COIL FIELD EXAMINER (QUICK, CHEAP, EFFECTIVE).

[Aurificus]

I have been looking for a way to detect the strength and extent of the fields produced by various coil configurations and Tx circuits.
Net searches suggested that a Hall Effect Sensor and a simple pre-amp circuit would work.
Scratching through the “junk box” for recyclable components to build one, produced an instant solution…a long forgotten, portable cassette player. Nothing wrong with it, just superseded by the Walkman and then again by the IPod.
It’s small, 120 x 80 x 30mm and powered by 2 AA batteries.
The only modification I have performed has been to cut the wires to the DC motor to reduce power consumption. This might also reduce noise too, but I’m not going to re-connect it to check.
With a reasonable pair of ear-buds (IPod) and the play button pressed the output is a “whitish” noise, adjustable with the volume control.
Bringing the unit into the coil field produces an additional audible high frequency signal . The strength varies with distance and position over the coil. COOL!
The player has a significant metallic content and will obviously produce a Rx signal (turn down the sensitivity or detector volume as applicable) My next mod will be to remove the tape head (2 small screws), extend the shielded cable and mount it to a wooden or plastic rule to make some quantitative magnetic field measurements.

Aurificus

[Carl-NC]

What a clever approach! Instead of driving an audio amp, rectify/filter and drive a meter for numerical readings. Time to dig through the junk boxes for an old cassette player.

[J_Player]

The strength varies with distance and position over the coil. COOL!
The player has a significant metallic content and will obviously produce a Rx signal (turn down the sensitivity or detector volume as applicable) My next mod will be to remove the tape head (2 small screws), extend the shielded cable and mount it to a wooden or plastic rule to make some quantitative magnetic field measurements.

Yes, very COOL.
In order to reduce the effects from the metal in the tape head, you could wind a small coil around the end of a soda straw using #32 wire or smaller. I expect a coil would have better sensitivity than a hall effect sensor, and the metal in a small coil may have similar effect to the RX signal compared to the metal in the leads to the hall effect sensor.

Best wishes,
J_P

[B^C]

Over the years i have used a few things from the very basic to the more technical.

Lately my thinking has turned to modifying police radar detectors, these are basically an EM sensor & with a bit of work should give nice accurate results especially with an array of them setup.
Maybe some discussion on the subject may prove to be very informative to all. We could start with a basic circuit & everyone can add there input, schematics etc. I have a few idea's as well.

Just a thought.

B^C

[WM6]

I have been looking for a way to detect the strength and extent of the fields produced by various coil configurations and Tx circuits.
The player has a significant metallic content and will obviously produce a Rx signal (turn down the sensitivity or detector volume as applicable) My next mod will be to remove the tape head (2 small screws), extend the shielded cable and mount it to a wooden or plastic rule to make some quantitative magnetic field measurements.
Aurificus

Good idea, but maybe still better to measure magnetic field in Gauss by (AVO) meter than by ear (but it is enough a coin to hearing and testing of coil reach, no need cassette recorder, which say to us nothing more)?

Measurement solution on this simple circuit with Hall effect sensor and meter:

http://web-ee.com/schematics/instrum...ld-gaussmeter/

another well descriptive ear "measurement" solution with Hall effect sensor:

http://www.instructables.com/id/Magn...-Whistler-Thi/

[WM6]

Over the years i have used a few things from the very basic to the more technical.
Maybe some discussion on the subject may prove to be very informative to all. We could start with a basic circuit & everyone can add there input, schematics etc. I have a few idea's as well.
B^C

To measure density of magnetic field on TX coils out of complete MD configuration, we can use very simple measurement configuration consist of universal AVO meter (200 mV AC range) and dynamic ear-piece (min 20 Ohm better more, suggest > 40 Ohm). Measured readings on AVO meter in connection with distance in cm, are repeatedly comparable to other coils (although not in Gauss but in induced mV on AVO meter AC range).



http://farm4.static.flickr.com/3119/...8f8f32.jpg?v=0

[Aurificus]

Multimeter & earphone gaussmeter

Yes, I like this one too. Quick, cheap & simple... just like me!

