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Audio Power Meter(BC547)

2014-11-07 05:13  
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This article describes the Audio Power Meter (BC547). The principle is very simple, very practical. The circuit components can help you understand better grasp this principle. For example, in this circuit, you can go to find and buy these components: BC547.

This simple circuit indicates the amount of power that goes to a loudspeaker. The dual-colorLEDshows green at an applied power level of about 1 watt. At 1.5 watts it glows orange and above 3 watts it is bright red. The circuit is connected in parallel with the loudspeaker connections and is powered from the audio signal. The additional load that this represents is 470 Ohm (R1//R3) will not be a problem for any amplifier. During the positive half cycle of the output signal the greenLEDin the dual-colorLEDwill be turned on, provided the voltage is sufficiently high.

At higher output voltages, T1 (depending on the voltage divider R2/R1) will begin to conduct and the greenLEDwill go out. During the negative half cycle the redLEDis driven via R3 and will turn on when the voltage is high enough. In the transition region (where T1 conducts more and more and ‘throttles’ the greenLEDas a result) the combination of red/green gives the orange colour of the dual-LED. By choosing appropriate values for the resistors the power levels can be adjusted to suit.

Figure:1 Audio Power Meter Circuit Diagram

Figure 1 Audio Power Meter Circuit Diagram

The values selected here are for typical living room use. You will be surprised at how loud you have to turn your amplifier up before you get theLEDs to go! The resistors can be 0.25 W types, provided the amplifier does not deliver more than 40 W continuously. Above this power the transistor will not be that happy either, so watch out for that too. Because T1 is used in saturation, the gain (Hfe) is not at all important and any similar type can be used. The power levels mentioned are valid for 4-Ohm speakers. For 8-Ohm speakers all the resistor values have to be divided by two.



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