Encyclopedia of radio electronics and electrical engineering / Civil radio communications
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Most car and landline radios operating on the 2 meter amateur band and CB band have a multi-function liquid crystal display that is backlit. The proposed modification to such radio stations will make it possible to turn off the backlight when it is not needed.
Several miniature incandescent lamps are used to illuminate the display, knobs and radio control buttons, but not every model has a backlight switch. This leads to the fact that in good external lighting, when such illumination is not required, the power of the power source is wasted (which is unprofitable when powered by a battery) and the life of incandescent lamps.
By adding a simple device to the radio station, you can, if necessary, turn off the backlight. It works like this (Fig. 1).
When the radio is turned on, capacitor C1 is discharged and transistor VT1 is closed. Current. which flows through resistor R4. will go to the control electrode of the thyristor VS1. and it will open - the backlight lamps will light up. A few seconds after turning on the power, capacitor C1 will charge, transistor VT1 will open and the current of resistor R4 will begin to flow primarily through the transistor, and not through the control electrode VS1, but its state will not change, and the lamps will continue to light.
To turn off the backlight, you need to turn off the power to the radio station using a standard switch, and then immediately turn it on. During this time, the thyristor will close. and capacitor C1 will not have time to discharge. The current in resistor R4 will not flow through the control electrode of the thyristor, but through the open transistor VT1. The thyristor will remain closed, and the backlight, naturally, will not turn on.
When the backlight is needed again, you will need to turn off the radio, but return it to working condition only after a few seconds (so that capacitor C1 has time to discharge). Capacitor C2 may be needed to increase the noise immunity of the device in conditions of strong power supply noise.
In the device it is permissible to use thyristors from the KU101, 2U107 series or other low-power ones with the lowest possible voltage in the open state. Instead of the VT1 transistor, any low-power silicon one with a base current transfer coefficient of at least 150...200 and a low reverse current will do. Capacitors - K50 series. K52, resistors - MLT, S2-33.
The device is assembled on a printed circuit board made of single-sided foil fiberglass, a sketch of which is shown in Fig. 2. It is placed in any convenient place.
The setup comes down to selecting resistor R4 until stable operation of the thyristor is obtained with capacitor C1 closed. The maximum time interval between turning on and off the radio station, during which the backlight does not turn on, can be changed by selecting capacitor C1.
Author: I.Nechaev (UA3WIA)
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