The use of LEDs in electronic circuits

Everyone is familiar with LEDs now. Without them, modern technology is simply unthinkable. These are LED lights and lamps, an indication of the operating modes of various household appliances, the illumination of screens of computer monitors, TVs, and many other things that you can’t even remember right away. All of these devices contain LEDs in the visible radiation range of various colors: red, green, blue (RGB), yellow, white. Modern technology allows you to get almost any color.

In addition to LEDs in the visible range, there are LEDs for infrared and ultraviolet light. The main field of application of such LEDs is automation and control devices. Just remember Remote control of various household appliances. If the first remote control models were used exclusively for controlling TVs, now they can be used to control wall heaters, air conditioners, fans, and even kitchen appliances, such as crock-pots and bread machines.

So what is an LED?

In fact, Light-emitting diode not much different from usual rectifier diode, - all the same p-n junction, and all the same basic property, one-sided conductivity. As we studied the pn junction, it turned out that in addition to the one-sided conductivity, this very junction also has several additional properties. In the process of evolution of semiconductor technology, these properties have been studied, developed and improved.

Soviet radiophysicist made a great contribution to the development of semiconductors Oleg Vladimirovich Losev (1903 - 1942). In 1919 he entered the famous and still well-known Nizhny Novgorod radio laboratory, and since 1929 he worked at the Leningrad Physics and Technology Institute. One of the activities of the scientist was the study of a weak, slightly noticeable, glow of semiconductor crystals. It is on this effect that all modern LEDs work.

This weak luminescence occurs when the current is passed through the pn junction in the forward direction. But at present, this phenomenon has been studied and improved so much that the brightness of some LEDs is such that it can simply be blinded.

The color scheme of LEDs is very wide, almost all the colors of the rainbow. But the color is not obtained at all by changing the color of the LED housing. This is achieved by the fact that dopants are added to the pn junction. For example, the introduction of a small amount of phosphorus or aluminum allows you to get the colors of red and yellow, and gallium and indium emit light from green to blue. The LED housing can be transparent or matte, if the housing is colored, then it is just a light filter corresponding to the glow color of the p-n junction.

Another way to obtain the desired color is the introduction of a phosphor. Phosphor is a substance that gives visible light when exposed to it by other radiation, even infrared. A classic example is fluorescent lamps. In the case of LEDs, white is obtained by adding a phosphor to the blue crystal.

To increase the radiation intensity, almost all LEDs have a focusing lens. Often, the end face of a transparent body having a spherical shape is used as a lens. In infrared light emitting diodes, sometimes the lens appears to be opaque, smoky gray. Although in recent years, infrared LEDs are available simply in a transparent case, these are the ones used in various remote controls.

Bi-color LEDs

Also known to almost everyone. For example, a charger for a mobile phone: while charging, the indicator lights up in red, and at the end of charging it turns green.Such an indication is possible due to the existence of two-color LEDs, which can be of different types. The first type is three-output LEDs. One housing contains two LEDs, for example, green and red, as shown in Figure 1.

Figure 1. Connection diagram of a two-color LED

The figure shows a fragment of a circuit with a two-color LED. In this case, a three-output LED with a common cathode is shown (there are also with a common anode) and its connection to microcontroller. In this case, you can turn on either one or the other LED, or both at once. For example, it will be red or green, and when you turn on two LEDs at once, it turns yellow. If at the same time using PWM modulation to adjust the brightness of each LED, you can get several intermediate shades.

In this circuit, you should pay attention to the fact that the limiting resistors are included separately for each LED, although it would seem that you can do just one by including it in the general output. But with this inclusion, the brightness of the LEDs will change when one or two LEDs are turned on.

What voltage is needed for the LED? This question can be heard quite often, it is asked by those who are not familiar with the specifics of the LED or just people very far from electricity. At the same time, I have to explain that the LED is a device controlled by current, and not by voltage. You can turn on the LED at least 220V, but the current through it should not exceed the maximum permissible. This is achieved by turning on the ballast resistor in series with the LED.

