Signal level indicators

Encyclopedia of radio electronics and electrical engineering / Audio equipment

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It's no secret that the sound of the system largely depends on the signal level in its sections. By monitoring the signal at the transition sections of the circuit, we can judge the operation of various functional blocks: gain, introduced distortion, etc. There are also cases when the resulting signal is simply not possible to hear. In cases where it is not possible to control the signal by ear, various kinds of level indicators are used.

For observation, both pointer instruments and special devices that ensure the operation of "bar" indicators can be used. So, let's look at their work in more detail.

1. Dial indicators

1.1 The simplest bar graph

This type of indicators is the simplest of all existing ones. The scale indicator consists of a pointer device and a divider. A simplified diagram of the indicator is shown in Fig.1.

Ris.1

As meters, microammeters with a total deviation current of 100 - 500 μA are most often used. Such devices are designed for direct current, therefore, for their operation, the sound signal must be rectified by a diode. The resistor is designed to convert voltage to current. Strictly speaking, the device measures the current passing through the resistor. It is calculated elementarily, according to Ohm's law (there was such. Georgy Semenych Om) for a section of the circuit. In this case, it should be taken into account that the voltage after the diode will be 2 times less. The brand of the diode is not important, so any that operates at a frequency greater than 20 kHz will do.

So, calculation: R = 0.5U/I 

where: R is the resistance of the resistor (Ohm)

U - Maximum measured voltage (V)

I - indicator total deflection current (A)

It is much more convenient to evaluate the signal level by giving it some inertia. Those. the indicator shows the average value of the level. This can be easily achieved by connecting an electrolytic capacitor in parallel with the device, however, it should be noted that in this case the voltage on the device will increase by (root of 2) times. Such an indicator can be used to measure the output power of an amplifier. What to do if the level of the measured signal is not enough to "stir up" the device? In this case, guys like the transistor and the operational amplifier (hereinafter referred to as the op-amp) come to the rescue.

1.2 Transistor bar indicator

If you can measure the current through the resistor, then you can measure the collector current of the transistor. To do this, we need the transistor itself and the collector load (the same resistor). The diagram of a bar graph indicator on a transistor is shown in fig. 2.

Fig. 2

Here, too, everything is simple. The transistor amplifies the current signal, but otherwise everything works the same. The collector current of the transistor must exceed the total deflection current of the device by at least 2 times (this way it is calmer both for the transistor and for you), i.e. if the total deflection current is 100 µA, then the collector current must be at least 200 µA. As a matter of fact, this is true for milliammeters, because. through the weakest transistor "with a whistle" flies 50 mA. Now we look at the reference book and find in it the current transfer coefficient h_21э.

Calculate the input current: I_b = I_k/h_21Э  

where: I_b - input current

I_k - total deflection current = collector current

h_21Э - current transfer coefficient

R1 is calculated according to Ohm's law for the circuit section: R=U_e/I_k   

where: R - resistance R1

U_e - supply voltage

I_k - total deflection current = collector current

R2 is designed to suppress voltage at the base. Choosing it, you need to achieve maximum sensitivity with a minimum deviation of the arrow in the absence of a signal. R3 adjusts the sensitivity and its resistance is practically not critical.

There are times when the signal needs to be amplified not only in current, but also in voltage. In this case, the indicator circuit is supplemented with a cascade with OE. Such an indicator is used, for example, in the Comet 212 tape recorder. Its scheme is shown in Fig. 3.

Fig. 3

1.3 Scale indicator on the op-amp

Such indicators have high sensitivity and input resistance, therefore, they make a minimum of changes in the measured signal. One of the ways to use an op-amp - a voltage-to-current converter is shown in fig. four.

Fig. 4

Such an indicator has a lower input resistance, but it is very simple in calculations and manufacturing.

Calculate the resistance R1: R=U_s /I_Max   

where: R is the resistance of the input resistor

U_s - Maximum signal level

I_Max - total deflection current

Diodes are selected according to the same criteria as in other circuits.

If the signal level is low and/or high input impedance is required, a repeater can be used. Its scheme is shown in Fig. 5.

