Encyclopedia of radio electronics and electrical engineering / radio reception

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The scale is designed for a dual-band VHF FM broadcast receiver or tuner with electronic tuning. In practice, it is a voltmeter that measures the voltage across varicaps or a variable resistor - a tuning organ. The scale is linear discrete, containing ten luminous elements for each range. When tuning to a station, the length of the light line consisting of these elements changes.

As an indicator, an IV27M electroluminescent indicator is used, designed for dynamic indication of 13 digits, in each of which the figure is created from seven segments - anodes. The same segments of all thirteen digits are included in place, and the bits are switched by switching the control grids. Thus, to create a scale, you need to apply a positive voltage to one of the anodes, for example, to segment "A" and switch the grids. If you need to create two or three scales on one indicator, you can connect, in addition to segment "A", segments "Q" and "B" (all horizontal lines of the seven-segment code). In this case, there are two scales, according to the number of ranges; when switching ranges, segments "A*" and "B" are simultaneously switched.

Usually, an additional generator with an RF transformer is used to power the filament circuits of such indicators, which generates an alternating voltage of 3,1V. In this case, the filament is powered by a constant voltage of + 12V, which is reduced to 3,1V by simply connecting a constant resistor R16 in series with the filament. Such a decision may seem controversial, but the author has repeatedly used such a power supply circuit for the filament of fluorescent indicators in the repair of electronic watches and various measuring instruments, as a replacement for a failed generator.

Schematic diagram of the scale is shown in the figure. It is based on the principle of voltage measurement by the method of successive approximation of a step-increasing reference voltage to the measured one.

The role of the meter is performed by the comparator D3. Its inverse input receives voltage from the receiver tuning resistor, and its direct input receives a step-increasing voltage from the matrix of resistors R6-R15. These resistors are connected to the outputs of the decimal decoder of the counter D2. Its input receives pulses from a multivibrator on elements D1.1 and D1.2. As a result, the state of the counter changes stepwise from zero to nine. Accordingly, the voltage at the connection point of these resistors also changes. As a result, on the resistor R5, the voltage is stepped, ten steps, increases from the level of logical zero to the level of logical one.

With resistor R5, this voltage is supplied to the direct input of the comparator. As a result, at some point, the voltage at this input becomes greater than the voltage at its inverse input, and at this moment the comparator changes its state, the zero level at its output becomes one. This unit enters the input R of the counter D2 and sets it to zero. The voltage across the resistor R5 immediately drops to zero and the comparator returns to its original position. Now the counter starts working again, and counts until the voltage at the direct input of D3 again reaches the voltage at its inverse input. Thus, the comparator limits the count of the counter to a certain level corresponding to the level of the input measured voltage. As a result, segments of several discharges, for example four, are sequentially ignited. Since the switching frequency of the counter is quite high (600 Hz), such a switching is visually perceived as a constant glow of a certain ruler, consisting, for example, of four segments. Visually, it turns out that, corresponding to an increase in the input voltage, that is, the voltage supplied to the varicaps, the length of the luminous line also increases. Thus, when tuning to the highest frequency section of the range, a line of maximum length, consisting of ten segments, is lit, and only one extreme segment is lit in the lowest frequency section.

The scales are switched by switching the segment anodes using the elements D1.3 D1.4. Control - by changing the logic level at the input D1.3. In this design, the IV27M indicator is used as the cheapest (its retail price in stores like "Young Technician" is close to the cost of one LED) and the longest. It is clear that another indicator can be used, it is only important that it can work at a voltage on the anodes and grids from 10 to 15 volts. Otherwise, if the indicator only works at a higher voltage, you will have to provide a bipolar supply, and power the filament circuit from a negative voltage by connecting R16 between the filament and the common wire, or make a generator that generates a negative voltage.

In the diagram, the indicator pins are indicated by fractional numbers, the fact is that the IV27M indicator has two groups of pins located at the ends, there are 15 pins on one end, and 11 on the second, so "5/15" means the 5th pin of the 15-pin end , and "9/11" means the 9th pin of the 11-pin end.

The supply voltage of the scale with this indicator can be in the range of 11 ... 15V, since 10V is the minimum for the indicator, and 15V is the maximum for microcircuits. The scale is most suitable for car receiver.

The scale can be used both as a linear voltmeter and as a level meter, if supplemented with appropriate input circuits.

Literature

1. Andreev S. "Tuning scale with moving dot", Radioconstructor 12-98 pp. 28-29.

Author: Andreev S.

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