Electromagnetic field sensing

Hi All ,

I need professional advice on a problem I am trying to solve . I am trying to sense electromagnetic field created by an iron-core coil from a distance of 12-feet. The Transmitter coil is 6-inches in length , #30 AWG wire is winded on it up the thickness of about 0.5 inches ( I don’t remember the number of turns ), its total impedance is 240ohms. See pic attached . I have a derive circuit in which a 555 is generating 25Hz of square wave its output is amplified by a LM1875 which is supplied +/- 25V to output amplified signal . The signal is fed into the winding and the other end is just grounded .

My Receive coil is a small with total impedance of 2.4Kohm because #44 AWG wire in winded on it , this coil is air core .

When I do testing I just connect both ends of Rx coil to the scope , I get very strong signals when Rx coil is line with either end of Tx coil because iron core amplifies the Magnetic field on both ends of Tx coil . I did some FEMM simulation and it shows that magnetic field should extend outwards beyond 14-feet . But I don’t see any signal or the FFT function of the scope doesn’t show any spike at 25Hz beyond 1-feet , although when Rx coil is inline with the end of TX coil I do see signal embedded in the noise . But my requirement is that Rx coil should detect 25Hz when place 12-feet away in length-wise parallel to the Tx coil .

I thank in advance to all the experts and i am grateful to their valuable advice.

The concept looks like you are attempting wireless transmission of electric power.

It's essential that your detection circuit be tuned to the same frequency as your transmitter. Even then I'd be surprised if a homemade magnetic field can be detected more than a foot or two away.

Perhaps a huge electromagnet would produce a field strong enough to be detected several feet away. The size used in auto salvage yards to lift automobiles.

Ideally you would find a way to make the transmitting coil produce a narrow beam of magnetic flux. Then it might create a response in your receiving coil several feet away. I'm not saying it's impossible. There may be a particular physical shape, or frequency of transmission, etc., yet to be discovered which will make it efficient.

Tesla's bifilar pancake coil has been receiving notice, as the shape of antenna which might be the key to wireless power transmission.

I am trying to sense electromagnetic field created by an iron-core coil

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Magnetic field on both ends of Tx coil

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There is a difference between magnetic and electromagnetic.

In fact you might have more luck doing this with electromagnetic waves (radio waves, light waves, microwaves). It involves a totally different circuit design, of course.

As you have no doubt heard, Tesla is rumored to have transmitted electrical power, to light bulbs at a distance of several miles. If the tale is true, I imagine it required enormous power to be transmitted, while a comparatively small amount was received. In other words it was probably inefficient.

If you are able to light an led at 12 feet distance, I think that will be a major accomplishment. Worthy of favorable attention from spectators.

Hi BradtheRad, No i am not trying to light a bulb or charge something 8 feet away ,i just need to sense 25Hz from a distance of 8 to 12 feet away , i dont care even if its modulated RF i.e pulses of 25Hz but during 40ms of high its 4 or 5 Mhz pulses and when its low for another 40ms its silent .

How do i do that , any leads ???

I agree that you shouldn't care for high frequency transients, just sense the 25 Hz fundamental wave.

Although you said to have performed a FEMM simulation, you didn't manage to report the results, particularly expected field strength in 12 feet distance. With this information it would be much easier to determine which means are suitable to sense the field.

But generally speaking, I'm quite sure that the 25 Hz AC field can be sensed. You need however a suitable receiver coil and a selective amplifier.

OK , in that case i should test my Rx coil again with a 8 order active butterworth or other type of opamp filter or just a RC low pass filter . I also wanted to test this circuit see attached . i am just not sure which transistor to use , any suggestions to improve this ckt will be highly appreciated .

I don't understand the meaning of the 1 MHz generator related to your original question. Are you trying to generate 1 MHz ASK instead of 25 Hz field?

1 MHz transmission over 12 feet can be still treated as pure magnetical field problem, but both transmitter and receiver technique would be different.

Regarding post #5 circuit, current is limited by the 5 k resistor, any small signal general purpose transistor (e.g. BC547, 2N2222, 2N3904) will do.

Hi,

is it possible to detect that magnetic field of 25Hz in a distance of some feet?

There may be a lot of mains frequency around. And even an 8th order butterworth filter may have problems to suppress mains frequency. It´s only one octave away.
Maybe a high Q resonant filter could do this, but to be honest i have no experience on that.

Modulated RF seems to me a more practical way.

BTW there are cheap 433MHz RF moduls for this.

Klaus

I am trying to modulate the 25Hz high signal with 1Mhz , because i think since 25Hz is very low maybe 1Mhz will radiate out further and can be detected . I am planning to make another TX coil with just a few turns of wire around a round plastic cap and try to transmit signal from it , will it work ?
What kind of suitable Rx coil do i need ?

1MHz is in the middle of the AM broadcast band. Some AM radio stations are VERY powerful and your receiver might pick them up instead of your low power transmitter.
Your transmitter might cause illegal interference to an AM radio.

Your "transmission" system is frequency sensitive. Transmitting 25 Hz magnetic field faces much higher loss than if you use 1 MHz or 10 kHz.
The idea of using a sensitive selective amplifier sounds good.

As you have the basic components ready, do a simple experiment:

Start with the short distance, say one meter, and measure receive-coil output voltage (by a scope, e.g.). Then increase the distance by one , two, three meters. Put the output voltage in a graph, and extrapolate. This will show you how much loss to expect over larger distances, and how much amplifier gain you will need.
Using ferrite or iron core in both coils will also help to reduce the loss.

Thanks for the advice jiripolivka , i am going to build a 8-order butterworth active filter with a gain of 4 , connect Rx coil to it and do further testing as you have pointed out , and share the results .

Use a simple switched-capacitor lowpass filter IC. Some have their own built-in oscillator, and the cutoff frequency is determined by the value of a single resistor. But you might need a notch filter to attenuate the mains frequency.

i was reading the literature of one such switched-capacitor lowpass filter IC i.e MAX291 , sounds very tempting , but do they also provide gain setting ???

HWguru said:

i was reading the literature of one such switched-capacitor lowpass filter IC i.e MAX291 , sounds very tempting , but do they also provide gain setting ???

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The datasheet of the MAX291 shows an opamp that can be used for gain.

Here are the results of testing Rx coil with a 8-order Sallen-key active filter , on the last stage of the circuit i ran out of accurate values of the capacitors or the sales guy from where i bought it didnt had 5% capacitor for those values , bottom line is it did cleaned the signal , but time domain trace dosent show anything except for over-riding noise, only when i plot FFT i see a nice spike at 25Hz at a distance of 6ft . see pics attached

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Actually my approach is wrong to detect magnetic field at a distance of 12ft or more a Hall-effect sensor is used and to channel field into sensor metal strips are used to direct field into its sense axis , they work at low voltage and also Tx coil doesn't have to running on very high voltage and current , see pics attached

That is an achievement. Congratulations!

I have played with neodymium magnets on a string. I think the maximum distance I've observed is a foot or two, for one to be influenced by a distant magnet.

Actually my approach is wrong to detect magnetic field at a distance of 12ft or more a Hall-effect sensor is used

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To see schematics showing how to use a hall sensor, look at compass projects based on hall sensors.