LED indicators, like dial gauges, are widely used in various electronic equipment. They are used to visually display information about the signal level - voltage, current, frequency and other parameters.
This article will show an example of a simple LED indicator circuitry that can be used, for example, to indicate the volume (power) level of an audio amplifier. The schematic diagram of the device is shown in the figure below.
The scheme is not difficult to manufacture and is available for repetition to radio amateurs of any level.
It does not contain expensive or "rare" elements and allows the use of similar parts for a wide range of applications. In addition, the circuit does not have any active elements (transistors, microcircuits), so its separate power supply is not required either. The circuit receives power directly from the powerful output signal of the amplifier. Therefore, such a circuit has, of course, a limited dynamic range, but this is often sufficient for an elementary display of the amplifier output power level.
The resistance of the resistor R7 is selected exactly when the circuit is directly connected to the output of the amplifier. With a power of up to 70 watts, its rating will be about 200... 600 ohms. Any silicon diode (forward voltage drop across which is about 0.7 volts) can be used with an allowable forward current of 0.3 amperes or more. The VD7 diode is needed to rectify the alternating current of the audio signal and isolate its constant component.
Capacitor C1 "smoothes" sharp signal jumps at peak values and gives the indication process some "inertia", which provides more convenient visualization. Its capacitance can vary from 1 to 50 μF to provide the most acceptable operating mode. Any LEDs can also be used, with a "standard" operating current of 10... 20 mA.
The circuit uses six LEDs, a larger number of them may not provide a sufficient level of brightness. After all, the circuit is passive, that is, its power supply directly depends on the power of the amplifier's output stage and the level of the maximum operating current is limited by this factor. This "disadvantage" is due to the exceptional simplicity of the scheme presented here; to eliminate it, more "complex" options with the use of control active elements will be required.