Three Level Audio Power Indicator Circuit Design

Circuit Diagram

D5 illuminates at 5W
D4 illuminates at 18.75W
D3 illuminates at 37.5W

Parts:

R1 100K 1/4W Resistor
R2 50K 1/2W Trimmer Cermet
R3 330K 1/4W Resistor
R4 1M2 1/4W Resistor
R5 470K 1/4W Resistor
R6, R7 500K 1/2W Trimmers Cermet
R8 1K5 1/4W Resistor
R9-R11 470R 1/4W Resistors
C1 47pF 63V Ceramic Capacitor
C2 100nF 63V Polyester Capacitor
C3 47µF 25V Electrolytic Capacitor
C4 1µF 25V Electrolytic Capacitor
D1 BZX79C5V1 5.1V 500mW Zener Diode
D2 1N4148 75V 150mA Diode
D3-D5 3mm. Yellow LEDs
IC1 LM339 Quad Voltage Comparator IC
SW1 SPST Slider Switch
B1 9V PP3
Clip for 9V PP3 Battery

Circuit operation:

This circuit is intended to show the output power level of any audio amplifier. It is simple, portable, and shows three levels that can be set to your desired values. For a standard HiFi stereo amplifier like the 45W model described in these pages, the suggested power levels are as follows:

D5 illuminates at 10W
D4 illuminates at 22.5W
D3 illuminates at 45W

The above values were chosen for easy adjustment, but alternative settings are achievable. IC1 is the input buffer, feeding three voltage comparators and LED drivers via a variable DC voltage generated by R5 and C4’s smoothing effect. To maintain stable settings, the power supply of IC1 and the trimmers R6 & R7 are reduced and fixed at 5.1V with Zener diode D1.

Notes:

The most straightforward way to connect this circuit to the amplifier output is with a twisted pair cable concluded with two insulated crocodile clips.
Setup is best performed with an oscilloscope or an audio millivoltmeter, as described in these pages.
A 1KHz sine wave generator with adjustable output is also required (refer to a similar circuit on this website).
Connect the generator to the amplifier input and the Audio Power Indicator to the amplifier output, simultaneously with the oscilloscope probe or audio millivoltmeter input.
When using high output levels, disconnect the loudspeakers to safeguard Tweeter components; instead, use an 8 Ohm 30-40 Watt wirewound resistor.
Remember that VRMS output is equal to Peak-to-Peak Voltage divided by 2.828.
RMS power output in Watts is equal to VRMS2 divided by speaker impedance (typically 8 or 4 Ohms).
Example: Set the output of the 1KHz sinewave generator to read 15V on the audio millivoltmeter (37.5W 8 Ohms). Adjust R2 until D3 illuminates, ensuring that D3 turns off when diminishing the generator’s output slightly.
Do the same with R7 for D4 and R6 for D5. The readings of the audio millivoltmeter should be 8V (10W 8 Ohms) and 3V (5W 8 Ohms) respectively.

author: RED Free Circuit Designs
circuit from http://www.redcircuits.com/