Build a Proton Precession Magnetometer

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    An educational "backyard" project, constructed using easily obtained electronic parts. A

    frequency counter is used to measure the post-polarizing pulse proton precession frequency. The

    measured frequency is related, by a physical constant, to the magnitude of the local geomagnetic



    For some background information and a description of a practical application for a proton

    magnetometer, see "The Amateur Scientist "column in the February 1968 issue of Scientific

    American. Construction of a dual coil magnetometer is described. Information in that article

    formed a basis for the details shown here.

    I constructed a fluxgate magnetometer several years ago. It was based upon Richard Noble's

    article in the September 1991 issue of Electronics World + Wireless World. With a chart

    recorder, it is possible to see the dirunal changes in the east-west component of the earth's

    magnetic field, after nulling out the overpowering total and north-south components.

    After finding the February 1968 Scientific American article, I thought that it would be an

    interesting project to try adding a frequency counter to the proton magnetometer.It would be an

    interesting "backyard science" project to use it to provide a measure of the earth's total magnetic

    field. The addition of a digital to analog converter can provide a output suitable for a chart


    However, a suburban backyard environment is a rather noisy one. Harmonics of the power line

    frequency extend well up into the audio frequency range. These compete with the decaying

    precession frequency tone. Connecting the sensor coils in differential series, sensor orientation

    and instantaneous sampling of the audio signal help in contending with the noise.

    From the physical sciences a quantity called the"Larmor frequency" defines the angular

    momentum of protons precessing in the presence of a magnetic field.

    There are currently quantum-mechanical views that explain particle precession, but a classical

    explanation seems a bit easier to comprehend. A proton, a charged particle, may be thought of as

    having definite "spin" about an "axis" and acts as a small magnet. An externally applied

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    magnetic field does not alter the spin rate, but causes the particle to wobble at a slower rate about

    an axis of precession. This axis tends to align with an external magnetic field. However in weak

    magnetic fields, any alignment tends toward randomness due to thermal effects and other

    molecular interactions.

    The proton reacts to the perturbing effects of an externally applied magnetic force by precessing

    at a rate in accordance with a precise constant called the gyromagnetic ratio. For protons this

    quantity is equal to approximately 267.53 x 1E6 radians per second per Tesla or 42.58 mHz per


    In the northern latitudes of the U.S. the total magnetic field strength is in the order of 50,000 to

    55,000 nanoTesla and varies from location to location. Short period variations due to magnetic

    storms may reach several hundred nanoTesla. Diurnal variations caused by solar induced

    ionospheric currents are in the order of tens of nanoTesla. Presently, the long term trend of the

    total field is in the order of minus 90 nanoTesla per year ( steadily decreasing).

    The proton precession frequency detected by a suitable sensor in the geomagnetic field of the

    earth will be at a frequency in the audio range:


    42.58 mHz / Tesla x 52500 x 1E-9 Tesla= 2235 Hz

    In my northeast location the frequency readings currently average about 2271 Hz, corresponding

    to a total field of about 53,300 nanoTesla. This agrees quite well with the USGS readings shown

    for the Fredericksburg, VA monitoring station , 160 miles to the west. This figure also agrees

    with the value obtained using the fluxgate magnetometer that was calibrated using a Helmholtz

    coil. The fluxgate sensor was tipped upward from a horizontal position to nearly vertical to

    obtain the maximum reading of the earth field.

    I have noticed a decrease in the frequency readings of about six or seven Hertz over the past

    twelve months or so since the sensors have been in place in the backyard. Originally the

    frequency readings were around 2277 or 2278 Hz. This also seems to agree with the magnitude

    of the predicted long term variation shown by the USGS site.

    Return to main page

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    This is a block diagram of a "listen only" version. The frequency counting circuitry is not used.

    Only the senor coil(s) ,audio amplifier and dc power source are included. A timer IC is used to

    provide switching contol to a relay that alternately connects the sensing coil between a polarizing

    current source and the input to the audio amplifier.(Click figure for larger diagram.)

