LED indicators

The design of LED indicators is somewhat more complicated. Of course, when using a special control chip, it can be simplified to the limit, but here a little trouble 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.

The simplest indicator on LEDs (Fig. 4) does not contain active elements and therefore does not need power. Connection - to the radio according to the "mixed mono" scheme or with an isolation capacitor, to the amplifier - "mixed mono" or directly.

Fig. 4

The scheme is extremely simple and does not require adjustment. The only procedure is the selection of the resistor R7. The diagram shows the rating for working with the built-in amplifiers of the head unit. When working with an amplifier with a power of 40 ... 50 W, the resistance of this resistor should be 270 ... 470 Ohms. Diodes VD1 ... VD7 - any silicon with a direct voltage drop of 0,7 ... 1 V and a permissible current of at least 300 mA.

Any LEDs, but of the same type and color of glow with a working current of 10 ... 15 mA. Since the LEDs are "powered" by the output stage of the amplifier, their number and operating current cannot be increased in this circuit. Therefore, you will have to choose "bright" LEDs or find a place for the indicator where it will be protected from direct light. Another disadvantage of the simplest design is a small dynamic range.

To improve performance, an indicator with a control circuit is needed. 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 diagram of the indicator with a logarithmic scale is shown in fig. 5. Dotted lines show optional elements.

Rice. 5 (click to enlarge)

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.

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 allows you 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 neighboring 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.

This scheme can be somewhat simplified, and as a side effect, new properties appear that are very useful for our purposes (Fig. 6).

Fig. 6

Unlike the previous circuit, where the transistor cells were connected in parallel, here a "column" series connection is used. The threshold elements are the transistors themselves and they open in turn - "from the bottom up". But in this case, the response threshold depends on the supply voltage. The figure shows the approximate thresholds for the indicator to operate at a supply voltage of 11 V (left border of the rectangles) and 15 V (right border). It can be seen that with an increase in the supply voltage, the limit of maximum power indication shifts the most. In the case of using an amplifier whose power depends on the battery voltage (and there are many of them), such "auto-calibration" can be useful.

However, the price for this is an increased load on the transistors. The current of all LEDs flows through the lower transistor in the circuit, therefore, when using indicators with a current of more than 10 mA, transistors will also require the appropriate power. "Cloning" cells further increases the unevenness of the scale. Therefore, 6-7 cells is the limit. The purpose of the remaining elements and the requirements for them are the same as in the previous scheme.

Slightly modernizing this scheme, we obtain other properties (Fig. 7). In this scheme, unlike the previously considered, there is no luminous "ruler". Only one LED is lit at a time, simulating the movement of the arrow on the scale. Therefore, energy consumption is minimal and low-power transistors can be used in this circuit. Otherwise, the scheme does not differ from those considered earlier.

Threshold diodes VD1 ... VD6 are designed to reliably turn off idle LEDs, so if there is a weak illumination of extra segments, it is necessary to use diodes with a high forward voltage or connect two diodes in series. "Cloning" cells reduces the brightness of the glow of the upper segments according to the scheme, to eliminate this, instead of the resistor R9, you need to introduce a current generator. And we agreed - not to complicate. Therefore, in this case, 8 cells is the maximum.

Author: A. Shikhatov; Publication: bluesmobile.com/shikhman

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