Sound dial indicator. Amplifier output power indicators How to install a dial indicator in an amplifier

Visibility is a big deal. So popular wisdom says: “It’s better to see once than to hear a hundred times.” And in electronics, where the ongoing processes in the operation of a particular device are often confirmed indirectly, or even generally implied and even taken on faith, it is generally difficult to overestimate the visual display. It is not for nothing that oscilloscopes are so revered among radio amateurs, giving them the opportunity to “look” even into the process. But I won’t talk about the complex - I’d like to deal with the simple ones. I have assembled almost a dozen different chargers, and to charge batteries I increasingly use a simple laboratory power supply that has an output voltage and current. The measuring heads clearly inform how many volts and milliamps go to the battery being charged. But it’s not possible to use them everywhere; even the smallest of them will often still be prohibitively large for many amateur radio homemade products. But dial indicators from tape recorders and other radio devices of the last century, which have not been sold out in the bazaars to this day, will be just right here. Here are some of them:

Designed for operation in DC circuits, at any scale position. Total deflection current (depending on model) 40 - 300 µA. Internal resistance 4000 Ohm. Scale length - 28 mm, weight 25 g.

Designed to work with the scale in a vertical position. Deviation current 220 - 270 µA. Internal resistance 2800 Ohm. Dimensions 49 x 45 x 32 mm. Scale length - 34 mm.

designed to work at any scale position. The total deviation current is no more than 250 µA. Internal resistance 1000 Ohm. Dimensions 21.5 x 60 x 60.5 mm. Weight 30 gr. These indicators and others like them are united by:

• small size
• simplicity of design
• low cost
• and, of course, the principle of operation

The operating principle is based on the interaction of two magnetic fields. The fields of a permanent magnet and the field formed by a current passing through a frameless frame, which consists of a large number (115 - 150) turns of copper wire with a diameter of only 8 - 9 microns. Without delving into the nuances, we can name two main actions that need to be performed in order to make it possible to use the existing indicator:

1. Equip it with a shunt or additional resistance (used to change the upper limit of measurement), depending on how you will use it (voltmeter / ammeter).
2. Make a new scale.

Discuss the article POINT INSTRUMENTS - INDICATORS

While sorting through the trash in the closet, I accidentally found my last year (autumn 2013) craft - a dial indicator of the sound level on the K157UD2 microcircuit. For some reason, she didn’t want to work for me then, and I threw her far away. And now I’ve decided to finally figure out what’s the matter? After all, the first copy of the device, made that same summer, still works properly.
The article that describes the amplifier circuit on a microcircuit is located, option 2, “Single-supply circuit.” There you can also see the pinout of the K157UD2 microcircuit. I am attaching a diagram with my denominations, the main part of which is the M68501 indicator and its wiring.

I’ll note right away that it can be connected either to exit sound amplifier and entrance. In the first case, the dial indicator will show the power of the output signal (and, accordingly, when the volume is reduced by the regulator, the arrow will “fall”), and in the second case, the power of the input signal, which is sometimes more useful (for example, visually monitoring the power of the input signal, since if it too much comes in, the signal may begin to distort). In the diagram, some numbers of the pins of the microcircuit are indicated in brackets - this means that you can assemble two identical amplifiers on one chip, and, accordingly, connect two indicators: to the right and left channels (or to the input and output of the amplifier).
It turned out that the cannons did not fire for twenty reasons, and the first of them was that there were no shells. And if we talk about the microcircuit, there were serious problems with its power supply. I also had to replace both electrolytic capacitors (at that time I didn’t yet buy them in buckets, so I installed them pulled out from somewhere), deal with the falling leg of the 22 nF capacitor and connect it correctly. After this, the circuit worked, although I still don’t know where it can be adapted.
Diodes - D311. D18 will be a little worse.
Resistor R5 is trimmer and has an asterisk - this means that not only will it have to be adjusted to the signal level (so that, for example, at normal amplifier volume the needle dangles around 75% of the scale), it is also not a fact that 47 kOhm suitable for all occasions.
If you increase the value of resistor R4 (470 - 910k), you can raise the gain of the microcircuit and make it “feel” weaker signals (this is just useful if the indicator is connected to entrance sound amplifier). For example, to observe the sound output from the player I had to install a 1 MOhm resistor.
Some photos of my circuit:





