Magnetometer

Barnes & Associates Mission Viejo, CA, USA

Fax: 949 454-2910

E-Mail: [email protected]

Copyright 1995 - 2005

mailto:[email protected]

How to build a low cost Differential Proton Precession Magnetometer.

Page 2

Forward:

Commercial magnetometers are simply too expensive for the average individual. So, I tried to obtain plans for building a magnetometer and failed to find a source. So, I set out on the Internet and explored. I posted questions on many forums and got few or no replies. I searched college libraries for weeks on end and found some NASA disclosures which were interesting, but of no practical use. I searched microfilm going back into the 1960's and found an article by Mr. Wadsworth of England who showed how he made a proton magnetometer. I was elated. I studied his article then built his magnetometer and it worked. This project was both exciting and fun. However, building his device posed some modern day procurement problems. He had used European germanium transistors in his amplifiers and an adjustable ferrite pot core inductor for the 2 KHz audio filter. I used modern silicon transistors and after a long search, I obtain a pot core, but it was not adjustable. I fine-tuned the filter with external capacitors. This method of tuning is very inconvenient for users in other parts of the world where the Earth's magnetic flux is stronger or weaker than at my location thus changing the precession frequency and necessitate tuning the filter for their location. It was then that I felt the need to update Mr. Wadsworth's design. I started by designing an op-amp based amplifier with a low noise front end. I tried just about every low noise op amp and FET for the front end, but nothing seemed to make an improvement. I ended up using a low cost silicon NPN that I socketed so I could evaluate individual transistors. This worked satisfactorily. When you hear the sounds of the magnetometer on this CD, note that the signal to noise ratio is quite good. The next challenge was to deal with the ferrite pot core inductor. I chose a active filter called a state variable band pass filter, which could be tuned with a potentiometer and the Q could be set with a resistor. Another victory. After successful testing, I documented the project and here it is.... Have fun. Phil Barnes


Page 3

Introduction: The Vanguard III satellite measured the earth’s magnetic field in space. Naval aircraft search the seas

for submarines hidden deep beneath the ocean surface. Archeologists search for ancient lost cities

and Prospectors search for heavy iron deposits that will lead them to gold. All of these applications

use versions of the proton precession magnetometer, a simple, sensitive instrument that measures the

earth’s magnetic field.

What is a differential proton precession magnetometer?

A differential proton precession magnetometer is an instrument used to detect variations in the

earth’s local magnetic field. These variations are caused by the presence of iron and ferrous

compounds. Iron and ferrous compounds provide a path for the earth’s magnetic field, which is less

resistant than air. Thus the magnetic field bends and passes through the iron, magnetite and/or

blacksand causing a local distortion or anomaly in the natural magnetic field. The differential

magnetometer senses this variation and is thus an excellent prospecting tool. The operation of the

magnetometer is not affected by water or soil, only by iron and iron compounds.

Theory of operation:

The magnetometer is based upon the effect the earth’s magnetic field exerts on protons in the nuclei

of atoms in water. According to theory, the most elementary particles - including the protons in

water, spin on their axes like tops. They are also magnetized. In the presence of a magnetic field,

such as the earth’s, the spinning protons precess around the direction of the earth’s magnetic field in

the same way that a spinning top standing on its point at an angle precesses around the earth’s

gravitational field. The rate (or frequency) of precession is proportional to the strength of the

magnetic field.

Normally the protons are out of step with one another, so that their effects cancel and the precession

cannot be detected. If, however, all of the protons could be made to point in the same direction at

right angle to the earth’s magnetic field and were then released together, they would all precess

together. The effect of all would add, and the precession would be detectable for a few seconds until

the precession got out of step again. Unfortunately a magnetic field of more than a billion Oersteds


Page 4

would be needed to line up all of the protons at room temperature. This field is much larger than we

can produce for a hand held instrument for portable operation. However, much smaller fields, which

can be produced easily by a battery and a coil of wire can cause slightly more protons to point one

way than the other. The difference is sufficient to produce a detectable signal when they are

released. The precession frequency, and hence the frequency of the signal induced in the coil, is

around 2025 cycles per second. After amplification it can be heard in an earphone as a musical tone

about three octaves above middle C.