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    This is a block diagram of a magnetometer design that adds the capability to measure the

    frequency of the voltage induced in the sensor coil by the precessing protons after the application

    of a polarizing current several seconds in duration. A four decade BCD counter dis- plays

    frequency to a selectable resolution of 1 or 0.1 Hz. A frequency multiplier method employs a

    phase locked loop to provide these resolutions using counter gate intervals much less than one


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    I found the local super market to be a good source for coils forms on which to wind the

    magnetometer coils and contain the proton medium. Check the area where the spices are located.

    Particularly look for the store brand spices. I found that these use thin walled plastic containers

    that have encircling ridges at the bottom and just below the lid. These make a form on which a

    multilayer coil can be easily wound. (CLICK FIGURE FOR DETAILS )

    The above referenced page shows the particular size used. There are a number of sizes available.

    Also found some taller ones that would provide a coil length of about 3.75 inches. A somewhat

    larger container would conveniently allow the use of a larger wire size. There are advantages ---

    lower coil resistance, providing higher coil Q and possibly higher polarizing current (if the

    power supply can provide it ). A higher polarizing current increases the initial amplitude of the

    decay signal.

    The higher coil Q will sustain the ringing effect of induced by the decay signal for a longer

    period of time.Note that the coil inductance increases as function of the square of the number of

    turns while coil resistance increases as linear function of the number of turns. This suggests that

    the best results (high Q and tuned circuit selectivity) will be obtained using the largest number of

    turns and largest wire size that is practical.Also, and possibly most important, the coils will be

    tuned by the addition of a shunt capacitor---perhaps the most important component of all.

    The coil inductance should high enough to permit the use of a reasonably valued non-polarized

    capacitor. A higher Q will also aid in providing a narrower tuned circuit bandwidth--important in

    improving the signal to noise ratio and reducing the pickup of high order power line harmonics.

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    Notes on Sensor Construction

    1.It may be possible to place the 700 turns in four layers. However, as subsequent

    layers are added it becomes more difficult to maintain close spacing. Most likely it

    will take five layers. Actual turns count is not critical. If you have 700 turns before

    reaching the end of the bottle, continue winding to complete the final layer.

    2. Coil constructed as shown will provide an inductance of about 10 millihenries. An

    approximate formula (neglects a small multilayer correction factor of about negative 5

    percent) for calculating the inductance is:


    where: r=one half the bottle diameter in inches n= number of turns l= coil length


    3. A coil tuning capacitor for two sensor bottles as shown, connected in series, will be

    about 0.25 microfarads.

    4. After winding, fill the bottle with a "proton rich" fluid. Distilled water, kerosene,

    methanol have been used. Common isopropyl alcohol will work.

    5. Spice bottles are not designed to hold liquids. The lids may have a paper inner liner

    that should be discarded. If needed to stop leaking, try making a gasket from bicycle

    inner tube or similar material.

    In my backyard environment, for the best signal to noise ratio, I found that two

    identical coils were useful. These were connected in series and oriented for

    minimizing the level of power line harmonics. An orientation with the coil axes in line

    and electrically series opposing provided a degree of cancellation of common-mode

    power line noise pick up.

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    The audio amplifier uses four bipolar transistors and one dual operational amplifier integrated

    circuit. The block diagram at the left shows the stage gain distribution. The operational amplifier

    provides a two stage active bandpass filter centered at the expected frequency of the proton

    precession. Maximum available gain is in excess of 130 dB.

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    The theoretical gain vs. frequency is shown in the figure below.

    With such high gain careful construction is required to prevent oscillation

    The figure at the left briefly outlines physical details. The amplifier was built on double sided

    copper clad PCB material. Components are soldered to standoff terminals. A push-in type nylon

    or teflon terminal is used. Vectorboard is difficult to use for a circuit made up entirely of discrete

    components. The circuit board is housed in a Radio Shack molded project case. The inside of the

    case is lined with adhesive backed aluminum tape.