I have 5 multimeters, only one will read small AC volts (3 volts to 3 decimal places)

Earphone (classic old type,.. I have a great junk box) is only 16 ohms and does not produce any voltage when exposed to coil or even florescent lights. (Tape player screams!). On the positive side the earphone does produce much less background noise from my tape player but response to coil signal is reduced.

I will now try wrapping coils on a straw until I get a reading.

Voltage measurement will solve a problem with the audio method.
At Tx frequencies of 15kHz and above some "experienced persons" may need a small child or a dog to listen for them.

Although the tone I get is quite distinct from the background noise I was not able to register a consistent frequency peak when put through a couple of sound-card audio analysers & scopes.

One FFT did produce an increased intensity, but over a 4-5 kHz range.

This might be a function of the $57 (eBay) detector, I was playing with.
Or, maybe I got a secret, prototype, broadband, multi frequency model.
Time to fire up the old 5kHz groundhog and see what happens.

Aurificus

[Aurificus]

The Groundhog is DEAD.....Long live the Groundhog

[WM6]

The Groundhog is DEAD.....Long live the Groundhog

Hi Aurificus.

Problem of tone check is that it is not useful comparable to compare different coils design solution.

If you measure 11mV at distance of 11 cm with one coil and 7mV at same distance on other coil, results are accurate comparable and usable in further construction decision. In tone check there is quite no difference between 7mV and 11mV, particularly when measurements are not taken at the same time.

My testing earpieces has 43 Ohms and can detect 1mV / 40cm apart from about 25cm MD coils (in vicinity of coil get over 180mV). AC AVO range 200 mV.

If you cannot find over 40 Ohms earpieces you can try with other suitable high Ohms inductor from your junk box e.g. relays (windings contact) and test your 3V AC AVO range applicability.

But I agree, tone sounds good, and it is usable, better than nothing.

[WM6]

The Groundhog is DEAD.....Long live the Groundhog

I am sorry for your Groundhog!

You mentioned FFT? Do you have digital scope?

[Aurificus]

Thanks for the tips WM6,
digging deeper in the box.

My scope, analysers and signal generators are all free/share programs running on an old WIN98 PC through a sound card pre-amp kit. I usually get reasonable results, the toughest problem is eliminating all the noise sources, particularly power supplies.
I'm now using a 12V SLA battery instead of plugpacks where possible and this helps a lot. Turning off the flouro light is a great noise reduction but introduces other problems. (can't see the bottom of the junk box)

The Groundhog will rise again, NOTHING gets thrown away.

Cheers, Aurificus.

[Aurificus]

Radar Detectors

Hi. B^C,
I didn't mean to neglect your post, but my brain was concentrating on a coil field detector using stuff in the box. I've got a sonar parking kit, printers, faxes, telephones, electronic ignitions, UPS's, monitors, photocopiers, car stereo's, bug zapper's, speakers, amplifiers...........no radar detectors!!... yet!

I have also had a good look into radar/microwaves for detectors. I hope to post some stuff in the LRL forum, soon. Plenty of "animated" discussion in that one, help me "stir the pot".

cheers, Aurificus

[B^C]

Aurificus

To eliminate your noise problems a common mode fliter after your output signal & before your pc may do the trick & will filter powerpac & fluro problems, cheap solution.


I think the strength of field is only one reading required, the SHAPE of the field is also very important.

May get a laugh out of this but years ago i used to look at Field shape by using an Aquarium with iron filings suspended in clear baby oil, some spectacular visuals were witnesed. You asked for cheap, this is a very simple solution & it does work although very crude. My Mrs has the smoothest Butt in the country with all the baby oil i had left over from testing.

Here is an EM Meter schematic that may work with some tweaking, the meter could be replaced & maybe connected to a pc or oscilliscope. Some input on this would be good!

A metel detector reciever circuit is also an EM Detector is it not?

Thanks

B^C

[Aurificus]

Pease post photos.....Aquarium &/or Mrs B^C's B.

[Esteban]

My simple and old method. With tunned cap., the detector "scream" at min sensibility causes the resonance of the checker coil. Also is waiting my project with bargraph leds (2 years ago) . I forgot this... I'm in other things, only need to insert correct capacitors in conjunction with rotative switch and drawn the frequencies in panel... Don't know if is useful for to patent this...