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
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.