EMF Meter Project

[blueroomelectronics]

The K-II wasn't designed to be a ghost detector, even though some ghost hunters (both real and fraudulent) do use them.

I'm curious, how do you tell the real ghost hunters from the frauds?

PS https://skeptoid.com/episodes/4081

 

[tcmtech]

I have the theory that the 'ghosts' they are chasing are the normal RF we have around us every day. Natural geomagnetic actions constantly warps the fields around us.
Northern lights are proof of it.

If you ever noticed a radio that was tuned to a far of station your own physical movement around it can make the reception weaker or stronger just buy standing at different places even at 10 feet or more away.
Your localized presence is also interacting with those radio waves too! Use the right sensing systems and you have a "ghost" moving around the room and even interacting with you!

Toss in some over active imagination with a general lack of scientific knowledge and your a ghost hunter!

Air, sound, light, magnetic fields all have naturally somewhat random ebb and flow patterns.
They are no where near static or constant at any point or time anywhere!

Much of the superstitions formed centuries ago are from a lack of understanding of the environmental variables around the observer.

Sound echoing and air movement distortions make 'whispering voices' that cant quite be understood.
Complex light reflections, refractions and thermal distortion from moving air temperature differences can make 'shadowy figures' that seem to move around a localized area.

I can go on! But I think you get the idea.

 

[tcmtech]

I'm curious, how do you tell the real ghost hunters from the frauds?

Thats simple! Your paying me to investigate so therefore I am real! The other guys are frauds!

Standard issue vehicle sales pitch stuff. This car is great, You should buy it! (because I get a much bigger kick back from it)
That car is junk, dont buy it. (because I get nothing from selling it)

 

[kpatz]

Isn't this thread supposed to be about my EMF meter????

There are other forums for discussing whether or not ghosts exist... let's stick to the original topic please...

Regardless of usage, I built a meter that I would like to use to pick up low to medium-frequency electrical fields. I don't care what's creating the fields, be it 120V wires in the walls, the TV, ghosts, leprechauns, spy satellites, what have you. If there's a field, I want to be able to measure it.

This *is* an electronics forum last I checked, so let's stick to the topic at hand, ok?

I want my meter to perform better at lower frequencies, that's it. I posted on an electronics forum in order to get advice on how to make my circuit work better. If I wanted a discussion on whether things you can't see with your naked eyes really exist or not, I would have posted on a skeptic's forum... or a paranormal forum...

Thanks...

 

[blueroomelectronics]

For VLF you'll need a much much larger coil.

 

[blueroomelectronics]

Here's some good reading on ULF coils.
www.vlf.it - Large Induction Coil for ULF monitoring

 

[kpatz]

For VLF you'll need a much much larger coil.

Yes, an on-topic answer! Thanks!

I did make a new coil using nearly 400' of wire, it's not in the box yet though. I'm going to experiment with amp stages on a breadboard before I install it permanently.

That said, if you look at the pics of the K-II meter I posted earlier, the closest thing to a "coil" that can be seen is a few long traces on the circuit board. And from what I've read the K-II can pick up frequencies down to 50hz or lower. Nothing with a million turns of wire in that box. Any theories on how it works? Maybe it really picks up electrostatic charges instead? Maybe I should just buy a K-II and take it apart to see how it works.

 

[blueroomelectronics]

50Hz / 60Hz is pretty easy to pick up, your AC lines are like big antennas. True low frequency magnetic pickups require large coil pickups. Some types of Geophysics devices do this.

 

[Sceadwian]

You'll also need to make sure that amplifier isn't getting rid of the low frequencies. Either use large coupling caps or make the whole thing DC.

 

[kpatz]

Update: I built another amp circuit (basically the op-amp section in the schematic on page 1) and tried it with my "Coil from Hell" and a pair of headphones and my scope, and it works great. Picks up 60Hz like it's going out of style, and I even get a signal when I wave a magnet over the coil (LOW frequency response!). Sweet. I'll have to play with the gain to minimize the hum, or maybe add a 60Hz notch filter.

