Processor configuration (installation of fuses, fuses)

CP:OFF, CCP1:RB0, DEBUG:OFF, WRT_PROTECT:OFF, CPD:OFF, LVP:OFF, BODEN:ON, MCLR:OFF, PWRTE:OFF, WDT:ON, OSC:INTRC_IO, IESO:OFF, FCMEN: OFF.

The modes in which the indicator can operate are shown in the table below. They can be combined by installing or removing jumpers (resistors). Resistor R1 changes the sensitivity of the indicator, changes the voltage at pin 2 of the microcontroller, and the lower the voltage at the pin, the higher the sensitivity. The optimal voltage at the output is 200-250 mV.

Table 1.Selecting display modes.

Resistor jumper	Absent	Present
	Running line	running point
	Logarithmic scale	Linear scale
	With peak indication	No peak indication
	Falling peaks	Simple peaks

Below in the archive there is a diagram and drawings printed circuit board, microcontroller firmware.

Do-it-yourself homemade stereo signal peak indication block, circuit diagram of a simple peak indicator. Peak indicators of audio signals indicate the fact that the AF signal level 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 connected to each other, so the elements work as inverters - Schmitt triggers.

Schematic diagram

The output signals of stereo channels from the ULF output are supplied through capacitors C1 and C2 to the inputs of elements D1.1 and D1.2, respectively. The inputs of these elements through resistors R2 and R3 receive constant voltage offset from trimming resistor R1.

At the inputs of logic elements, the DC offset voltage is added to the AC component of the audio signal. The task of resistor R1 is to set the optimal bias voltage at which the required sensitivity of the indicator will be achieved, that is, this resistor sets the same peak threshold.

Rice. 1. Schematic diagram homemade peak indicator.

The state at the outputs of elements D1.1 and D1.2 will change only when this threshold is exceeded; the set value of this circuit is converted into logic level pulses, which charge capacitors C3 and C4 through diodes VD1 and VD2. These circuits of diodes VD1, VD2, capacitors C3, C4 and resistors R4, R6 work as detectors.

And the voltage on capacitors C3 and C4 increases. This is especially important since the peak moment of the input signal may not last long. And the voltage in the form of charge is retained by these capacitors because they are charged quickly through the diodes and discharged slowly through the resistors.

As soon as the voltage at 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 causes the HL1 or HL2 LED to light up. The corresponding LED, or if the stereo signal is well balanced, both LEDs flash and remain on for at least the time required to discharge C3 or C4 through R4 or R6.

Details and setup

LEDs - any indicator, for example, AL307. Setting up - adjusting resistor R1 according to the operating threshold.

LM3915 – integrated circuit(IC) manufactured by Texas Instruments, 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. Because LED indicator based on LM3915 works on a logarithmic scale, he 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 linear characteristic and is ideal for measuring linear quantities (current, voltage), and the 3916 series IC is more universal and is capable of controlling different types of loads.

Brief description of LM3915

The block diagram of LM3915 consists of ten similar operational amplifiers, working 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. Thanks to this, the LEDs in the load light up according to a logarithmic dependence. Comes to inverse inputs input signal, which is processed by the 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 fundamental electrical diagram 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, 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, 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 top 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. Further growth 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, the LED current should not exceed 20 mA. After all, 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 technical description 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.