Sensitive Audio Power Meter Circuit diagram

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Sensitive Audio Power Meter  Circuit diagram: As a follow-up to the simple audio power  meter described in [1], the author has developed a more sensitive version. In practice,  you  rarely  use  more  than  1 watt  of  audio  power in a normal living-room environment.  The only time most people use more is at a  party when they want to show how loud their  stereo system is, in which case peaks of more  than 10 W are not uncommon. With this circuit, the dual LED starts to light up  green at around 0.1 watt into 8 ohms (0.2 watt  into 4 ohms). Naturally, this depends on the  specific type of LED that is used.
Sensitive Audio Power Meter  Circuit diagram:
Sensitive Audio Power Meter-Circuit-Diagram
Sensitive Audio Power Meter Circuit Diagram
Here it is  essential to use a low current type. The capacitor is first charged via D1 and then discharged via the green LED. This voltage-doubler effect  increases the sensitivity of the circuit. Above a level of 1 watt, the transistor limits the current through the green LED and the red LED con ducts enough to produce an orange hue.The red colour predominates above 5 watts. Of course, you can also use two separate ‘normal’ LEDs. However, this arrangement cannot generate an orange hue. For any testing that may be necessary, you should use  generator with a DC-coupled output. If there is a capacitor in the output path, it can cause misleading results. 
Reference: Simple Audio Power Meter, Elektor July & August 2008.

Author : Michiel Ter Burg Streampowers