Static Magnetic Field Monitor
Projects ----------------- Static E. M. Field Monitor |
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This project provides schematics and instructions to build a Magnetic Anomaly Detector. It provides a means of monitoring the background for changes in a steady magnetic field. Two modes of operation are available. One allows an absolute value to be set. Any deviation from that will sound an alarm or activate the unit. The second mode provides a "floating" baseline. This will not cause activation of the unit on slowly changing fields over a period of minutes. Only abrupt changes will result in an alarm condition. The first may require some occasional intervention on the part of the user as conditions such as solar flares and such cause deviation. The second mode, while not as sensitive will generally self-correct for these types of changes without causing an alarm condition. The system also includes a self-contained power source in the form of a rechargeable battery. As an option it can be operated from a 12 V DC source.
The sensor consists of a Hall Effect circuit. This chip is mounted between two pole pieces which increase the sensitivity of the device. Instructions are provided to build this as well.
The first drawing includes the schematics and board layouts for the signal processing section of the detector. It is divided into two sections, the first the actual amplifier and meter nulling. The second is a window detector which signals the presence of a magnetic disturbance. Two LED indicators show the field polarity and provide an audible alarm if a disturbance is detected.
This is a high gain device. Care must be given regarding shielding and minimizing lead length. The entire detector (except sensor) should be placed in a metal enclosure to prevent false activation by nearby magnetic devices. A single common point ground approach should also be taken since to do otherwise might cause unstable operation.
One additional area of concern is the meter null circuitry. The Hall Sensor output is approximately 2 1/2 volts when no magnetic field is present. However it is sensitive enough that the earth's magnetic field will cause a full scale meter deflection. This can lead to some initial set-up difficulties.
A "trim" resistor is shown on U2 from pin 2 to ground. The value of this is selected once the device has been built. Its value is determined as follows. Measure the voltage on pin 3 of U2. It should be near 2.5 Volts with the sensor connected and oriented so the pole piece is pointing north. If there is a discrepancy, re-orient the sensor until this voltage is present.
Next, while measuring pin 2, rotate the Meter Null control from one extreme to the other. The voltage on pin 2 should shift, first above the voltage on pin 3, then below. It should equal the voltage when the control is set midway. If this does not occur, place a trim resistor as shown, selecting a value which conforms to the stated conditions. Trim is complete.
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The next drawing shows the output and detection circuits. This board is shown separately, however should you want to do so, both may be placed on the same PC board. Jumpers are shown on the "component placement" drawing which provide connection between the two boards.
The only special concern on this board is the resistor placed in the meter lead. The value is selected based on two factors. Raising this value will allow a greater signal before full scale deflection is obtained. It must also be raised should a more sensitive meter movement be used. Changing the value does not affect the overall sensitivity, only the meter deflection.
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The final drawing below provides PC board layouts for the circuits described above. They may be printed out to create a mask for etching purposes. They are shown as attached together, however they can be separated if that is desired. The first is the mask, the second shows component placement as seen from the back. They have been reduced in size for the web page, note scale provided to insure proper size if a direct etching process is used.
Construction details for the Hall Sensor assembly are also included here. This is a basic sensor, if you wish to experiment with various pole arrangements these instructions can be adapted as needed.
The Magnetic Anomaly Detector is intended to operate from a 10 Volt DC, well regulated power source. This voltage must hold constant, otherwise drift will cause false activations. I recommend the unit be built using a rechargeable battery for ease of operation, but this is not a requirement.
| The circuit shown to the right will take a 12V DC source and regulate it down to the 10 Volts required. In addition, it provides a current limiting resistor to allow the use of an internal 12 Volt rechargeable battery. Simply supplying 12Volts DC or AC to the power input will charge the battery while preventing an overcharge from occurring. This circuit is only a suggested method, others will work too. The only requirement is that 10V, at a minimum of .5 amps be available to power the circuit. It must also be filtered to |
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Operation of the Detector
Set up and operation is easily accomplished. There are two modes the detector can utilize. Fixed baseline is where any change, even over a long term, will cause an activation. The floating baseline mode compensates for slow changes while still allowing a quick change to activate the detector. Most normal operation is done in the floating mode since this allows the system to correct for natural variations over time.
Connect the sensor to the unit. Place the sensor in the center of the area you wish to monitor with the arrow pointing north. Set the mode switch to the desired position, fixed or float. Turn the unit on.
It is likely the system will alarm at this point. Either the Red or Green alarm indicator will light. Adjust the Meter Null control until the pointer is centered. The alarm will clear. Turn on the audible alarm if you desire that function.
The Window Size control allows the alarm sensitivity to be set. When it is fully clockwise the window is large. A full scale meter deflection, strong field, will be required to trip the alarm. When fully counter-clockwise, the window is small and the alarm will react to a smaller deviation in the field.
If the mode is set to Fixed" the meter should stay at its center point. Any deviation is the result of a change in the magnetic field. It is possible over time that the meter null will need to be adjusted to compensate for this. However it should be noted that the system is most sensitive in the Fixed mode.
If the "Float" mode is selected, some minor adjustment of the Meter Null may be needed in the first few minutes as the system establishes its baseline. Once this is done and the meter becomes stable the system will correct for variations in the background field.