Saturday, March 14, 2015

EMC sound detector

I saw this youtube video a while ago. It shows an EMC meter using a piezoelectric summer, a coil and a fet.I thought that instead of  coil, I could do the same thing using a semi-rigid cable for a magnetic field probe, like the ones Doug Smith describes. I have previously written about a field probe that I have used with an oscilloscope. The signal from the probe that I have seen has been on the order of 10mV, max. This is far to low to make a summer sound. The summer needs a at least a few volts of signal to sound enough.
So to make this circuit work I needed to produce a gain of at least a few hundred. I started out trying to do this with an uA741. With a gain-bandwidth product of 1MHz, I could at most produce a gain of 100 at 10kHz, which was a bit weak. After that I tried using a . It had a better gain-bandwidth product of 3MHz, but I had a lot of problems with lock up and oscillations.
Finally I solved the gain-bandwidth issue using two op-amps, the LM358 (two amplifiers in one package).  Since the signal is low, and we are interested in the AC-signal, I AC-coupled the feedback loop in both amplifier stages. Final circuit looks like this:
EMC sound detector
EMC sound detector, actual build.

There was some trial and error and usage of what components was at hand. To determine the maximum gain of the second stage, I used a potentiometer where the 270 ohm resistor is now. I turned the potentiometer to just below the point where the circuit started to oscillate. The circuit is sensitive enough to sound out fields from, for example a cell phone, but the sound volume isn't that high. I think the circuit can achieve higher gain with a smaller cap on the output (22uF now). I would need to put a higher resistor after it though, to keep this stage from filtering out low, but audible frequencies.

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