LED design is somewhat more complicated. Of course, by using a special control circuit can be simplified to the limit, but there lurked a little trouble. Most of these chips produces the output current up to 10 mA, and the brightness of the LEDs under the car may not be enough. In addition, the most common chip in yields on 5 LEDs, and it is only "minimum program". Therefore, for our conditions diagram on discrete elements is preferable, it can be extended without much effort.
The simplest LED display (Fig. 4) does not contain active elements and thus does not need power. Connection - to the unit under the scheme "mixed mono" or split capacitor, an amplifier - "mixed mono" or directly.
Fig. 4
The scheme is very simple and does not require the establishment. The only procedure - selection of R7. On the face value of the scheme to work with built-in amplifiers head unit. When working with power amplifier 40 ... 50 W resistance of this resistor should be 270 ... 470 ohms.Diodes VD1 ... VD7 - any silicon with forward voltage drop of 0.7 ... 1 and permissible current of at least 300 mA.
Any LEDs, but the same type and color glow with operating current 10 ... 15 mA. Since LEDs are "fed" by the amplifier output stage, their number and increase the operating current in this circuit is impossible. So we have to choose the "bright" LED indicator or find a place where it will be protected from direct light. Another disadvantage of the simplest design - small dynamic range.
To improve the work required indicator to the control circuit. In addition to greater freedom in the choice of the LEDs can be simple means to generate any type of scale - from linear to logarithmic, or "stretch" the only one site. Indicator diagram with a logarithmic scale is shown in Fig.5. The dotted line shows the optional items.
Fig. 5
LEDs in this scheme the control keys on the transistors VT1 ... VT5. Thresholds options specify diodes VD3 ... VD9. Picking up their number, you can change the dynamic range and scale type. Overall sensitivity of the indicator is determined at the input resistors. The figure shows the approximate thresholds for the two variants of the scheme - with single and "twin" diodes. The basic version of the measuring range - up to 30 watts into 4 ohms, with single diodes - up to 18 watts.
HL1 LED lights continuously, it means the beginning of the scale, HL6 - overload indicator. Delay capacitor C4 0.3 ... 0.5 seconds extinction LED that allows even notice briefly overloaded. Storage capacitor C3 determines the return stroke. It is, by the way, depends on the number of LEDs glowing - "column" of the maximum begins to subside quickly, and then "brakes". Capacitors C1, C2 at the input only when working with integrated amplifier recorder. When dealing with a "normal" amplifier exclude them. Number of input signals can be increased by adding a chain of resistors and a diode. The number of cells can be increased simply indicating "cloning", a major limitation - the "threshold" diodes should be no more than 10 and between the bases of adjacent transistors must be at least one diode.
LEDs can be any, depending on requirements - from single LED to LED assemblies and panels, high-brightness. Therefore, the diagram shows the current-limiting resistors ratings for different operating currents. The remaining details of any special requirements are not established, transistors can be used almost any structure with npn collector power dissipation of 150 mW at least twice and reserve current collector. Transfer coefficient of the base current of these transistors must be at least 50, but better - more than 100.
This scheme can be simplified somewhat, with the side effect of new properties appear, very useful for our purposes (Fig. 6).
Fig. 6
Unlike previous schemes, where transistor cell were connected in parallel, series connection is used here "column". Threshold elements are transistors and open themselves, they in turn - "bottom up." But in this case depends on the threshold voltage. The figure shows the approximate indicator thresholds at a supply voltage 11 V (the left edge of the rectangle) and 15 (right border). It is seen that with increasing voltage is shifted most border indicating the maximum power. In the case of the amplifier, the power of which depends on the voltage of the battery (and there are many), this "auto-calibration" can benefit.
However, the fee is - increased load transistors. Through the lower scheme transistor current flows all the LEDs, so the use of indicators with a current 10 mA transistors also require appropriate power. "Cloning" cell further increases the scale unevenness. Therefore, 6-7 cells - is the limit. Assignment of the remaining elements and their requirements - are the same as in the previous scheme.
Slightly modernized this scheme, we obtain other properties (Fig. 7). In this scheme, unlike previously discussed, there is a glowing "ruler". At any time, only one LED lights, simulating the movement of arrows on a scale. Therefore, the power consumption is minimal and in this scheme can be applied low-power transistors. The rest of the scheme does not differ from those discussed previously.
Threshold diodes VD1 ... VD6 designed to safely disconnect idle LEDs, so if there will be a weak illumination superfluous segments, you must use diodes with a large forward voltage connected in series or two diodes. "Cloning" cells reduces the brightness of the upper segments of the scheme, to address this instead of resistor R9 need to enter the current generator. And we agreed - not to complicate.Therefore, in this case, an 8-cell - is the maximum.
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