The heart of the instrument is two plastic bottles wrapped with insulated copper wire and filled with

distilled water. A switch enables the operator to connect the two coils to either a battery or the input

terminals of an amplifier. The bottles are mounted in PVC TEEs on the ends of a six foot PVC pole

and at right angles to the pole. This pole (or beam) also supports the switch, battery and amplifier.

Usually the coils are connected to the input of the amplifier. Pressing the switch transfers their

connections to the battery. When the switch is released, the battery circuit opens first, and then the

contacts transfer the coil leads back to the amplifier.

The earth’s magnetic field has a force of around 0.5 Oersted. So, at a potential of three volts the

current in the coil develops a magnetizing force in the water of about 30 Oersteds. When the field is

turned off, the protons precess around the earth’s field for about 3 seconds and induce an alternating

voltage into the windings on the bottles. The windings are connected in reverse polarity. For this

reason the induced voltages cancel each other if the earth’s field is uniform in strength and direction

at both bottles. Nothing is heard in the earphone except the residual noise of the amplifier. This

differential connection also reduces common mode noise signals induced into the coils by external

phenomenon such as magnetic storms, engine ignition noise, lighting and such.

On the other hand, if the magnetic field differs at the two bottles, the induced frequencies will differ

and the net difference in voltage appears at the terminals of the amplifier. Such differences are

observed when one bottle is closer than the other to a piece of iron or inhomogeneities of the soil.

Beats or wavers then appear in the signal. They reflect the difference frequency of the current in the

two sets of coils. The 2025 cycle note from the earphone gets alternately louder and softer. The


Page 5

magnetometer can detect a minimum difference in field strength of about -53 10x Oersted. The

smallest difference that can definitely identified as arising from a buried test magnet is about 2x10-4

Orsted, which give about one beat per second.

How it is used.

The operator should always orientate the coils with the earth’s magnetic field to produce the loudest

and longest duration tones. When both coils of the magnetometer are in equal magnetic field

strengths they each produce the same tone or frequency which cancel each other. When the

magnetic field differs between the coils, each coil produces a different tone. The tones then “Beat”

thus producing a tone that rises and falls slowly. (EEE..EEe..Eee..eee....) As the difference in

magnetic field increases so does the beat frequency. This is demonstrated on the CD ROM.

When searching in an open area, hold the pole horizontally at right angles to your path. If you are

moving east or west, the pole points north and south and the bottles point east and west. Press the

switch for about three seconds then release it. If you hear no beats or one that persists for more than

a second, conclude that no target is in the vicinity. Press the switch while taking three slow paces

forward, Stop and listen again. In this manner, search a strip about 6 to 8 feet wide. If you find no

target, similarly scan parallel strips. Thus you can quickly search a large area, listening at points six

feet apart in a series of 6-foot strips. When used vertically, one coil is very near the earth or stream

and the other coil is up in the air. This technique is used to search for smaller objects nearer the

surface.

This project should be undertaken by individuals who have experience in electronic assembly, can

read schematic diagrams and know how to solder. This is not a difficult project for an experienced

person.

The Sensors: The bottles: Both sensor coils are wound on matching polyethylene bottles. Each bottle is fully

filled with distilled water and the top is sealed with RTV silicone. This gives an otherwise soft

bottle a firmness that facilitates handling when winding the coils on the bottles. The required bottles


Page 6

each have a capacity of 2 fl. Ozs. and a diameter of 1 3/8 inches. This size bottle is very common

and can be found in drug stores and craft stores often containing shampoo samples or craft paints.

Transparent or translucent bottles are preferred over opaque bottles thus allowing the water level to

be checked from time to time and topped-off if necessary. Always use distilled water.

CAUTION: Do not use a bottle having a larger or smaller diameter than specified above. To do so

changes the magnetic flux density during polarization and the resonance frequency of the coils

wound on the bottles.

