Pulse generators on a flashing LED. Encyclopedia of radio electronics and electrical engineering

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So-called "Blinking LED Lamps" appeared in the catalogs of foreign firms manufacturing and selling semiconductor devices - light-emitting diodes that look ordinary, but when connected to a constant voltage source, flash and go out about twice a second. These devices can often be purchased at radio markets. This article describes several simple devices in which a "flashing" LED serves as a generator of not only light, but also electrical impulses.

First of all, let's answer the question, why does such an LED blink? Inside it, as shown in the diagram (Fig. 1), in addition to the actual light-emitting semiconductor structure HL1, there are a pulse generator and an electronic key. Sometimes a quenching resistor R1 is provided, in other cases, its functions are performed by the internal resistance of the key. Diode VD1 protects the device from supply voltage of reverse polarity.

By the way, it is this diode that causes the device to fail. It often happens that when checking an LED, a relatively powerful 9 V battery is connected to it with the polarity reversed. As a result, a current of hundreds of milliamps heats up the protective diode to a temperature that is dangerous not only for itself, but also for other components of the device. Therefore, when checking the LED in series with it, it is necessary to include a resistor with a resistance of 100 ... 200 Ohms. During operation, when the voltage applied to the LED has the correct polarity and is within acceptable limits, an additional resistor is not needed.

The most common are "flashing" LEDs of the V621, V622, V623 series (Diverse); LTL 4213, LTL 4223, LTL 4233 (Lite On Opto); TLBG5410, TLBR5410, TLBY5410 (Temic Telefunken); L-36, L-56, L-616, L-796, L-816 (Kingbright Reinhold). In appearance, they resemble the usual AL307BM, they have a body with a diameter of 3 ... 10 mm, a viewing angle of 40 ... 1400, the color of the glow is red, orange, yellow or green. Their typical parameters are as follows: operating voltage - 3,5 ... 13 V, maximum forward current - 60 ... 70 mA, maximum power dissipation - 200 mW, flash frequency - 1,5 ... 2,5 (sometimes up to 5 Hz), brightness - 1,3 ... 1000 mcd.

In the luminous state, the properties of a "blinking" LED are similar to those of an ordinary LED. The experimentally taken initial segment of its current-voltage characteristic is shown in Fig. 2 (curve 1). In the intervals between flashes, the "LED" circuit is broken and, at the same voltage, the current flowing through the device is much less, since only the internal generator consumes it. Curve 2 corresponds to this state.

If a resistor is connected in series with the "blinking" LED, the voltage drop across it will change in time with the flashes. Using an oscilloscope, you can make sure that the generation continues even when the resistance of the resistor increases to a value at which flashes of light are no longer visible. Carried out in fig. 2 load line (3) corresponds to a resistor with a resistance of 33 kΩ and a supply voltage of 5 V. The difference in voltage drops across the resistor during flash and pause AU exceeds 2 V. This is sufficient, for example, to trigger a logic element.

Devices, the schemes of which are shown in fig. 3 and 4, by analogy with RC oscillators, one could call RHL oscillators. The types of LEDs and logic elements are not indicated in the diagrams, since a variety of combinations of them have been tested and work stably. The duration of the high logic level at the output is 280...320, low - 340...370 ms. These values ​​within a small range depend on the resistance of the resistor R1 and the type of logic element used. In the device according to the scheme in Fig. 3, the interval of possible resistances of the resistor R1 in kiloohms when using microcircuits of the series indicated in brackets is 0,1 ... 1,8 (K155). 0,1...5,6 (K555). 0,15...30 (KR1533) or 0,15...91 (K561). When the resistance approaches one of the boundary values, the complete breakdown of the oscillations is often preceded by a "bounce" - the generation of bursts of short pulses at the fronts of the main ones. In the generator according to the scheme of Fig. 4, only microcircuits of the CMOS structure (K561 series and the like) can work, and the resistance R1 must be in the range of 0,8 ... 300 kOhm.

On fig. 5 shows a diagram of an economical burst generator containing only one logic element - a Schmitt trigger. During the flash of the "flashing" LED HL1, the voltage level at input 1 of the DD1.1 element corresponds to logic 0. In the pause between flashes, this voltage increases to the level of logic 1 and the RC generator starts to work. formed by the elements R2, C1, DD1.1. At the output, you can observe bursts of pulses following with the frequency of LED flashes. The signal can be heard by connecting a BF1 acoustic transducer to the output of the generator, for example, a piezo emitter ZP - 1, ZP - 19 or ZP - 22. The values ​​\u2b\u500bof the elements indicated in the diagram correspond to a pulse frequency in a pack of 230 kHz. the repetition period of the bursts is 1. and the duration of each of them is 620 ms. With an increase in the resistance of the resistor R150 from 450 Ohm to 600 kOhm, the burst repetition period increases from 2,2 to 1,5 ms, and their filling frequency decreases from 135 to 1 kHz. You can pick up such a resistance (approximately 1 kOhm). at which a sequential melodic triad is generated. By swapping RXNUMX and HLXNUMX, by selecting the same resistor, they achieve such an interesting effect as "glissando" - a smooth change in pitch.

It should be borne in mind that for all the generators considered here, with large values ​​of the load resistor, the brightness of the light pulses decreases so much that they become invisible. However, the generation of electrical impulses continues.

Author: S.Ryumik, Chernihiv, Ukraine

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