Output power LED indicator. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Audio equipment
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The design of LED indicators is more complicated than conventional ones. Of course, when using a special control chip, it can be simplified to the limit, but here a small nuisance lurks. Most of these microcircuits develop a current of no more than 10 mA at the output, and the brightness of the LEDs in a car may be insufficient. In addition, microcircuits with outputs for 5 LEDs are most common, and this is only the "minimum program". Therefore, for our conditions, a circuit based on discrete elements is preferable; it can be expanded without much effort.
In addition to greater freedom in the choice of LEDs, it is possible to form a scale of any type - from linear to logarithmic, or "stretch" only one section by simple means.
The LEDs in this circuit are controlled by keys on transistors VT1 ... VT5. Switching thresholds are set by diodes VD3...VD9. By selecting their number, you can change the dynamic range and scale type. The overall sensitivity of the indicator is determined by the input resistors. The figure shows approximate response thresholds for two circuit options - with single and "double" diodes. In the basic version, the measurement range is up to 30 W at a load of 4 ohms, with single diodes - up to 18 W.
(click to enlarge)
The HL1 LED is constantly on, it indicates the beginning of the scale, HL6 is an overload indicator. Capacitor C4 delays the extinction of the LED by 0,3 ... 0,5 seconds, which makes it possible to notice even a short-term overload. The storage capacitor C3 determines the flyback time. By the way, it depends on the number of luminous LEDs - the "bar" from the maximum starts to fall off quickly, and then "slows down". Capacitors C1, C2 at the input of the device are needed only when working with the built-in amplifier of the radio. When working with a "normal" amplifier, they are excluded. The number of signals at the input can be increased by adding chains of a resistor and a diode. The number of indication cells can be increased by simple "cloning", the main limitation is that there should be no more than 10 "threshold" diodes, and there should be at least one diode between the bases of adjacent transistors.
LEDs can be used in any way depending on the requirements - from single LEDs to LED arrays and high-brightness panels. Therefore, the diagram shows the values of current-limiting resistors for different operating currents. There are no special requirements for the rest of the details, transistors can be used in almost any npn structure with a collector dissipation power of at least 150 mW and a double collector current margin. The current transfer coefficient of the base of these transistors must be at least 50, and better - more than 100.
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