The coils: Each bottle is to be wound with 520 turns of # 24 enameled magnet wire. The turns are

divided into four groups having 130 turns each. The groups are evenly spaced and separated to

reduce capacity between windings. To facilitate winding, fit each bottle with 5 spacers (rings) cut

0.2” wide from a PVC pipe “slip coupler” which is normally used to join 1” PVC pipes. The

spacers should be smoothly finished and snugly fitted onto the bottle. Slide these spacer rings onto

the filled bottle and layer wind the coils carefully and smoothly. The end spacers should fit tight

and be glued with super glue to secure them to the bottle. Do not rush the job, the coils must be

layer wound very carefully. Ideally, the first layer should have 20 turns. Winding the coils is

possibly the most difficult part of constructing the magnetometer. Some practice and a lot of

patience is required. After the winding is complete, carefully strengthen the whole coil assembly by

wrapping with black PVC electrical tape stretched moderately tight at the bottle ends. The goal here

is to keep the end spacers from popping off of the bottle. Finish the coil leads by soldering flexible

24 gauge speaker wire to the coils of sufficient length to reach the electronics package at the center

of the beam, or you may wish to solder small nonmagnetic connectors to the coil leads at this

juncture to facilitate removing the sensors and disassembling the beam for travel convenience.

The beam: The beam is made of two sections of 1 1/2” PVC pipe some 3 to 4.5 feet in length. A

“TEE” connector in the center attaches the electronics to the beam and a TEE on each end holds the

sensor coils. The sensors are wrapped in foam and slipped into the TEE’s with their connecting

wires fed into the beam center pipe for easy access. Since the coils are designed for “out of water”

use there is no need to glue the TEEs onto the ends of the beam. They may be slipped on and off as


Page 7

required for storage and travel convenience. If you plan to use the coils underwater a whole new

beam design would be required.

The Amplifier: The signals from the coils amount to about 10-16 watt, equivalent to the radiant

energy that enters the eye from a candle 16 miles away. Normal electron noise generated by the

copper coils approaches this 10-16 watt level. The amplifier must therefore be of the type which

develops high gain and low noise at the operating frequency of 2025 cycles. The amplifier is

compact, lightweight, and imposes a small load on the 9-volt battery.

The low noise pre-amplifier: Because a signal of 10-16 watt can just be heard over the noise of the

coils, an amplifier having an amplification factor of 10 to 20 million is required for satisfactory

performance. With this much gain, the first amplifier stage must necessarily generate very little

noise of its own. The input transistor is socketed and individually selected for best signal to noise

performance. The amplifier assembly is constructed on a Radio Shack PC Board part # 276-170. An

in-line design is used to isolate the output as far as possible from the input to prevent oscillation.

Although 8 pin DIP 741 IC’s may be used, the TO-99 metal can units are preferred if available. Pin

#8 of each metal package should be grounded to provide added shielding of each stage and thus

improve stability and freedom from oscillation.

The band pass filter: Due to the high gain of the audio amplifier, wideband noise generated by the

“front end” of the amplifier is undesirable to the operator and only serves to mask the desired signal.

The band pass filter is tuned to the signal frequency of 2025 Hz and eliminates much of the

undesirable noise while passing and amplifying the desired signal. The filter used in this design is a

tunable, active, band pass filter consisting of three (3) op-amps producing a gain of 6 to 8 with a high

Q. It is easily tuned to the proper frequency using the trimmer potentiometer and an audio generator

set at 2025 Hz. When testing the amplifier set the tuning potentiometer to the center of its range and

select R14 so the filter peaks at 2025 Hz. This is accomplished by using a decade resistor box and

audio oscillator for this operation. The filter may then be fine tuned to the actual signal during

operation using the potentiometer.


Page 8

The output stage: The audio output stage is preceded by the volume control and consists of a 741

op-amp having a voltage gain of 220. Like the earlier stages, it draws very little supply current.