Which brings me to the next stage. I have the output of the op-amp going into 2 separate transistor amplifiers. One to drive the headphone jack and another to provide a signal for the comparator. The issue I'm having is an oscillation at around 3-4 KHz that seems to be brought on by/rides atop the 60Hz hum picked up by the coil. I took a .1uf capacitor and stuck it at various points in the circuit but had no luck suppressing this noise. Then I added a capacitor (100uf) to the output of the transistor amplifier circuit that will be providing the comparator input, since I want that to be a DC signal corresponding to signal strength, and then the entire circuit stopped picking up signals, and oscillated at a much lower frequency (click every couple seconds or so).

I'll have to sketch/post a schematic, but any suggestions off the top of your heads? The op-amp circuit alone works great, it's when I add the transistor stages that I run into the issue...

EDIT: I doctored up and attached a schematic (may not be exact, once again I'm at work and the circuit is at home).

 

[Dick Cappels]

1. When you disconnect the coil from ground, you turn it from being mostly a time varying magentic field transducer to an electric field transducer. From the schematic, it was originally intended to respond to magnetic fields.

2. Threading the coil through a ferrite toriod coil assures that it will not respond as a magnetic pickup. Wind it on an open core, or better yet, make it air core.

 

[Fuego]

I've used a variant on that circuit in a few of my projects--works a treat! But the inductor winding gets a bit tedious, not to mention calibrating several circuits. And determining the source of the field with only one axis is a touch tricky.

I've spent the last month looking for/simulating an inductor-less version, largely to no avail. Something using a hall effect or AMR sensor (perhaps the HMC1052L) could vastly improve low frequency response, provide the option of removing the static field component, and have a smaller foot print to boot. Any suggestions?

 

[Dick Cappels]

The commercially available hall sensors are so small that they do not give good sensitivity compared to inductors. If you were to use an magnetic antenna, made of magnetic material, that might improve the sensitivity by effectively increasing the sensor area. A long iron rod with a gap in the middle, and the sensor placed in that gap could be an example of such an antenna.


I would think that an air core inductor driving a dead short (such the input of a transimpedance amplfier) could give a pretty wide bandwidth, mostly limited by the Q of the inductor and the loop gain of the amplifier.

A variation on this idea is the subject of U.S. patent 5,296,866. Perhaps you would want to download a copy from Google Patents and have a look at the circuit.
https://www.google.com/patents


You could place use three small loops, one for each axis as used by Combinova in their magnetic field meter. The outputs from the preamp for each loop could either be combined (the output being the square root of the sum of the squares of the outputs from each axis) to provide total magnetic field or they could be used separately.



**broken link removed**
(Photo above by Combinova)


By the way, when making my earlier comment, I failed to notice that one end of the sense coil is bypassed to ground with a 100 uf capacitor. This would reduce the sensitivity to electric fields. Still, its a good idea to shield the inductor, leaving a small gap in the shield to prevent a shorted turn, in order to further reduce susceptibility to electric fields.

 

[Fuego]

The commercially available hall sensors are so small that they do not give good sensitivity compared to inductors. If you were to use an magnetic antenna, made of magnetic material, that might improve the sensitivity by effectively increasing the sensor area. A long iron rod with a gap in the middle, and the sensor placed in that gap could be an example of such an antenna.


I would think that an air core inductor driving a dead short (such the input of a transimpedance amplfier) could give a pretty wide bandwidth, mostly limited by the Q of the inductor and the loop gain of the amplifier.

A variation on this idea is the subject of U.S. patent 5,296,866. Perhaps you would want to download a copy from Google Patents and have a look at the circuit.
Google Patents


You could place use three small loops, one for each axis as used by Combinova in their magnetic field meter. The outputs from the preamp for each loop could either be combined (the output being the square root of the sum of the squares of the outputs from each axis) to provide total magnetic field or they could be used separately.