But still, remembering the voltage, it should be noted that it also plays a big role, because the LEDs have a large forward voltage. If for a conventional silicon diode this voltage is on the order of 0.6 ... 0.7 V, then for a LED this threshold starts from two volts and above. Therefore from one galvanic cell With a voltage of 1.5V, the LED does not light.

But with this inclusion, we mean 220V, we should not forget that the reverse voltage of the LED is quite small, not more than several tens of volts. Therefore, in order to protect the LED from high reverse voltage, special measures are taken. The easiest way is counter-parallel connection of a protective diode, which may also not be very high-voltage, for example, KD521. Under the influence of alternating voltage, the diodes open alternately, thereby protecting each other from high reverse voltage. The protective diode switching circuit is shown in Figure 2.

Figure 2 Wiring diagramparallel to the LEDprotective diode

Two-color LEDs are also available in a two-pin package. A change in the color of the glow in this case occurs when the direction of the current changes. A classic example is an indication of the direction of rotation of a DC motor. At the same time, one should not forget that the limiting resistor is necessarily switched on in series with the LED.

Recently, a limiting resistor is simply built into the LED, and then, for example, on the price tags in the store they simply write that this LED is 12V. Also, blinking LEDs are marked by voltage: 3V, 6V, 12V. Inside such LEDs there is a microcontroller (it can even be seen through a transparent case), so any attempts to change the blinking frequency do not give results. With this marking, you can turn on the LED directly to the power supply at the specified voltage.

Developments of Japanese amateur radio

Radio amateur, it turns out, is engaged not only in the countries of the former USSR, but also in such an "electronic country" as Japan. Of course, even a Japanese ordinary amateur radio amateur cannot create very complex devices, but individual circuitry solutions deserve attention. You never know in which scheme these solutions can come in handy.

Here is an overview of relatively simple devices that use LEDs.In most cases, control is carried out from microcontrollers, and you can’t get anywhere. Even for a simple circuit, it’s easier to write a short program and solder the controller in the DIP-8 package than to solder several microcircuits, capacitors and transistors. It is also attractive that some microcontrollers can work without any attachments at all.

Two-color LED control circuit

An interesting scheme for controlling a powerful two-color LED is offered by Japanese hams. More precisely, two powerful LEDs with a current of up to 1A are used here. But, it must be assumed that there are powerful two-color LEDs. The diagram is shown in Figure 3.

Figure 3. Powerful two-color LED control circuit

Chip TA7291P is designed to control DC motors of small power. It provides several modes, namely: rotation forward, backward, stop and braking. The output stage of the microcircuit is assembled according to the bridge circuit, which allows you to perform all of the above operations. But it was worth making some imagination and now, please, the microcircuit has a new profession.

The logic of the chip is quite simple. As can be seen in Figure 3, the microcircuit has 2 inputs (IN1, IN2) and two outputs (OUT1, OUT2), to which two powerful LEDs are connected. When the logic levels at inputs 1 and 2 are the same (no matter 00 or 11), then the potentials of the outputs are equal, both LEDs are off.

At different logical levels at the inputs, the microcircuit works as follows. If one of the inputs, for example, IN1 has a low logic level, then the output OUT1 is connected to a common wire. The cathode of the HL2 LED through the resistor R2 is also connected to a common wire. The voltage at the output OUT2 (if there is a logical unit at input IN2) in this case depends on the voltage at the input V_ref, which allows you to adjust the brightness of the LED HL2.

In this case, the voltage V_ref is obtained from the PWM pulses from the microcontroller using the integrating chain R1C1, which controls the brightness of the LED connected to the output. The microcontroller also controls the inputs IN1 and IN2, which allows you to get a wide variety of shades of light and algorithms for controlling LEDs. The resistance of the resistor R2 is calculated based on the maximum permissible current of the LEDs. How to do this will be described below.

Figure 4 shows the internal structure of the TA7291P chip, its structural diagram. The circuit was taken directly from the datasheet, therefore, an electric motor is depicted as a load on it.

