Encyclopedia of radio electronics and electrical engineering / radio reception

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Amateur radio design is a rather specific hobby, because it is much easier for a radio amateur to solder a complex circuit than to make a simple mechanical device. A good example is the design of a radio receiver. The whole circuit can be very well assembled and debugged, but the scale-vernier device spoils everything. Some pulleys, rollers, springs are required. And there is nowhere to take them. You have to select something, grind something, file it, use parts from a children's designer, a broken alarm clock, etc. The result is a clumsy and extremely short-lived design that creaks and seizes all the time. In addition, it takes several times more time than assembly, installation and adjustment of the circuit itself. Of course, one can say that it is necessary to master "parallel specialties", but we are "electronics engineers", not "mechanics". Therefore, it is easier for us to assemble an electronic circuit than a mechanical vernier.

Using multi-turn variable resistors (such as those used in fixed-tuning units of Soviet color TVs) and bar display chips like LM3914, you can make a good electronic-mechanical vernier-scale device for a homemade radio receiver, which will be reliable, modern and very simple (from the point of view of a radio amateur , Certainly).

A schematic diagram of such a device is shown in Figure 1.

Fig. 1

Microcircuit A1 is switched on in the indication mode with an intermittent luminous dot (pin 9 is not connected anywhere). If pin 9 is connected to pin 3 (to + power), then the indication will be a luminous column, it may be beautiful, but not economically viable, because several LEDs will be constantly on. and in one of the extreme positions of the scale, in general, all ten. The current consumption, in this case, will be too high for battery power. And so, with a luminous dot, only one LED is constantly on and the additional current is the same as from the LED indicator for fine tuning or power on.

The mechanical part turns out to be very simple - you just need to choose a suitable handle and attach it to the small disk end handle of a multi-turn variable resistor. The scale consists of a line of LEDs located along the edge of the printed circuit board to the left of the variable tuning resistor.

The layout of the printed circuit board and its wiring diagram is shown in Figure 2.

Fig. 2

The printed circuit board is mounted on the front panel of the radio receiver. The board is located perpendicular to the panel and up printed tracks. In the frontmost panel there should be a rectangular transparent window measuring approximately 55x5 mm. The easiest way to digitize the scale is this: you need to prepare a drawing of the scale on a personal computer. The easiest way to do this is in a Word text editor of any version (the editor has a page layout with rulers in millimeters horizontally and vertically). Then purchase a transparent film of the Transparencies type, and print the prepared scale pattern on it using a laser printer. Then cut out the scale pattern and stick it on the inside of the transparent window with adhesive tape.

Of course, you can make a scale and engraving filled with black paint, or using a photo method. It all depends on the specific "technological" capabilities. However, now, the option with a laser printer is the most affordable.

The variable resistor R1 is a tuning organ; it is a multi-turn variable resistor of the SP3-36 type. AL307 LEDs can be replaced with any other small-sized LEDs.

From the engine of the variable resistor R1, the voltage is supplied to the receiver varicap.

Establishing the circuit consists in selecting the value of the resistor R2 in such a way that when the knob of the resistor R1 is rotated from one limit position to another, all the LEDs turn on and off in turn. Moreover, in the extreme positions of R1, the extreme LEDs were on.

The scale can be powered by voltage from 4 to 15V. In such a power supply circuit as in Figure 1, it is desirable to power the scale from a stabilized source. Or you need to apply voltage to the resistor R1 from a stabilized source (for example, from a zener diode), and power the A1 chip itself and the LEDs from an unstabilized source.

Author: A.Ivanov

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