This simple circuit
indicates the amount of power that goes to a loudspeaker. The dual-color
LED shows 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 green
LED in the dual-color LED will 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 green LED will go out. During the negative half cycle the red LED
is 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 green LED as 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.
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 the LEDs 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.