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    The input stage uses a 100 ohm unbypassed emitter resistor to raise the input impedance to about

    12 kilohms to reduce loading on the tuned sensor coils. The tuned circuit formed by the coils and

    resonating capacitor present a parallel impedance of about 3000 ohms. A number of different

    devices were randomly selected and tried at the input stage in order to find one providing the

    best signal to noise ratio. The noise contribution from a 560 ohm resistor soldered across the

    input terminal can be detected. However, noise from the sensor coils and external pickup exceed

    the intrinsic amplifier noise contribution.

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    The following page links to the schematic of a counter implemenation that measures the

    precession frequency. It was intended as a educational project to attempt to provide a

    measurement of the magnitude of the local geomagnetic field. It is offered for informational

    purposes only. Others may find it of interest or may adapt it to a specific practical application.

    One of my objectives was economy, to use parts that were on hand or easily obtained standard

    components. For operation from a battery source lower power dissipation equivalent CMOS

    logic elements can be substituted for the TTL elements shown.

    Counter Circuit Description

    The circuit shown requires twelve integrated circuits in addition to other discrete components.

    Integrated circuit choice was based on economy--- that is, using parts that were on hand. There

    are many alternate ICs that may be substituted for the NAND gates, counters and multivibrator.

    The 4060 counter /oscillator and 4046 Phase Locked Loop IC are probably good choices in any

    event, but there are other possibilities there also. If power is to be obtained from batteries,

    substitution of equivalent CMOS logic ICs in place of TTL types will reduce dc current


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    (There is another circuit shown in a separate segment that is a simpler LISTEN ONLY version. It

    eliminates the frequency counter and uses a timer to cycle the polarizing current to the sensor

    coils on and off.)

    Timing for polarizing the sensors and measuring frequency is derived from a watch crystal.

    These are the tiny cylindrical units found in some digital wrist watches. They sell for about two

    for a dollar at Active Electronics or a dollar each at Radio Shack.

    The oscillator circuit is pretty much per CD4060/MC14060 application note. The oscillator

    portion produces an output frequency of 32.768 kHz that is applied to a fourteen stage counter.

    The final output of the last stage is 2 Hz or a pulse repetition rate of 0.5 seconds. This drives a 4

    stage binary counter whose last stage provides a four second high / four second low logic level.

    For simplicity, the full count cycle of the 4 stage binary counter is used. If the intent is to use the

    magnetometer in a portable search mode, it would probably be useful to shorten the four second

    (listen) non polarizing interval to a half second. This will require the addition of at least one four

    input NAND gate to decode the counter state (10 count ) and reset the counter.

    Polarizing current should be applied to the sensing coils for several seconds in order to maximize

    the amplitude of the precession signal. Three seconds appears to be sufficient. After removal of

    the polarizing current the the relay connects the coil(s) to the input of an audio amplifier. The

    output of the audio amplifier is a ringing tone at the precession frequency, whose amplitude

    rapidly decreases into the background noise level. In order to obtain an accurate measurement of

    the frequency, the counter should begin sampling immediately after the removal of polarizing

    current. Also counting should only be done when the signal amplitude is well above the noise


    Measuring the audio amplifier output directly would require a 1 second counting interval to

    resolve to 1 Hz at the expected relaxation frequency, and 10 seconds to resolve frequency to 0.1

    Hz. Certainly, in the last case, the signal would have long decayed below amplifier noise or local

    power line harmonics. And, in a backyard environment, after one second, the signal is competing

    with ac power line harmonics.

    A phase locked loop is used to permit measuring the precession frequency to 1 an 0.1 Hz

    resolutions using counting intervals much less than one second. One input to the phase detector

    is the output of the audio amplifier. The other input to the phase detector is derived from the

    voltage controlled oscillator (VCO) whose frequency is divided down by two intervening digital

    dividers; a divide by 10 and a divide by 8 in series. When in lock the VCO frequency is then

    equal to the audio amplifier output frequency multiplied by a factor equal to the total division ( 8

    x 10 =80). Measuring the VCO output frequency at the output of the divide by ten counter using

    a counting interval of one-eighth second allows resolution to 1 HZ. Measuring the VCO

    frequency directly ( ahead of the divide by ten counter ) allows resolution to 0.1 Hz. In this case,

    the most significant digit (thousands) overflows the fourth stage of the counter leaving the

    display of hundreds, tens , ones and tenths Hz.

