Diagram Guide

The problem with level indicators is that an amplifiers maximum output power is not constant. Thats because the amplifiers supply rails are not regulated and so the maximum power available at any given instant varies, depending on the applied signal. The schema is quite simple and is based on two BD140 PNP transistors and zener diode ZD1. During normal operation, Q1 is turned on via ZD1 and R1. As a result, Q2 is held off (since its base is pulled high) and so LED1 is also off. However, if the output signal subsequently rises to within 4.7V of the positive supply rail, Q1 turns off since it no longer has any forward bias on its base.

As a result Q2s base is now pulled low via R2 and so Q2 turns on and lights LED1. (Note: the 0.6V drop across Q1s base/emitter is ignored here because ZD1 conducts before its rated voltage due to the very low current involved). Why choose 4.7V below the power rail as the turn-on point? The reason is that, due to the drive limitations and the nature of emitter followers, they can be expected to have at least 4V across them when they saturate (ie, clip). ZD1 can be increased to a 5V or 6.2V type if the schema is to be used with a monster amplifier.

The value of R3 should be customized according to the amplifiers supply rail, so that LED1 operates with the correct brightness. To do that, first measure the amplifiers positive supply voltage, then use Ohms Law (R = V/I) to calculate the value of R3 for a current of about 20mA. As it stands, this schema can only be used to monitor the positive-going half-cycles of the audio waveform. If you want to monitor the negative half-cycles as well, you will have to build a second schema with the following changes: (1) reverse both LED1 and ZD1; and (2) use BD139 (NPN) transistors for Q1 & Q2. Note that, in both cases, you should use the earth inside the amplifier, as the speaker negative may not be earth (such as in a bridged output).

Author: Philip Chugg - Copyright: Silicon Chip Electronics Magazine