Fig. 5

For reliable operation of the diodes, it is recommended to raise the output voltage to 2-3 V. So, in the calculations, we start from the output voltage of the op-amp. First of all, let's find out the gain we need: K \uXNUMXd U_O/U_vh.

Now let's calculate the resistors R1 and R2: K=1+(R2/R1) 

It would seem that there are no restrictions in the choice of ratings, but it is not recommended to set R1 less than 1 kOhm.

Now calculate R3: R=U_o/I 

where: R - resistance R3

U_o - output voltage of the op-amp

I - total deflection current

2. Peak (LED) indicators

2.1 Analog indicator

Perhaps the most popular type of indicators at present. Let's start with the simplest ones. On the pic.6 the diagram of the "signal/peak" indicator based on the comparator is shown. Consider the principle of action. The response threshold is set by the reference voltage, which is set at the inverting input of the op-amp by the divider R1R2. When the signal at the direct input exceeds the reference voltage, +U appears at the output of the op-amp_п, VT1 opens and VD2 lights up. When the signal is below the reference voltage, the output of the op-amp is -U_п. In this case, VT2 is open and VD2 is lit. Now let's calculate this miracle. Let's start with the comparator. To begin with, we select the operation voltage (reference voltage) and the resistor R2 in the range of 3 - 68 kOhm.

Calculate the current in the reference voltage source

I_to=U_op/R_б

where: I_to - current through R2 (the current of the inverting input can be neglected)

U_op - reference voltage

R_б - resistance R2

Fig. 6

Now let's calculate

R1. R1=(U_e-U_op)/ I_to 

where: U_e - power supply voltage

U_op - reference voltage (trip voltage)

I_to - current through R2

The limiting resistor R6 is selected according to the formula

R1=U_e/ I_LED   

where: R - resistance R6

U_e - supply voltage

I_LED - direct current of the LED (it is recommended to choose within 5 - 15 mA)

Compensating resistors R4, R5 are selected from the reference book and correspond to the minimum load resistance for the selected op-amp.

2.2 Indicators on logic elements

Let's start with the limit indicator with one LED (fig. 7). This indicator is based on the Schmitt trigger. As you know, the Schmitt trigger has some hysteresis i.e. the trigger threshold is different from the release threshold. The difference between these thresholds (width of the hysteresis loop) is determined by the ratio of R2 to R1 since The Schmitt trigger is a positive feedback amplifier. The limiting resistor R4 is calculated according to the same principle as in the previous circuit. The limiting resistor in the base circuit is calculated based on the load capacity of the LE. For CMOS (CMOS logic is recommended), the output current is approximately 1,5 mA.

Ris.7

First, let's calculate the input current of the transistor stage:

I_b=I_LED /h_21Э 

where: I_b - input current of the transistor stage

I_LED - direct current of the LED (it is recommended to set 5 - 15 mA)

h_21Э - current transfer coefficient

Now we can roughly calculate the input impedance:

Z=E/I_b  

where: Z - input impedance

E - supply voltage

I_b - input current of the transistor stage

If the input current does not exceed the load capacity of the LE, you can do without R3, otherwise it can be calculated using the formula:

R=(E/I_b)-Z 

where: R - R3

E - supply voltage

I_b - input current

Z - input impedance of the cascade

To measure the "bar" signal, you can assemble a multi-level indicator (Fig. 8). Such an indicator is simple, but its sensitivity is low and is only suitable for measuring signals from 3 volts and above. LE operation thresholds are set by tuning resistors. The indicator uses TTL elements, in the case of CMOS, an amplifier stage should be installed at the output of each LE.

Ris.8

2.3. Peak indicators on specialized microcircuits

The easiest way to make them. Some schemes are shown in fig. 9

Fig.9 (click to enlarge)

You can also use other display amplifiers. You can ask for connection schemes for them in the store or from Yandex. You can also order ready-made kits from Masterkit, masterkit.ru/main/bycat.php?num=15

3. Peak (luminescent) indicators

At one time they were used in domestic technology, now they are widely used in music centers. Such indicators are very difficult to manufacture (include specialized microcircuits and microcontrollers) and to connect (require several power supplies). I do not recommend using them in amateur technology.

Author: Pavel Ulitin, Overlord7[doggy]yandex.ru, ICQ#: 322-026-295; Publication: cxem.net

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