    For simplicity and economy, individual light emitting diodes are used to display the state of the

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    decade counter. The schematic shows four LEDs at the most significant digit, two or three

    should be sufficient since this stage will normally always read the BCD equivalent of a 2 ( two

    thousand or two hundred depending upon the resolution selected). Under stable conditions, there

    is only a variation in the least significant digit when using one Hertz resolution or the last two

    significant digits when resolving to one-tenth Hz. If the intended use is for portable searching, I

    suspect that it would be desirable to use a decimal display so that changes in the reading may be

    easily seen. (Although just listening to the audio output may be sufficient to detect magnetic

    anomalies. ) There are many choices to implement this - - - -composite LCD display, seven

    segment LCD, etc. These will require the addition of the appropriate BCD to segment

    decoder/drivers or the use of the expensive integrated counter/display ICs.

    For economy, minimizing interconnecting wiring and component count , monostable

    multivibrators (one shots) are used to set the decade frequency counter gating and timing

    intervals. This is probably easier than decoding the states of the CD4060 and 74197 counters (U1

    and U2) that derive the time base from the 32.768 crystal. Straight decoding would require

    several multiple input NAND gates as well as inverters ( since the counters do not provide

    complementary logic outputs- - - - Q and inverted Q.)

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    The periods of the multivibrators must be set with some degree of accuracy since the tolerance

    on the nominal values of the timing components is insufficient to guarantee correct time delays.

    The existing accurate time base waveforms and the decade counter are used to set the time delays

    accurately. The timing resistor values, R3 and R4, are varied as needed to provide the correct

    time delays.

    Time Delay Adjustment

    The fourth binary stage of the CD4060 oscillator/counter output (Q4) is available at pin 7. The

    oscillator frequency has been divided by a factor of 16 at this point, resulting in a frequency of

    2048 Hz.

    U3A Delay

    Make the following temporary connections:

    1. Open connection between points A1 and A2. Connect A2 to the 2048 test signal at U1 pin 7.

    2. Open the connection between points TC1 and TC2. This places an enabling signal to the

    decade counter input gate that is equal to the time delay of multivibrator U3A.

    Adjust the value of R12 at pin 11 of U10 to some value around 8000 ohms or so. This should

    adjust the free run frequency of the CD4046 VCO to value that will allow it to phase lock to the

    test signal. At lock the VCO frequency should be 80 times that of the test signal or 163840 Hz.

    Set the resolution switch (S1) to 1 Hz. This connects the divided (by 10) VCO frequency of

    16384 to the decade counter input gate.

    Use a nominal value of 56 kohms or 62 kohms as the timing resistor for R3.

    The counter display should produce a new reading every eight seconds. The desired counter

    display should be equivalent to 0.2 seconds which will be displayed as 0.2 X 16384 or 3277.

    Select a value of resistor to give time delay of 190 to 210 milliseconds or a counter reading

    between 3112 and 3440.

    U3B Delay:

    Leave previous test connections as they were. Make the following additional temporary test


    1. Open connection between points D1 and D2.

    2. Open connection between points B1 and B2. Connect a short insulated wire to B1 such that

    you can manually touch it to ground to reset the decade counter zero.

    3. Temporary connection from point E1 to D1.

    Manually reset the counter by manually grounding B1. Observe counter reading as before and

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    reset counter manually as needed. Use an initial value of 27 kohms for R4.

    Adjust the value of R4 to provide a delay time between 90 and 100 milliseconds, equivalent to

    counter readings between 1475 and 1638.

    Restore all connections to normal per schematic.

    VCO Frequency:

    Temporarily connect point A1/A2 to ground. Adjust value of R12 to produce a counter reading

    of 2230 to 2250. Remove temporary ground.





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