LED beacon. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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It will become easier to find various objects and objects at night, including moving ones (for example, pets), if you attach an economical beacon to them, the description of which is given below: when it gets dark, it automatically turns on and starts to give light signals.

The beacon scheme is shown in fig. one.

In fact, this is an asymmetric multivibrator based on transistors of different structures VT2, VT3, which generates short pulses with an interval of several seconds. The light source is the emitting diode HL1, the light sensor is the phototransistor VT1.

The device works as follows. As can be seen from the diagram, the emitter-collector section of the phototransistor VT1, together with the resistors R1, R2, forms a voltage divider in the base circuit of the transistor VT2. During daylight hours, the resistance of this section is small, so the voltage at the emitter junction of the transistor VT2 is small, and it is closed. The transistor VT3 is also closed, since the bias voltage at its base, which depends on the collector current VT2, is zero. In other words, the multivibrator does not work and the current consumed by it does not exceed 2 ... 3 μA.

With the onset of darkness, when, due to a decrease in illumination, the resistance of the emitter-collector section of the phototransistor VT1 increases so much that the voltage drop across it reaches approximately 0,6 V, the transistor VT2 begins to open. An increase in the voltage drop across the resistor R4 created by its collector current leads to the fact that the transistor VT3 also starts to open. As a result, the voltage on its collector decreases and the capacitor C1 begins to charge. The charging current flows through the resistor R1, the emitter-collector section VT1 and the emitter junction of the transistor VT2, so the latter opens even more and its collector current grows, which leads to an even greater opening of the transistor VT3, etc. The process proceeds like an avalanche, and the HL1 LED is bright flashes.

As the capacitor C1 charges, the charging current decreases, and at some point the transistor VT2, followed by VT3, begins to close. This happens quickly, so the LED goes out abruptly. Next, the capacitor is discharged through the HL1 LED, resistor R5 and high-resistance resistor R2, and as soon as the voltage across it drops to a certain value, transistor VT2 will start to open again and the whole process will repeat. Due to the high resistance of the discharge circuit, the duration of discharging the capacitor is much longer than charging, so the interval between LED flashes reaches several seconds.

To make the flashes more visible, the device uses a super-bright LED. To minimize the supply voltage, the TLWR9622 LED (red glow) of the Y group was selected (forward voltage - 1,83.-.2,07 V). This allows you to keep the beacon working when the supply voltage drops to about 2,3 V.

All parts of the device are placed on a printed circuit board made of one-sided foil fiberglass, a sketch of which is shown in Fig. 2.

In addition to the transistors indicated in the diagram, KT361V, KT361G and KT315V, KT315G, as well as KT3107 (VT2) and KT3102 (VT3) series transistors with any letter index can be used in the beacon. LED HL1 - any super-bright red glow with the lowest possible forward voltage and, preferably, with a large radiation angle. You can use a super-bright LED and a white glow, but then you will have to increase the supply voltage (it must be at least 3,5 V). Capacitors C1, C2 - any oxide in a cylindrical case with a diameter of 5 mm (for example, the TK series from Jamicon), resistors - MLT, C2-33, P1-4. Switch SA1 - any small-sized.

To expand the angle of emission of the LED, you can attach a light-diffusing plastic cap (opaque or transparent with a corrugated surface) to it.

The beacon power battery can be made up of various galvanic or rechargeable cells. For example, if it is intended for installation on small moving objects, then it is convenient to use small-sized and lightweight disk elements of size 357A, in other cases it is advisable to use AAA finger-type elements with a larger capacity.

If all the parts are in good order and there are no installation errors, the beacon starts working immediately after the power is turned on - it is enough to close the phototransistor window with an opaque curtain. The required brightness of the flashes is achieved by selecting the resistor R5. The duration of the flashes depends on the resistance of the resistor R1 and the capacitance of the capacitor C1, and the pauses between them depend on the capacitance of the same capacitor and the resistance of the resistor R2.

To increase the detection range of the beacon, the number of LEDs can be increased, for example, up to four, by connecting them in series and placing them in the structure in such a way that they emit light in different directions. In this case, of course, the supply voltage must be increased to 12 V and the resistance of the resistors R1, R2 must be proportionally increased, and the resistor R5 must be selected according to the required brightness of the flashes.

Author: I. Nechaev, Moscow

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