This low current is attributed to the use of a crystal earphone as the output transducer. If a magnetic

output device such as a speaker or phones is used, the output amplifier should be changed to a type

386 IC audio power amplifier and rewired accordingly. This will result in a significant increase in

battery power consumption.

Tuning the Sensors: The sensor coils need to be tuned to 2025 Hz to optimize their sensitivity to

the proton frequency. When the unit is assembled and operational, connect an AC voltmeter across

the piezo earphone terminals. Locate the sensor coils so as to just pickup the 2025 Hz tone from a

“transmitter” coupling coil hooked up to the output of the audio signal generator. Select the sensor

coil tuning capacitors to peak at 2025 Hz and solder them permanently in place. Mine were 0.47

Mfd, 0.1 Mfd & .01Mfd.

The Crystal Earphone: vs. headphones or speaker.

The crystal earphone used in this design offers several advantages over the more conventional

magnetic headphone or speaker. The crystal earphone is quite sensitive requiring very little power to

produce an acceptable output volume, it often resonates near the magnetometer output frequency and

is free of ferrous metals or magnets which could influence the magnetometer. Replace the earphone

leads with a thin shielded microphone cable to prevent feedback to the sensor coils and low noise

amplifier. The coils in magnetic headphones can cause magnetically coupled feedback directly to the

sensor coils with resulting oscillation when the head is turned. If you expect to be operating in a

quiet environment, you may wish to use a piezo speaker such as Radio Shack Part # 273-073. This

transducer can be tuned to 2025 Hz by adjusting the output opening hole by partially covering the

hole, as one would tune an organ pipe, and listening for the output to peak.

Battery life: The amplifier draws around 6 to 7 milliamperes from the 9-volt battery. This assures a

battery life of several days of continuous usage from a conventional 9-volt alkaline battery.

The polarizing battery is a three cell nickel-cadmium type, size AA, and is field rechargeable

overnight. The coil polarization current is around 360 to 390 milliamperes with a 50% duty cycle


Page 9

bringing the average current to 180 milliamperes. The size AA cells are rated at 500 milliampere-

hours. This estimates to be just short of 2-3 hours of service prior to the need to recharge. This

battery is not critical and it may be desirable to substituted it with a higher Ampere-Hour unit. Size

and weight are a consideration. If a 12-volt battery is used, performance will improve because

polarization time is reduced, polarization current will increase to around 1 ampere and signal output

level will increase. This trade-off may be worthwhile to many users. A small six-volt motorcycle

battery may be ideal with its iron free construction and lightweight. These typically have a 4AH

rating.

Enhancements: This magnetometer is a useful instrument as built. Here are some thoughts regarding enhancements:

A 556 timer IC and 3PDT relay may replace the hand-operated switch. The timer can have both adjustable polarize time and listen time. A manual mode can operate the relay using a switch. I prefer the hand-operated switch as shown in the schematic. A 567 PLL tone decoder IC may be used to detect the 2025 Hz tone and switch the output transducer directly to the 567 PLL VCO for a loud clean output. This device replaces the 741 output amplifier. After trying this I preferred the normal analog output over the switched output of the 567.

Notes: • The magnetometer will usually not work indoors because of the inhomogeneities of the magnetic fields present there. Do all testing in an open outdoor area. • This magnetometer is a very sensitive instrument. It is our wish that you enjoy building and using this instrument. Have fun.....

• Phil Barnes, E-mail: [email protected] FAX: (949) 454-2910


Page 10

•

BEAM CENTER TEE (NOT SHOWN) IS CONNECTED TO BOX AND TWO BEAM PIPES.

THIS ALLOWS EASY BREAKDOWN FOR TRAVEL AND STORAGE.

PC BOARD IS MOUNTED TO THE BOX METAL COVER WHICH PROVIDESADDITIONAL SHIELDING FOR AMPLIFIER STABILITY.

THE POLARIZING SWITCH IS SEEN IN THIS PICTURE AS A CONVENIENT FINGER TRIGGER FOR THE HAND HOLDING THE BEAM.

