It has been a while since I posted on the forum. I fell in love with another hobby and although I enjoyed my audio system, I didn't do much with my test gear. I wanted to buy a bench meter that could display audio Watts. Most have a math dB or dBm function, but the references usually only go to 50 Ohms. There are a few that do measure at 4/8/16 Ohms, but they were not in my price range. So I decided to build my own using an Arduino. I'm not much of a coder, but I did fall in love with the Arduino because of the amount of information (books, kits, forums, etc.). So here is my project in it's finished form. Due to the 4 line display having such a large circuit board, the final enclosure is a bit larger than I prefer. I did find an OLED display that has a large display and small board footprint. So there will be another revision forth coming!
The small 2 line display under the finished 4 line completed unit is my single channel prototype. I created my own circuit board and then populated myself. The fab house had some minimums, so I just had the board made. The magic of the circuit is the LTC1968 RMS to DC converter. Unfortunately, the max input voltage is 1V AC. Therefore, reducing the input requires quite a ratio. I buffer the output with a simple gain of 5 buffer to give my ADC full range. There are some other minor tricks in there, but not much else. I could have added another unity gain stage, but it didn't improve the performance. The most important thing in all of this was to have my bench meter calibrated. It was only a few years, but I sent it out for official calibration. There is absolutely no way to build and calibrate this without an accurate meter. For the final unit, I have a SS amp on the bench and I'm measuring both channels under load.
Below are some pics. The board on the bench and the displays show how accurate the circuit can be. Don't mind the decimal point on the measurement; I was messing around with the scale in the software. I can measure against 8/4/2 Ohm loads, there is a mode to measure in single channel only and then you push-hold the buttons for other modes.
The small 2 line display under the finished 4 line completed unit is my single channel prototype. I created my own circuit board and then populated myself. The fab house had some minimums, so I just had the board made. The magic of the circuit is the LTC1968 RMS to DC converter. Unfortunately, the max input voltage is 1V AC. Therefore, reducing the input requires quite a ratio. I buffer the output with a simple gain of 5 buffer to give my ADC full range. There are some other minor tricks in there, but not much else. I could have added another unity gain stage, but it didn't improve the performance. The most important thing in all of this was to have my bench meter calibrated. It was only a few years, but I sent it out for official calibration. There is absolutely no way to build and calibrate this without an accurate meter. For the final unit, I have a SS amp on the bench and I'm measuring both channels under load.
Below are some pics. The board on the bench and the displays show how accurate the circuit can be. Don't mind the decimal point on the measurement; I was messing around with the scale in the software. I can measure against 8/4/2 Ohm loads, there is a mode to measure in single channel only and then you push-hold the buttons for other modes.
Congratulations,nice!
What is your plan?
Is it for share,will you post files,like pcb,software,e.t.c?
What is your plan?
Is it for share,will you post files,like pcb,software,e.t.c?
Thanks @thimios . My code a is a bit hacky, the board requires soldering two MSOP-8 devices and the calibration does require a meter that is calibrated (not close, but calibrated). It would make for a great article in the magazines, but I don't think a great kit. I could furnish Arduino Nanos with the code loaded, but then I'm getting into a world of supply and support.
There were so many people on Arduino forums stating that the ADC in the Arduino didn't have the speed or resolution to accurately measure. One goal was to prove that was not true...and I did.
There were so many people on Arduino forums stating that the ADC in the Arduino didn't have the speed or resolution to accurately measure. One goal was to prove that was not true...and I did.