Figure 4Internal device chip TA7291P

According to the structural scheme, it is easy to trace the current paths through the load and the methods for controlling the output transistors. Transistors are turned on in pairs, along the diagonal: (upper left + lower right) or (upper right + lower left), which allows you to change the direction and speed of the engine. In our case, light one of the LEDs and control its brightness.

The lower transistors are controlled by the signals IN1, IN2 and are designed simply to turn on / off the diagonals of the bridge. The upper transistors are controlled by the Vref signal, they regulate the output current. The control circuit, shown simply as a square, also contains a short circuit protection circuit and other unforeseen circumstances.

How to calculate a limiting resistor

Ohm's law will always help in these calculations. The initial data for the calculation let them be the following: the supply voltage (U) is 12V, the current through the LED (I_HL) is 10mA, the LED is connected to a voltage source without any transistors and microcircuits as an indicator of inclusion. Voltage drop on the LED (U_HL) 2V.

Then it is quite obvious that the voltage (U-U_HL) will be necessary for the limiting resistor, - the LED itself “ate” two volts. Then the resistance of the limiting resistor is

R_o = (U-U_HL) / I_HL = (12 - 2) / 0.010 = 1000 (Ω) or 1KΩ.

Do not forget about the SI system: voltage in volts, current in amperes, the result in Ohms. If the LED is turned on by the transistor, then in the first bracket, the voltage of the collector - emitter section of the open transistor should be subtracted from the supply voltage. But this, as a rule, no one ever does, accuracy to hundredths of a percent is not needed here, and it won’t work out due to the spread of details of the parts. All calculations in electronic circuits give approximate results, the rest has to be achieved by debugging and tuning.

Tri-color LEDs

In addition to two-tone lately, widespread tri-color RGB LEDs. Their main purpose is decorative lighting on stages, at parties, at New Year's celebrations or at discos. Such LEDs have a four-pin housing, one of which is a common anode or cathode, depending on the specific model.

But one or two LEDs, even tri-color ones, are of little use, so you have to combine them into garlands, and to control garlands use all kinds of control devices, which are most often called controllers.

Assembling garlands from individual LEDs is boring and of little interest. Therefore, in recent years, industry began to produce LED strips in different colorsas well as tapes based on tri-color (RGB) LEDs. If single-color tapes are produced at a voltage of 12V, then the operating voltage of three-color tapes is often 24V.

LED strips are marked by voltage, because they already contain limit resistors, so they can be connected directly to a voltage source. Sources for power led strip sold in the same place as the tape.

To control three-color LEDs and ribbons, to create various lighting effects, special controllers are used. With their help, you can easily switch the LEDs, adjust the brightness, create various dynamic effects, as well as draw patterns and even paintings. The creation of such controllers attracts many hams, naturally those who can write programs for microcontrollers.

Using a three-color LED, you can get almost any color, because the color on the TV screen is also obtained by mixing only three colors. Here it is appropriate to recall another development of Japanese amateur radio. Its circuit diagram is shown in Figure 5.

Figure 5. Connection diagram of a three-color LED

Powerful 1W three-color LED contains three emitters. When the resistors are indicated on the diagram, the color of the glow is white. By selecting the values ​​of the resistors, a slight change in shade is possible: from white to white to warm white. In the author's design, the lamp is designed to illuminate the interior of the car. Will they (the Japanese) be sad! In order not to worry about observing the polarity, a diode bridge is provided at the input of the device. The device is mounted on a breadboard and shown in Figure 6.

Figure 6. Development board

The next development of Japanese radio amateurs is also automotive. This device for illuminating the room, of course, on white LEDs is shown in Figure 7.

Figure 7. Scheme of the device for highlighting the number on white LEDs

The design used 6 high-power ultra-bright LEDs with a limiting current of 35 mA and a luminous flux of 4 lm. To increase the reliability of the LEDs, the current through them is limited to 27 mA using a voltage regulator chip included in the current stabilizer circuit.