**broken link removed**
(Photo above by Combinova)

Thanks for the reply. As previously mentioned I'm trying for an inductor-less design. I should have emphasized the want of a smaller footprint. SOIC is a vast improvement from even a 2+ inch diameter sphere. From what I've read, the sensitivity of the HMC1052L and resolution (120µG) are acceptable for measuring between 1-200mG. The magnetic antenna seems unnecessary in this regard.

I suppose I'm asking for suggestion for good filtering and amplification techniques when trying to amplify low frequency (say, fc <= 5Hz HP) ac signal in the 1-200µV range with a DC component of .6V (which I'm trying to remove.) Spice simulations have gotten me precisely nowhere.

DC blocking caps will crush the low frequency response, and something like an isolation transformer will likely skew the results by its very nature--even with proper shielding.

Thoughts or suggestions?

 

[Dick Cappels]

You said you want to measure 5 Hz and below, but you did not say how low. If you want to measure close to DC, then an auto-zero technique would be your best bet for removing the offset. You may need to use a shielded enclosure in which to place the probe during the zeroing process.

 

[Fuego]

HP = High pass, sorry. 5Hz and above. Heck, measuring the DC component where 1mV = 1G (from the HMC1052L's data sheet), is easy. Unless I was working in an environment with a strong AC field on the order of a gauss I wouldn't really worry about it all that terribly much.

But it's the AC field that I'm interested with in this instance--at a rather low cutoff frequency. And due to variations in the earth's magnetic field, the DC component (that I listed as roughly .6V) is not consistent so I can't just use a unity difference amp or fixed offset.

Still plugging away on the simulations and crossing fingers.

 

[kpatz]

I know I haven't posted in this thread in a while, but I thought I'd post my latest updates.

My current EMF meter version uses my "Coil from Hell" and it picks up lower frequencies beautifully. Though 60Hz hum drives it nuts, it has provided some useful information on what I need to be looking for in the signal for my application. I need to detect narrow pulses of energy (little clicks and pops when heard in headphones), and I'm designing "version 3" of the circuit to do this.

Using op-amps alone proved to not work very well, probably due to the extremely low voltage of the pulses, and I needed insane amounts of voltage gain to get them to a level that would be useful. The other day I had a thought... maybe I should be amplifying current instead of voltage, in the initial stages of the circuit. So I breadboarded a circuit using transistors in the input stage. Voila... I can get some nice amplification of the weak pulses this way, and then the op-amp can convert the current into voltage with a moderate amount of gain (about 5-10X). I then run though another op-amp stage with a capacitor to "stretch" the pulses out (a peak hold circuit) so the slow microcontroller I'm using has a chance to read them.

With high-pass filtering (a small series capacitor) inline with the coil, it eliminates much of the 60hz and other miscellaneous noise one gets near anything electric, and so far this circuit is working great for picking up narrow pulses (at least, from the narrow pulse generator circuit I made). I still need to test it in the field, but once I have it perfected, I'll post a schematic.

 

[Dick Cappels]

Hmmmm...one wonders whether you are making an ultrawideband communications channel operating a extremely low frequencies.

It seems that you have solved the gain problem. Good going. I don't know whether I had mentioned this in an earlier post, but you might find that shielding the loop will reduce the 60 Hz further. I found it to be beneficial on one project.

**broken link removed**

 

[mrx3010]

New here but just want to say your build is quite well done IMO.

 

[kpatz]

Thanks... I'll have to post some pics sometime. The amplifier stage is still undergoing revisions. I just came up with a new amp circuit that is really, REALLY sensitive and has less noise than my last attempt, so I'll be trying that out (with a gain control!)

Update: Those pics I posted are OLD! The one I'm working on now uses a PIC16F818 and has a temperature display as well as the EMF. Plus I made it modular so I can swap out the amp board when I come up with new designs.