Audio dial indicator
Many sound reproducing devices, whether tape recorders or amplifiers of the end of the last century, were equipped with a dial gauge on the front panel. His arrow moved to the beat of the music, and although it had no practical significance, it looked very beautiful. Modern equipment, in which compactness comes first and high functionality no longer has such luxury as a dial gauge of sound. However, it is quite realistic to find an arrow head now, which means that such an indicator can be easily assembled with your own hands.It is based on the Soviet K157DA1 microcircuit, a two-channel, full-wave medium value rectifier. The supply voltage of the circuit lies in a wide voltage range, from 12 to 16 volts, because The circuit contains a stabilizer of 9 volts (VR1 on the circuit). If you use a stabilizer in the metal case TO-220, then the voltage can be supplied up to 30 volts. Trimming resistors R1 and R2 regulate the signal level at the input of the chip.The circuit is not critical to the ratings of the components used. You can experiment with capacitors C9, C10, which affect the smoothness of the arrow, as well as with the resistors R7 and R8, which specify the time of the reverse arrow. In L and In R on the circuit are connected to a sound source, which can be any device with a linear output - be it a computer, a player or a phone.
Circuit assemblyThe indicator board is manufactured using LUT method on a piece of textolite with dimensions of 30 x 50 mm. Just in case, the microcircuit should be installed in the panel, then it can be replaced at any time. After etching, the charge must be tinned, then it will look beautiful from the side of the tracks, and the copper itself will not oxidize. First of all, small parts are soldered - resistors, ceramic capacitors, and only then electrolytic capacitors, trimming resistors, microcircuit. Lastly, all connecting wires are soldered. The board contains two channels at once and assumes the use of two switch heads - on the right and left channels,however, you can use one arrow head, then the input and output contacts for another channel on the board can be simply left blank, as I did. After installing all the parts on the board, it is imperative to wash off all remaining flux, check the adjacent tracks for short circuit. It is most convenient to use jack 3,5 for connecting the board to the signal source. In this case, if the length of the wires from the board will be large (more than 15 cm), you should use shielded wire.
Arrow headFinding the sale of Soviet arrow heads now is not difficult, there are many types, different shapes and sizes. I used a small arrowhead M42008, it does not take up much space and looks nice. For this scheme any head with a current of a total deviation of 10-100 microamps is suitable. For completeness, you can also replace the native scale, graduated in microamperes, with a special sound scale,graduated in decibels. However, it is not necessary to directly connect the dial head to the circuit, but through a trimmer resistor of 1–2 megohms. Its middle contact is connected to any of the extreme ones and is connected to the board, and the remaining contact is connected directly to the head, as seen in the photo below.
Indicator settingWhen the board is assembled, the switch head is connected, you can begin the tests. First of all, by applying power to the board, check the voltage at the 11th output of the chip, there should be 9 volts. If the supply voltage is normal, you can apply a signal from the sound source to the board input. Then, using resistors R1 and R2 on the board and a trimming resistor at the switch head, achieve the desired sensitivity so that the arrow goes off scale and is approximately in the middle of the scale. This completes the basic setup, the arrow will move smoothly to the beat of the music. If you want to achieve a sharper behavior of the arrow, you can install resistors with a resistance of 330-500 Ohm parallel to the switch heads.Such an indicator will look great in the case of a self-made amplifier, or as an independent device, especially if you highlight the indicator with a pair of LEDs. Happy assembly!
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