And a demonstration of the work when the output of “VEF 216” is monitored:

A special feature of the circuit is its low sensitivity to high-frequency signals (the needle moves with greater pleasure from drums and bass guitars than from voices and guitar solos).
And for the night, I built two blue five-millimeter LEDs into the indicator housing. Normally they light from five volts, if less, then only one works, the second one turned out to be burnt. For compatibility with other supply voltages, the backlight is turned on through a 500 Ohm trimming resistor - you can easily power the entire circuit from 5 to 9 volts, you just need to adjust the voltage.

Output indicators are currently very popular, especially for their use in modernizing rare equipment. Many radio amateurs remember very well the Soviet power amplifier Radiotehnika U-101 from the Riga plant of the same name. In the early 80s, the plant began producing a new model, the international standard (dimensional) music complex “Radiotehnika K-101 stereo”. Overall, this combine was a very good complex. But the amplifier, or rather the output power indicator built into it, was either imperfect or there were design errors.

Nevertheless, when the device was new, it did not cause any complaints, but over time it began to cause some inconvenience with its not clear and dim glow of the scale, or in general some element in the control circuit failed. Recently I also became the owner of such an amplifier. Of course, I had no desire to restore the standard indicator, and initially I already intended to install pointers in the device. Moreover, I had several of these in stock, and in my opinion it’s not difficult to find them on radio markets. But be that as it may, I began restoration and partial modernization in order to establish dial indicators of the output signal Radiotehnika U-101 on K157DA1. p>

First, I took three-millimeter plastic and cut out 3 rectangular pieces from it, and then glued the indicators together using dichloroethane. Plastic strips should be adjusted so that they are the same width as the indicators and do not protrude beyond the perimeter. Here the photo shows a design with a natural window size in the front panel of the power amplifier.

I made windows in the glass from the standard indicator and put them on new dial indicators. It is advisable to process the glass with a small fine file or needle so that it fits tightly into place. Then I glued it all together again with dichloroethane. Of course, this whole operation must be done very carefully, since this is a front panel and should look accordingly.

Here comes a crucial stage.
There is a small gap on top of the indicators, relative to the window in the glass. So let it remain like that, it will be convenient to place SMD LEDs there for illumination.

Now you need to solder the wires to the LEDs and place them in the gap between the indicator and the glass with a small amount of super glue.

I also cut out a strip of plastic and attached it to the side walls. After it is still attached to the glue, the structure will acquire even greater rigidity and will serve as the basis for installing a control board on it.

This photo shows the standard installation location for the indicator. There you can also see a red connector with wires; it is designed to supply power to the control board. It will certainly be needed in the future.

At this stage, it is necessary to try on the assembled module to see how it becomes. The fact is that this design is not fastened with any screws, but is simply pressed against the chassis by the front panel power amplifier. Therefore, it is necessary to ensure the tightest possible fit. Under the wires coming from the LEDs, use a round needle file to make a small cut in the chassis.

Schematic diagram and printed circuit board of the control module

The circuit is powered unipolarly - 9 volts, and is made using a simple voltage stabilizer made on the 78L09 microcircuit - it is shown in the diagram.

The device is connected to the output of a power amplifier, although its sensitivity is quite sufficient for picking up sound from the linear input.

The device is configured using variable resistors with a nominal value of 30K and capacitors C7 and C8. Variable resistors adjust the position of the needle at maximum power, and capacitors adjust the return time of the needle.

This dial indicator is assembled on a printed circuit board, which is mounted on the housing of the indicator heads.

