[Solved] EMF detector using LM339

Hi guys!


There are easiest magnetic field detector scheme from encyclopedia. I decided to build something like emf detector using same principle, and got some strange results. I used 1N4148 diodes as detectors, they worked well in the other dummiest circuit that I tested with my 5W walkie-talkie at 446 MHz. About 4-inches wire from arduino kit acts as antenna.


First I built this one, but somewhy LED is always shine somewhere in the middle of range, more brighter than dimmed. Am I right, that there’s always potential presents on wire so OpAmp is always open? But no reaction to any radiations, tried with cellular and walkie-talkie, always same brightness.
Have tried with just a wire, without diodes - same result.


Then I tried to set an “antenna” to positive input. And what I’ve got - LED normally is turned off, but when I’m touching an isolated part of wire with fingers, LED is turning on slowly, the closer and more I handle, the shine goes brighter, and become brightest when I touch the metallic wire. Tried with a wire without diode, then LED lights brighter on a longer distances, and very bright when touching metallic end. (Was this diode voltage resistance effect?) And, of course, no reaction to radiations. Am I acted as grounding and created more negative potential, than power source’s 0V? Or what is happening? Any recommendations to build working circuit?



The LM339 is completely useless at much above a few MHz. Hence you need a detector in front of it.

1N4148’s are fast enough but need about 0.7v to turn on, so are insensitive.

Even so, something like the enclosed would ( sort of) work…


Edit - silly mistake. The diode in the output of the LM339 should be your LED…

detector.pdf (16.6 KB)

Thank you for fast reply!
Aha, this is my scheme #1, just you've added capacitor and pull-down resistor, what are their roles? Should capacitor charge and then slowly discharge to LM339 input at emissions? Will not current drains thru resistor instead of OpAmp as it has high input impedance? I am just trying to understand physical level of what is happening in scheme, and at all of my modification cases.

The capacitor and resistor act as a leaky integrator - holding the vey short positive peaks of the rf to allow the much slower LM339 to catch up…

Actually, the enclosed should be a lot more sensitive - balances out the diode drop…


ps there are professional devices to do exactly this job - see the AD AD8314 for example. I’ve used them and they’re excellent. Your mobile phone will have something like it for accurately controlling the power output level on transmit ( which must be within 0.5 dB for 3G phones)…

Before such devices were available designers used detectors based on schottky diodes - much faster and lower voltage drop. eg the BAS17 etc

detector1.pdf (17.2 KB)

Hi, You might also learn more about the LM339 and comparators. See THIS page.

You probably want the comparator reference to be other than ground or +

Terry, yes, I want comparator reference to be a "global radio ground", and then catch detected small positive voltages at comparing input. I read the whole Horowitz/Hill book, now I'm trying to do some practice, and the results sometimes interestingish and unexplainable at my still low knowledge level.

I want comparator reference to be a "global radio ground"

I think you don't understand electronics very much, but it made me laugh all the same. Such a thing is a stake in the ground but that will not help you.

A comparator looks at the two inputs, and outputs either a HIGH or a LOW, depending on which input has the higher voltage. If you connect one input to ground then the other input will always be higher. So you need to bias one input to sit up above the ground of your circuit. Something like a potential divider, formed by two 1K resistors will sit it up at 2.5V. Then the input signal from your detector can change as the field changes and take the other input above and below this threshold point. Then you will see a change in the output of the comparator.
Note this is an open collector output so to see it go HIGH you need a pull up resistor.

I read the whole Horowitz/Hill book,

What all of it? That is a big book. May I suggest you start again and try to understand what it says a bit at a time. Did you do the questions at the end of each chapter? Did you get them right?

Mike, thank you for explanations, I will try to build this circuit.
I see, that both inputs needs to be balanced [that's what I did not understand earlier] and when signal from detector will take -input above referenced voltage, the open collector goes LOW, and led turns on. When I will connect it to arduino input, I will use pull-up resistor to see HIGH level. I also planning to use germanium diode for detector, that drops about 0.2 of incoming Volts instead of 0.7V silicone diodes to increase input sensitivity.

And yes, I read all of the big book, and some other, but that surely doesn't mean that I see everything clear. I will read it again and again, doing all the exercises. I am newbie enough in electronics, just soldered some multivibrators and radio receivers about 25 years ago. Nowadays I've successfully built some digital arduino projects; but still have troubles understanding analog and radio circuits, that's why I started to learn again with serious literature. I also does not have professor or friends who shares my hobby and whom I may ask, so I am asking here :slight_smile:

I also does not have professor nor friends who shares my hobby,

You have now with us. :slight_smile:

I was a professor ( as you call them ) for 21 years.

Thank you very much! :slight_smile:

Finally got it working, thanks to Mike again!


I have tuned negative input using pot and stopped immediately when the led turned off, so there is +2.5V at positive input and something about +2.49(9)V :slight_smile: taken at negative. Now it detects cellular in 2G mode very well [even 20-30cm distance], same with 5W walkie-talkie [5-10cm], and gives just a very dim flashes for 3G and WiFi cellular [2-5cm]. Also shines well when antenna is near laptop’s PSU, also at 2-5 centimeters. Also some shines when I touching isolated wire, and bright shines when I touch the metallic end, I assume, from my own potential, acting myself as antenna.

By the way, I’ve built also detector.pdf and it was not working :frowning:

Well done. :slight_smile:

You could try an experiment with a feedback resistor from the o/p to i/p+

Try 10meg.


By the way, I've built also detector.pdf and it was not working :frowning:

It would be very insensitive.

Grumpy_mike's idea is better. And adding a little hysteresis would be interesting.

If you want a wide range sensitive device, look at the log detectors suggested above.

But , of course, very wideband.


Yes, I know this thing is hysteresis, we will get Schmitt trigger with this positive feedback and led will switch more digitally, visually without dim state, only on or off. And thanks to Allan, ordered AD8314 for experiments.

Feed the AD8314 output to an arduino analog input, and you'll get a pretty good idea of signal strength.

Add a tunable filter at the input ( or a superhet with a vco, mixer and lowpass filter) and you've got the crude basics of a spectrum analyser...


I prefer to do FFT or FHT in software using big brother to get the wideband spectrum analyser data when needed :slight_smile: But it is still very interesting to calculate and build own tx/rx circuits in hardware.

@ megavoid What has that to do with the question?

FFT's with a 400MHz signal takes a pretty hairy a/d and processor...