Arduino Audio Wattmeter.

Many, many moons ago I needed something to measure audio power in a hurry. So I lashed up a quick and dirty audio power meter.

It was nasty....

... very nasty, but did the job at the time.

It was simply a switchable 4 or 8 ohm load (actually 4.7 or 8 ohms)  which was rectified, and used to drive two meters, each measuring full scale at 10 watts. The fan was driven from rectified audio. Most unpleasant.




Anyway, the load was good, as was the fan, but I needed some sort of better instrument, so a plan was dreamt up. Measure the peak voltage across the load, do a bit of maths and get the results displayed on a 1602 display, driven by an arduino. Great ....

So audio comes into the load (R1 & R2), and is rectified by a bridge. There's a small bit of smoothing on the output, so we can get a steady-ish DC. This is divided by a voltage divider, R3 and R4 and the resultant is fed to the arduino's A-D converter. Couple of things to note here, the load is switched, either 8.0 Ohms, or 4.7 Ohms, by S1. This also lets the arduino know which load is selected. S2 is a "BTL safety switch". It completely isolates the left and right channel. It also stops the arduino reading the right channel. If you're testing a BTL(bridge tied load) amp, or any type of amp which may not take kindly to commoning up of the negative poles of the speaker, best click this across. It also grounds the second analogue input to prevent spurious readings. A PWM signal is output from D6, and is used to drive a FET to run a fan. I chose a stupidly large FET here, you can pick something more sensible. I just happen to have a bucket of them. 

This will only give accurate results when feeding in a sine wave.

That's it really....

 Dummy load salvaged from the old unit...

Front panel coming together.

... sizing it all up....

... starting the wiring...

Looking good until I noticed that I'd mistakenly picked up a 16x1 display instead of a 16x2!... and the 16x2 requires a slightly smaller opening. Drat! ... some hot melt later will have to do (and it was all going so well ...)

Rectifier board and PWM FET

Rectifier was a little overkill! Still I had two identical, salvaged from a switched mode PSU.











Setting up ready for calibration...

Apply 28.28VDC accross the outputs of the two bridge rectifiers, and with the unit set to 8 Ohms, adjust each pot until the display reads 50.0W. Easy... (Why 28.28V ? 20 VRMS into 8ohms is 50Watts. 20*1.414 = 28.28Vpk... see here for more)





All calibrated and ready for action!
Top line on the display reads left channel, bottom reads right. The three readings are instantaneous power, average and peak.











And last, but by no means least... the code....

/* The "I'm sick of that analogue meter" Audio wattmeter (c) A.G.Doswell 2nd April 2016 License: The MIT License (See full license at the bottom of this file) V1.0 Arduino measures the DC output from a diode bridge across the load resistor, does the maths and displays the results. Peak-hold feature, averaging and PWM fan control too! This is only going to give true readings with a sine wave input. VRMS = VPk + 1.41 / 1.414 Power = VRMS^2 / R R is either 4 or 8 ohms (switchable) */ boolean rPin; int sensorValue; float voltagePk; float voltageRMS; float powerR; float powerL; float loadR; float peakR = 0.0; float peakL = 0.0; int loopCounter = 0; float averageL = 0; float averageR = 0; int averageCounter = 0; float averagePowerL = 0; float averagePowerR = 0; int fanPWM = 0; int fanPWM2 = 0; #include <LiquidCrystal.h> LiquidCrystal lcd(12, 11, 5, 4, 3, 2); // initialise the LCD void setup() { pinMode (7, INPUT_PULLUP); // switch connected to 7 defines load resistance, Open for ~8 ohms, short to GND for ~4 Ohms. lcd.begin (16, 2); } void loop() { rPin = digitalRead (7); if (rPin == false) { loadR = 4.7; // My 4 ohm load is actually 4.7 ohms } else { loadR = 8.0; // .. but my 8 ohm load is spot on! } sensorValue = analogRead(A0); // Output from Right channel connected to A0 voltagePk = sensorValue * (35.0 / 1023.0); //Convert the analogue reading to a voltage voltageRMS = (voltagePk) / 1.414; // Vpk to VRMS powerR = (sq(voltageRMS)) / loadR; // VRMS and load to power if (powerR > peakR) { // update peak value peakR = powerR; } averageR = averageR + powerR; //running average sensorValue = analogRead(A1); // Output from Left channel connected to A0 voltagePk = sensorValue * (35.0 / 1023.0); //Convert the analogue reading to a voltage voltageRMS = (voltagePk) / 1.414; // Vpk to VRMS powerL = (sq(voltageRMS)) / loadR; // VRMS and load to power if (powerL > peakL) { // update peak value peakL = powerL; } averageL = averageL + powerL; //running average fanPWM2 = (powerR + powerL) * 8; // get a value for the fan speed PWM output if (fanPWM2 > fanPWM) { // if it's greater than the current PWM value, then up date it fanPWM = fanPWM2; } if (fanPWM > 255) { // limit the maximum PWM value to 255 (flat out) fanPWM=255; } analogWrite (6, fanPWM); // write it to the if (averageCounter == 100) { // sets the Average reset averagePowerL = averageL / averageCounter; // create the two averages from the running value averagePowerR = averageR / averageCounter; averageCounter = 0; // reset the counter averageL = 0; // reset the running averages averageR = 0; fanPWM --; // decrement the PWM value for the fan if this are cooling down! } if (loopCounter == 1000) { // when this has run 1000 times reset the peak value. loopCounter = 0; peakL = 0; peakR = 0; } lcd.setCursor (0, 0); lcd.print (powerL, 1); lcd.print ("W "); lcd.print (averagePowerL, 1); lcd.print ("W "); lcd.print (peakL, 1); lcd.print ("W "); lcd.setCursor (0, 1); lcd.print (powerR, 1); lcd.print ("W "); lcd.print (averagePowerR, 1); lcd.print ("W "); lcd.print (peakR, 1); lcd.print ("W "); lcd.print (loadR); loopCounter++; // decrement the counters averageCounter ++; } /* Copyright (c) 2016 Andrew Doswell Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permiSerialion notice shall be included in all copies or substantial portions of the Software. Any commercial use is prohibited without prior arrangement. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESerial FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHOR(S) OR COPYRIGHT HOLDER(S) BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. */