LEDs EL1 ... EL3, resistor R1 together with the DA1 chip form a current stabilizer. A stable current through the resistor R1, supports a voltage drop of 1.25V on it. The second group of LEDs is connected to the stabilizer through exactly the same resistor R2, so the current through the group of LEDs EL4 ... EL6 will also be stabilized at the same level.

Figure 8 shows a converter circuit for powering a white LED from a single galvanic cell with a voltage of 1.5V, which is clearly not enough to ignite the LED. The converter circuit is very simple and controlled by a microcontroller. In fact, the microcontroller is ordinary multivibrator with a pulse frequency of about 40KHz. To increase the load capacity, the outputs of the microcontroller are paired in parallel.

Figure 8Converter circuit for powering a white LED

The scheme works as follows. When the outputs PB1, PB2 are low, the outputs PB0, PB4 are high. At this time, the capacitors C1, C2 are charged through the diodes VD1, VD2 to about 1.4V. When the status of the controller outputs is reversed, the sum of the voltages of two charged capacitors plus the voltage of the battery will be applied to the LED. Thus, almost 4.5V will be applied to the LED in the forward direction, which is enough to ignite the LED.

A similar converter can be assembled without a microcontroller, just on a logic chip. Such a circuit is shown in Figure 9.

Figure 9

A rectangular oscillation generator is assembled on element DD1.1, the frequency of which is determined by the values ​​of R1, C1. It is with this frequency that the LED will flash.

When the output of element DD1.1 is high, the output of DD1.2 is naturally high. At this time, the capacitor C2 is charged through the diode VD1 from the power source. The charge path is as follows: plus the power source - DD1.1 - C2 - VD1 - DD1.2 - minus the power source. At this time, only the battery voltage is applied to the white LED, which is not enough to light the LED.

When the level becomes low at the output of the element DD1.1, a high level appears at the output of DD1.2, which leads to the blocking of the diode VD1. Therefore, the voltage across capacitor C2 is added to the voltage of the battery and this amount is applied to resistor R1 and LED HL1. This sum of voltages is enough to turn on the HL1 LED. Next, the cycle repeats.

How to check the LED

If the LED is new, then everything is simple: that conclusion, which is slightly longer, is a plus or anode. It is it that must be included in the plus of the power supply, naturally not forgetting the limiting resistor. But in some cases, for example, the LED has been removed from the old board and the conclusions are the same length, a call is required.

Multimeters in this situation behave somewhat incomprehensibly. For example, a DT838 multimeter in the semiconductor test mode may simply slightly illuminate the LED under test, but at the same time an open circuit is shown on the indicator.

Therefore, in some cases, it is better to check the LEDs by connecting them through the limiting resistor to the power source, as shown in Figure 10. The resistor value is 200 ... 500 Ohm.

Figure 10. LED test circuit

LED sequential

Figure 11. Sequential inclusion of LEDs

It is not difficult to calculate the resistance of the limiting resistor. To do this, add the direct voltage to all the LEDs, subtract it from the voltage of the power source, and divide the resulting residue by the given current.

R = (U - (U_HL_1 + U_HL_2 + U_HL_3)) / I

Suppose that the voltage of the power supply is 12V, and the voltage drop across the LEDs is 2V, 2.5V and 1.8V. Even if the LEDs are taken from one box, there can still be such a spread!

By condition of the task, a current of 20 mA is set. It remains to substitute all the values ​​in the formula and teach the answer.

R = (12– (2 + 2.5 + 1.8)) / 0.02 = 285Ω

LED parallel

Figure 12. Parallel activation of LEDs

On the left fragment, all three LEDs are connected through one current-limiting resistor. But why is this scheme crossed out, what are its drawbacks?

It affects the spread of the LEDs. The greatest current will go through the LED, in which the voltage drop is less, that is, the internal resistance is less.Therefore, with this inclusion, it will not be possible to achieve a uniform glow of the LEDs. Therefore, the scheme shown in Figure 12 on the right should be recognized as the correct circuit.

 

Boris Aladyshkin