Arrow indicators

Arrow indicators are the simplest. Their manufacture requires a minimum of details and qualifications, especially if you use a "proprietary" measuring device with a beautiful scale. However, in our time, making a home-made scale is not difficult - it can be printed on a printer and pasted over the old one. As a basis, it is easiest to use pointer indicators from old-type tape recorders or small-sized panel meters of a magnetoelectric system with a total deviation current of 0,25 ... 1 mA. Devices of the electromagnetic system (for example, automobile voltmeters) and milliammeters with a total deviation current of more than 5 mA are unsuitable for our purposes.

Since simple dial indicator circuits do not require power, they can be connected to the outputs of the amplifier in a "mixed mono" circuit, which allows for some reduction in the number of parts (Fig. 1).

Fig. 1

Fig. 2

On fig. 2 shows a diagram of the simplest indicator. If necessary, the number of channels can be increased by adding resistors and diodes, as shown by the dotted line. When using the indicator together with the radio amplifier, in series with resistors R1, R2, electrolytic capacitors with a capacity of 47 ... 100 microfarads must be connected ("plus" to the radio). You can also use "mixed mono" (see fig. 1), in this case no capacitors are required, and the R2VD2 chain can be omitted.

The resistance of a resistor in series with the instrument depends on the total deflection current. The approximate resistance value can be found using the formula shown in the figure. The exact value should be corrected when adjusting to the required pointer deflection at a given power. The rest of the parts can be used of any type. The smoothing electrolytic capacitor must be designed for an operating voltage of at least 25 volts when measuring power up to 15 W and at least 50 volts for higher power. The voltage margin is needed because the capacitor is used in the AC circuit. By selecting its capacitance in the range of 1 ... 100 microfarads, you can adjust the return time of the arrow for every taste.

The disadvantage of the circuit is a small dynamic range, not exceeding 10 dB. This is enough for the radio, but when working with a high-power amplifier, the arrow will deviate only at signal peaks. In this case, it is better to apply the scheme shown in Fig. 3.

Fig. 3

Its main difference is the dynamic range expander on the VD1 diode and the HL1 LED. As soon as the rectified voltage across the capacitor C1 reaches a value of 0,7 V, the diode opens and a further increase in voltage is slowed down by the resistor R3. By selecting its resistance in the range of 100 Ohm ... 10 kOhm, you can adjust the "travel" of the scale in the middle part. The next limitation occurs at the moment the LED is lit, and further voltage growth practically stops. The LED can also be used as an overload indicator. The resistance of the input resistors is determined by the maximum power of the amplifier and the current of the applied LED. The calculation formula is shown in the figure, the exact resistance value should be adjusted according to the moment of ignition of the LED at maximum power.

The resistance of a resistor connected in series with the device can be found using the second formula. The exact value should be corrected when adjusting to the required pointer deflection at the moment the LED is lit. The voltage on the red LED is approximately 1,6 V, on the brighter yellow-orange - approximately 2,5 V. The rest of the parts can be used of any type. The smoothing electrolytic capacitor must be rated for an operating voltage of 6,3 ... 10 V, since the voltage on it is limited by the LED. The indicator is connected in the same way as the previous one.

The dynamic range of such an indicator can easily be brought up to 20 dB, further expansion of the dynamic range already requires a special control circuit with a logarithmic amplifier, and such a circuit is already beyond the simplest.

Author: A. Shikhatov; Publication: bluesmobile.com/shikhman

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