The backlight of the house number will turn on automatically. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Lighting

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Lighting the house number and street name in the countryside is not an empty whim, but an elementary necessity. At night, the postman will be able to deliver the telegram faster, and the operational services will be able to come to the rescue. This is especially convenient when the backlight lights up automatically at dusk and does not require human intervention. Perhaps, anyone familiar with the basics of electrical engineering can make such a device.

The electrical circuit of the automatic (light-controlled) switch for backlighting the house number, which is triggered when the working surface of the photoresistor is darkened, is shown in fig. 1.

Rice. Fig. 1. The electrical circuit of the automatic (light-controlled) switch for backlighting the house number, which is activated when the working surface of the photoresistor is darkened

The incandescent lamp is installed inside the plastic matte housing of the plate with the house number, and the elements of the device are installed in the wall of the house, in a closed and moisture-proof housing.

In natural light, the lamp does not light. At dusk, when the lighting falls, the incandescent lamp lights up, highlighting the sign with the house number.

The photoelectric sensor is a photoresistor, on which an external light flux penetrates through the lens. Photoresistor FR1 - brand RFT-01, used as a photo sensor in the rotation controller of the record player "Arctur-004". Instead, it is also permissible to use photoresistors installed in OEP-1 optocouplers.

When the photoresistor is illuminated (bright sunlight), its resistance drops to 12 kOhm and shunts the transition "control electrode - cathode" of the thyristor VS1. In this case, the thyristor closes and de-energizes the incandescent lamp EL1.

For such illumination, a low-power lamp of 7 - 15 watts is sufficient.

It is impossible to use a more powerful incandescent lamp in the device because of the possibility of overheating and melting of the plastic case of the plate.

With a 7 W incandescent lamp to illuminate refrigerators, sewing machines and other household appliances, the device was operated in the fall for two days (with the EL1 lamp constantly turned on), while the temperature around the lamp bulb did not rise above 30 ° C, which is quite acceptable.

With insufficient illumination of the sensor, which occurs at night, the resistance of the photoresistor is high (more than 1 MΩ). The current passing through the limiting resistor R1 is sufficient to open the thyristor.

Establishment. To increase the photosensitivity of the node, you can replace the constant resistor (R1 * with an equivalent circuit as shown in Fig. 2 by introducing an adjustment into the node. The possibilities of changing the resistance R1 are achieved by increasing or decreasing the current through R1 and changing the potential at the midpoint of the voltage divider R1FR1. Due to Therefore, when the current through R1 increases (its resistance decreases), the node's sensitivity to external darkness increases many times over.Now the lamp works faster at dusk (turns on the backlight).

Rice. 2. Equivalent circuit of resistor R1*

With a decrease in the current through the resistor R1 (increase in its resistance), the opposite will happen - the lamp becomes more sensitive to external lighting and turns off the backlight even with a small impact on the working surface of the light flux photosensor. Any variable resistor is used in the process of setting the sensitivity threshold (after adjustment, having measured its equivalent resistance, R1 is replaced by a constant one).

To install it in the device case, the minimum dimensions of the variable resistor are required. If you need to retain the ability to adjust, a multi-turn variable resistor SPZ-1BV or a similar tuned resistor will do.

The phasing of the connection to the 220 V network is not important for the device.

About details. Thyristor VS1 - type MCR-106-8 (marked MCR-106 in the diagram). This designation is not given by chance, since the thyristor for this circuit can be selected and another one, guided by the reference material on the electrical characteristics of thyristors and triacs from Motorola.

Capacitor C1 - type MBM or similar for an operating voltage of at least 300 V.

With an increase in load power of more than 60 W, the thyristor must be installed on a heat sink.

Issues of application and prospects. The electrical circuit shown in the figure can be assembled independently for use in other designs as a simple photo relay. In this case, the following points are taken into account: the VS1 thyristor is replaced by the domestic triac KU208G. The fixed resistor R2 is excluded from the circuit, a jumper is installed instead. Resistor R1 is replaced by another with a dissipation power of 2 W and a resistance of 12-18 kOhm or an equivalent circuit (if sensitivity and threshold adjustment of the photorelay is required) with the same total resistance. Photoresistor FR1 - the same as in the basic circuit. When replacing it with a different type, the resistance of the resistor R1 will have to be selected additionally and within other limits.

The experiments were not without incidents. So, we managed to find the only normally working triac of 6 (!) copies - mind you, absolutely new and purchased in one batch! The main "marriage" among these devices is reduced to an increased leakage current - visually, this is noticeable as a flickering lamp even in the absence of a control signal on the control electrode. Such a triac must be replaced, despite the "novelty".

In some cases, if the lamp burns constantly and does not respond to a change in the resistance of the photoresistor, it will be necessary to "flip" the triac by connecting it the other way around (the control electrode is not changed). There is no practical explanation for this phenomenon, perhaps this is another surprise from the KU208G thyristors.

Author: A.Petrovich

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