Most low-cost home stereo power amplifiers don't have indicators for the level of the output. Each channel of these stereo power amplifiers can have an output power level indicator added to it. Low levels of output power don't bother or hurt people, so you don't need to add a preamplifier and low- level detector before IC LM3915. But you should know when the power output gets to be a lot. Here, we show you a very simple, low-cost stereo-level indicator circuit for home power amplifiers with a power rating of about 0.5W. Two LM3915 dot/bar display driver ICs are the main parts of the circuit (IC1 and IC2). The LM3915 measures analog voltage levels and drives ten LEDs to make a logarithmic 3dB/step analog display.

Voltages below 1V don't matter because they represent low levels of the audio signal. In the same way, input voltage levels above 30V result in output power levels that are too high for home power amplifiers. So we're interested in voltage levels from 1V to 30V, which LM3915 can handle right away. The circuit is easier to build because the LM3915 does not need protection against inputs of up to 35V.

Most audio power amplifiers can power loads from 2 ohms to about 32 ohms. A load of a few thousand ohms won't change the way the amplifier works. CON1 is the input connector for the speaker or headphones, and CON2 is the output connector. Each channel has its own LM3915 and ten

LEDs that show how much power is being used. You can choose between three different colored LEDs to show the different sound levels. For instance, you could have 5 green LEDs, 3 yellow LEDs, and 2 red LEDs. If you don't have the right signal generators and measuring tools, you can calibrate the level indicator based on what you see. For example, the audio signal should be in the green LEDs zone when it is loud enough but not annoying, in the yellow zone when it is annoying or starting to get distorted, and in the red zone when it is heavily distorted or too loud to listen to in the room. Potentiometers VR1 and VR2, which are not required, can be used to calibrate. With the S1 and S2 switches, you can choose between the bar mode and the dot mode. If you don't need them, you can take them off or replace them with jumpers. Leave pins 9 of IC1 and IC2 open when the switches are taken out. The circuit works with 12V that has been fixed. You can also power it with a 12V battery that you can charge. Put the circuit together on a PCB that can be used for many things and put it in a suitable cabinet. Put all of the LEDs in two rows on the front of the cabinet. Attach the two ports for input and output to the back of the cabinet as well.