The indicator heads were taken from an old Soviet tape recorder. Also, almost any beautiful switches with a total deflection current of 50-200 μA are suitable here. If you wish, as is now fashionable, you can make the scale blue or green. Author of the article: M. Pelekh

I remember a carefree childhood - while visiting a classmate, we listened to music. Amplifier “Radiotekhnika-001-stereo”, the indicators sway gently to the beat of the music... Then it was the ultimate dream. And it seemed blasphemous when the father of a classmate (the man was fond of amateur radio) replaced the standard dial indicators with a luminescent one of an ugly green color. And the amplifier lost some of its charm, and I didn’t want to listen to it anymore...

I want a switch!

A legend of deep antiquity... K157DA1

Numero due - LM324

Century XXI, Attyny13

And this is where the fun begins for me. The natural logarithm function is available in the library of mathematical functions for Atmel controllers and is located in the file math.h. But it just doesn’t fit into this controller - there’s not enough memory. It’s not possible to solve the problem head-on, so we begin to wrinkle our forehead. The use of a more powerful controller was not considered - not interesting. There seems to be enough memory, and it’s convenient, and inexpensive, and the dimensions are not large. The first thing that came to mind was to replace this function with a similar one, but simpler. And give it shape by playing with the coefficients. Let us recall the graph of the inverse function. Not “screw it!”, but remember! If you move the lower right square upward relative to the X axis, and slightly move the coefficients back and forth, then it is quite possible to adjust it to the desired shape. Here it is, a formula that replaces the logarithm: Y=-8196/(X+28)+284. Can you imagine the horror of a controller doomed to calculate these values ​​thousands of times per second at the whim of the owner, who wanted to remember his “golden childhood”?

But unpleasant emotions were also guaranteed for the owner of the controller. Short integer values ​​were not enough to process the results, and the input and output had to be just that. For me, translating data presentation formats in controllers from one to another has always been difficult. The wrinkles on my forehead multiplied.

The second option was born- calculate everything in advance, and the controller will simply select data from the array that corresponds to the input values ​​and throw them out. Preparing values, setting an array - compilation error. The array dimension is too large for this controller. But making several arrays and tinkering with them depending on the input value of the ADC is not kosher. Thoughts about Newton's binomial swarmed, but were rejected due to non-constructiveness.

Here a phrase from a lecturer in higher mathematics from a university came to mind: “Using a cubic spline approximation, you can describe any function.” Well, we don’t need a cubic one, but a linear spline will do just fine! Thus, I practiced a little in OO Calc, and wrote a system of equations that fairly accurately replicate the graph of a logarithmic function using line segments:
if (n>=141) x=2*n+2020; else if (n>=66) x=5*n+1600; else if (n>=38) x=9*n+1330; else if (n>=21) x=15*n+1110; else if (n>=5) x=40*n+600; else if (n>0) x=160*n+50; if (n==0) x=0;
Everything is intentionally multiplied by 10 so that the discarded “tails” are smaller. I then divide it in the program before displaying it on indicators.
And here are the graphs:

I am sure that such a solution will immediately come to mind for many of you and seem obvious. However, I am sure that this will be new to someone and will be useful in the future. At least as a tool in your arsenal it will not be superfluous to have.

Video

Summary and notes on the diagram

The circuit can be tuned. Tinka has 2 legs left free. No one is stopping you from sticking LEDs there to indicate overload, it was once fashionable. Not my thing - I don’t like it when something on the amplifier blinks, that’s why I didn’t do it. The implementation is elementary: at a certain level we light the LED and keep it lit for N milliseconds. Level and N are adjusted to taste, like salt and pepper. Just remember that one of the free legs is Reset. This means that you should do your experiments on one channel, because if you install the appropriate fuse when flashing the firmware, Reset will become just a port, and you won’t be able to change the controller after that.

Files

Good! The freebie is over. If you want files and useful articles, help me!