DIFFERENTIAL PROTON PRECESSION MAGNETOMETER

6 -9 FEET


Page 11

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Finished Sensor Coil wrapped with tape to secure coil and spacers. Do your best work with the sensor coils.

Coil, TEE and

Page 12

Tupperware end covers.

Leads from the electronics box are fed through the

beam and are soldered to the sensor coil leads. You may wish to use small non-magnetic connectors at this

junction for easy disconnect.


Page 13

•

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Page 14

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Page 15


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Page 17

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Page 18

Proton Magnetometer - Parts List

SYMBOL QTY DESCRIPTION P/N NOTE C1 1 I.0 mFd. electrolytic C2 1 6.0 mFd. electrolytic C3 1 2.0 mFd. electrolytic C4 1 .05 mFd. disc. (.047 ) C5 1 2.0 mFd. electrolytic C6 1 0.1 mFd. disc. C7 1 0.1 mFd. disc. C8 1 0.01 mFd. disc. C9 1 0.01 mFd. disc. C10 1 0.1 mFd. disc. C11 1 470 pF. disc. C12 1 100 mFd. electrolytic C13 1 22 mFd. electrolytic C100 1 0.47 mFd. Polystyrene TYPICAL VALUEC101 1 0.1 mFd. Polystyrene TYPICAL VALUE

R1 1 120 K 1/8W CARBON FILM R2 1 47 K 1/8W CARBON FILM R3 1 6.8 K 1/8W CARBON FILM R4 1 8.2 K 1/8W CARBON FILM R5 1 4.3 K 1/8W CARBON FILM R6 1 1.0 K 1/8W CARBON FILM R7 1 470 K 1/8W CARBON FILM R8 1 5.1 K 1/8W CARBON FILM R9 1 220 1/8W CARBON FILM R10 1 100 K 1/8W CARBON FILM R11 1 10 K 1/8W CARBON FILM R12 1 100 K 1/8W CARBON FILM R13 1 2.2 K 1/8W CARBON FILM R14 1 SELECTED VALUE (~2-3K) R15 1 2.2 K 1/8W CARBON FILM R16 1 1.0 K 1/8W CARBON FILM R17 1 220 K 1/8W CARBON FILM R18 1 100 K 1/8W CARBON FILM R19 1 100 K 1/8W CARBON FILM R20 1 10 K - POT. VOLUME CONTROL R21 1 200 OHMS - TRIMMER

IC1 1 OP AMP. TO-99 741 IC2 1 OP AMP. DUAL, 8 DIP 1458 IC3 1 OP AMP. TO-99 741

IC4 1 OP AMP. TO-99 741

Q1 1 NPN TRANSISTOR - TO92 MPS A05 MOTOROLA


Page 19

D1 1 DIODE, SILICON 1N4001 RADIO SHK. D2 1 DIODE, ZENER 12-20V

EARPHONE 1 CRYSTAL EARPHONE 25CR25 MOUSER

SOCKET 4 8-PIN DIP SOCKET 276-1995 RADIO SHK.

PC BOARD 1 EXPERIMENTERS P.C.BD. 276-170 RADIO SHK. BOX 1 6.26 x 3.75 x 2.0 - METAL PANEL 270-627 RADIO SHK. SW1 1 S.P.S.T. ON/OFF SWITCH RADIO SHK. SW2 1 3 P.D.T. MONENTARY SWITCH

WIRE ~ 22GA. STRANDED, Misc colors. RADIO SHK.

BOTTLE 2 2 oz. POLYETHELENE, 1 3/8"D. Read text.

WIRE ~ MAGNET WIRE #24 AWG. Enameled L3-612 GC Tech TAPE 6' PVC ELECTRICAL

Misc. Case, Knob, PVC tubing and fittings, Batteries, Solder Etc... PIPE 6' PVC, 1 1/2" I.D. TEE 3 PVC, 1 1/2" TEE CONNECTOR COVER 4 TEE END HOLE PLUGS 201-28 TUPPERWARE

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