Flashlight brightness control. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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The article "Lamplight Dimmer", published in Radio No. 7, 1986, described an electronic device for controlling the brightness of a flashlight. Today, the author of the named article offers an improved version of the device, which makes it possible to give the lamp an additional function of a light beacon.

You can, of course, adjust the brightness of a flashlight lamp with a variable resistor connected in series with it. But, unfortunately, significant power is wasted on the resistor and the efficiency of such a regulator will be low. The key regulator is more economical, the principle of its operation is based on the fact that the load is connected to the power source (battery) not constantly, but periodically - for time intervals that can be smoothly changed. As a result, the average current through the incandescent lamp will change, and hence its brightness.

The proposed regulator (Fig. 1), like the one mentioned above, is built into the body of the flashlight and allows you not only to adjust the brightness of the incandescent lamp from maximum to low luminescence. With its help, the lantern can be easily turned into a light beacon.

The basis of such a regulator is the integrated timer DD1. It has a pulse generator. Their repetition frequency (from 200 to 400 Hz) and duty cycle can be changed. Transistor VT1 acts as an electronic key - its operation is controlled by a generator. The principle of operation of the regulator is explained by the oscillograms shown in fig. 2.

In the brightness control mode, the contacts of the switch SA1, combined with a variable resistor R3, are closed. By moving the resistor slider, the duration of charging and discharging the capacitor C1 is changed, and charging is carried out through the VD2 diode, and discharging through VD3. Resistors R1 and R2 of relatively high resistance have practically no effect on the operation of the generator.

In one of the extreme positions of the resistor engine at the output of the generator (pin 4), short voltage pulses are formed that open the transistor switch (Fig. 2, a). In this case, the lamp is connected to the battery for a short time, the brightness of its glow is minimal.

In the middle position of the resistor slider, the duration of time while the lamp is connected to the battery is equal to the duration of the pause (Fig. 2, b). As a result, a power equal to about half the maximum is released on the lamp, i.e. the lamp will light up.

In the other extreme position of the slider, most of the time the lamp remains connected to the battery and turns off only for a short time (Fig. 2, c). Therefore, the lamp will shine with almost maximum brightness.

On the transistor switch in the open state, the voltage drop is approximately 0,2 V, which indicates a fairly high efficiency of such a regulator.

In the light beacon mode, the contacts of the switch SA1 are open, and the charging of the capacitor C1 is carried out mainly through the resistor R2 and the diode VD1, and the discharge through the resistor R1. In this mode, the lamp is connected to the battery for a few tenths of a second at intervals of several seconds.

Switch SA2 is the lamp's own switch, capacitor C2 acts as a buffer energy storage device that facilitates the operation of the GB1 battery.

Tests of the regulator showed that it works normally when the supply voltage drops to 2,2 ... 2,1 V, so it can be used in flashlights even with batteries of two galvanic cells. For the transistor indicated in the diagram, an incandescent lamp can be with a current of up to 400 mA.

It is permissible to use the KR1006VI1 timer in the device, diodes KD103A, KD103B, KD104A, KD522B, as well as a transistor specially designed for operation in key or pulse circuits - with a collector-emitter voltage in saturation mode of 0,2 ... 0,3 V, the maximum collector current is not less than the current consumed by the incandescent lamp, and the current transfer coefficient is not less than 40. For an incandescent lamp with a current of up to 300 mA, transistors KT630A - KT630E, KT815A - KT815G, KT817A - KT817G are suitable, in addition to those indicated in the diagram. It is advisable to use small-sized oxide capacitors, for example, the K52, K53, K50 - 16 series, a variable resistor - SDR - 3 with a switch, constant - MLT, C2 - 33. Resistor R3 can also be used with a several times higher value, for example 10, 22 , 33, 47 kOhm, but in this case it will be necessary to proportionally reduce the capacitance of the capacitor C1 so that the generator frequency remains practically the same.

Structurally, the regulator is easier to install in a lantern with a so-called "square" case, designed to use batteries 3336, "Ruby" and their foreign analogues, as well as in a "round" lantern with collapsible halves of a plastic case. In this case, the resistor R3 is first fixed on the case, and then the rest of the parts are placed. Moreover, in any case, it is more convenient to install them using the surface mounting method: diodes and resistors R1, R2 can be soldered to the terminals of the resistor R3 and switch SA1. After mounting and checking, the parts must be fixed and insulated, for example, with epoxy glue.

If the light beacon mode is not required, the regulator can be simplified by excluding the elements R1, R2, VD1 and using the resistor R3 without the SA1 switch.

Establishing the device is reduced to the selection of resistors R1, R2, R5. In the beacon mode, by selecting the resistor R1, the duration of the pause between flashes is set, and the resistor R2 is the duration of the flash. The value of the resistor R5 depends on the type and parameters of the transistor, as well as the voltage of the power supply. To pick it up, you need to apply a supply voltage about two times less than the maximum or minimum, at which the regulator works stably. After that, the resistor R3 is set to the maximum brightness position and a voltmeter is connected to the collector and emitter terminals of the transistor. Between the base of the transistor and pin 4 of the microcircuit, a chain of series-connected constant resistors with a resistance of 30 ohms and an alternating resistor of 2,2 kOhm is temporarily installed. By changing the resistance of the variable resistor from maximum to minimum, the voltage on the collector of the transistor is controlled. The position of the engine is noted, at which a further decrease in the resistance of the resistor does not lead to a noticeable decrease in the voltage on the collector. After that, the resulting total resistance of the chain is measured, and a constant resistor of the same rating is installed.

In order for the regulator to be able to work with powerful incandescent lamps that consume a current of 1 A or more at a supply voltage of up to 10 ... 15 V, it is enough to use a powerful composite transistor with a current transfer coefficient of several hundred as VT1 (from small ones, KT829A - KT829G KT973A, KT973B ). It is only necessary that the supply voltage does not exceed the maximum allowable for the microcircuit. Of course, you will have to use oxide capacitors with the appropriate rated voltage.

Author: I. Nechaev, Kursk

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