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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING LED signal level meters. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Audio equipment In recent years, signal level meters with indicators in the form of a line of LEDs have been widely used in tape recorders and AF amplifiers. Readers are offered three variants of the measuring part of these devices, different in complexity and capabilities. All of them are designed to measure positive voltage, therefore, if it is necessary to control the level of alternating voltage, they must be supplemented with a detector that selects half-waves of this polarity. Since the shape of the musical signals is not symmetrical, it is desirable to use a full-wave detector.
A schematic diagram of one of the simplest meters is shown in fig. 1. As you can see, it is made on a single chip containing six inverters. In the absence of input voltage and as long as it is less than 0,14 V, the voltage at the outputs of the inverters is high and none of the LEDs HL1 - HL6 is off. As the controlled voltage increases, the high level, first at the output of the inverter DD1.1, and then the rest, is replaced by a low one and the indicator diodes begin to glow. The disadvantage of the device, due to the peculiarity of the "switching" characteristics of inverters, is a rather large measurement error. So, the HL1 LED starts to glow at an input voltage of about +0,14 V, and lights up as brightly as possible at +0,5 V. The HL2 LED actually indicates the voltage range of +0,5 ... 0,85 V, etc. This shortcoming is deprived of the meter, the scheme of which is shown in Fig. 2. Here, one of the inputs of all elements 2I is connected to the collector of the transistor that forms the initial logic level, so the interval of input voltages corresponding to levels 0 and 1 at the output of the element is much narrower. The second input of each element is connected to the output of the next one, due to which the appearance of level 0 at the output of any of them fixes the same level at the outputs of all previous ones.
In the initial state (until the input voltage has reached the lower limit of the measured interval), the transistors VT1-VT8 are closed, the voltage at the inputs, and therefore the outputs of the elements DD1.1 - DD1.4, DD2.1 - DD2.4 have a logic level of 1, LEDs HL1 - HL8 are off. At an input voltage of about 0,6 V, the transistor VT1 opens, and the voltage at its collector and the input of the DD1.1 element connected to it drops to level 0. As a result, the same level is set at the output of the element, and the HL1 LED lights up. A further increase in the controlled voltage leads to the sequential opening of the transistors VT2 - VT8 and the ignition of the LEDs HL2 - HL8. To control voltages that differ by tens and hundreds of times, it is advisable to use a meter made according to the circuit in Fig. 3. It contains a clock generator (DD2.1, DD2.2), a binary counter DD1, a digital-to-analog converter (DAC), made on inverters DD2.3 - DD2.6 and resistors R7-R11, an emitter follower (VT1), comparator (DA1), electronic key (VT2) and decoder DD3 with LEDs HL1-HL16 connected to its outputs. The measurement cycle consists of sixteen cycles. Each pulse of the clock generator changes the state of the counter DD1. Its output signals are fed to the DAC and the DD3 decoder, which in turn connects the cathodes of the LEDs to a common wire. The reference voltage generated by the DAC is taken from the resistor R11 and through the emitter follower on the transistor VT1 is fed to the inverting input of the comparator DA1, which compares it with the level of the controlled voltage at the non-inverting input.
At input voltages less than the reference voltage, the output voltage of the comparator is negative, the transistor VT2 is closed and level 1 is applied to the enable inputs E2, E3 of the DD1 chip, which prohibits decoding the states of the counter DD1 (not a single LED is on). Exceeding the input signal voltage level at the inverting input changes the polarity of the output voltage of the comparator, and the transistor VT2 opens. As a result, level 1 at the outputs of the resolution of the decoder DD3 is replaced by level 0, the voltage of the same level appears at its corresponding output, and the LED connected to it lights up. The range of levels indicated by the meter is about 50 dB, their measured values are given in the table (in one case, the 0 dB level corresponds to the lighting of the HL13 LED, in the other - HL14).
The meter does not require adjustment. The only thing that may have to be done is to select the capacitor C1, which determines the pulse repetition rate of the clock generator. In this case, it should be remembered that an excessive decrease in frequency leads to a noticeable blinking of the LEDs, and an excessive increase in frequency leads to a decrease in the brightness of the HL1 LED. Author: V.Dimov
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