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Adjustment and Cal
At this point you could run off and start using the mag, but for optimal performance we need to make one easy mechanical
adjustment. Recall that a single sensor is sensitive to its orientation within the magnetic field. For the two-sensor arrangement, the
mixed-frequency output is sensitive to their misalignment to each other. If they are perfectly aligned, then each sensor will detect
the exact same magnetic field (in the absence of a target) and the gradient will be zero even if the whole tube is rotated. If they are
not aligned, then one sensor could detect a slightly different field, and this will change with the movement and orientation of the
tube. All calibration and alignment procedures should be done well away from undesirable magnetic fields, including iron targets
and electrical (AC) lines.
Before we can align the sensors, we first need to allow the SCL007 to perform a sensor calibration. According the literature,
the calibration is done during the first 10 seconds or so after power-up during which time it expects the sensors to see a minimum
and maximum earth field. The described method is to hold the tube in a north-south orientation pointing the north end upwards at
the angle of the field’s inclination, about 67° to the horizontal in the UK, a little less in the US, and 0° at the equator. Switch on the
power and slowly rotate the tube, head over heels, through 360° during the first 10 seconds. That’s all there is to it; this procedure
should be done every time the mag is turned on for use.
To align the sensors, turn on the unit and go through the sensor calibration described above. Place the tube in an east-west
orientation (and level) in a way that you can rotate or roll the tube. I built a simple wooden “U” stand with “V” notches cut in the
uprights. With the tube in a starting position listen to the output frequency. If there is no output sound or the frequency is less than
10Hz you may need to go ahead and adjust the VCO to get an appropriate frequency. Now slowly rotate the tube by about 180° and
listen for a frequency change. If there is one, it indicates that the sensors are misaligned. I found an effective method is to rotate the
tube to find the highest output frequency; with the tube held at this position adjust the sensor to reduce the frequency. Continue this
until you can rotate the tube with little or no frequency change.
FIGURE 12. Adjustable Sensor Assembly
FIGURE 13. Sensor Connection
56nH
33mF
VCC1
GND
Ouput
+
ã1999-2002 Carl W. Moreland Revision 1.2 Page 8
Options
This mag has only two knobs: volume and “threshold.” The threshold knob can be used to reduce the minimum frequency at
idle for better sensitivity. Doing this creates non-linearities in the DAC voltage to VCO current translation and reduces the
maximum frequency somewhat, but neither effect is important for an audio mag. You may choose to omit this potentiometer and
put a short in its place. As mentioned before the PC board includes extra solder pads at the digital outputs of the SCL007 for
interfacing to a computer. This permits use of the mag design for more accurate mapping with appropriate software. The analog
output of the DAC also has convenient solder pads for driving another analog indicator but it is important to use a high input
impedance buffer for this signal as any current drawn from this node will get applied directly to the VCO capacitor and cause large
frequency errors.
The Motorola MC34119 was used for the audio amplifier and is the
amp supplied by Fat Quarters. This device is a little difficult to find so the
PC board has been laid out to accommodate the more popular (and slightly
higher power) LM386. Figure 14 shows the schematic for this as well as
the parts placement. Note that you need to clip pin 7 off the LM386 and
add a ground short for pins 2 and 4 (pads are provided).
The fluxgate sensors are extremely sensitive to magnetic fields and the
use of a moving coil speaker caused initial concern. However, I’ve found
that the dynamic electromagnetic field of the speaker does not seem to
affect the sensors. You may choose to use headphones instead of or in
addition to the speaker, or to use a piezo-based audio device or a meter
driven from the DAC analog output.
Usage
You can carry the mag one of two ways: horizontal or vertical. Vertical orientation seems to be more sensitive while horizontal
can allow you to cover more ground by making a wider swath. In the vertical position the sensor with the wires coming out towards
the end cap (the adjustable module) is more sensitive and should be the one nearest the ground. When using the mag in the
horizontal position, remember that a target signal could be coming from either of the two sensors. You will either have to
determine this by moving the mag around, or you can use the sign bit from the SCL007 to drive an LED to distinguish the sensors.
With a little practice you should be proficient in no time. The fluxgate mag is extremely easy to use and will make a nice (and
inexpensive!) addition to your treasure hunting arsenal.
Adjustment and Cal
At this point you could run off and start using the mag, but for optimal performance we need to make one easy mechanical
adjustment. Recall that a single sensor is sensitive to its orientation within the magnetic field. For the two-sensor arrangement, the
mixed-frequency output is sensitive to their misalignment to each other. If they are perfectly aligned, then each sensor will detect
the exact same magnetic field (in the absence of a target) and the gradient will be zero even if the whole tube is rotated. If they are
not aligned, then one sensor could detect a slightly different field, and this will change with the movement and orientation of the
tube. All calibration and alignment procedures should be done well away from undesirable magnetic fields, including iron targets
and electrical (AC) lines.
Before we can align the sensors, we first need to allow the SCL007 to perform a sensor calibration. According the literature,
the calibration is done during the first 10 seconds or so after power-up during which time it expects the sensors to see a minimum
and maximum earth field. The described method is to hold the tube in a north-south orientation pointing the north end upwards at
the angle of the field’s inclination, about 67° to the horizontal in the UK, a little less in the US, and 0° at the equator. Switch on the
power and slowly rotate the tube, head over heels, through 360° during the first 10 seconds. That’s all there is to it; this procedure
should be done every time the mag is turned on for use.
To align the sensors, turn on the unit and go through the sensor calibration described above. Place the tube in an east-west
orientation (and level) in a way that you can rotate or roll the tube. I built a simple wooden “U” stand with “V” notches cut in the
uprights. With the tube in a starting position listen to the output frequency. If there is no output sound or the frequency is less than
10Hz you may need to go ahead and adjust the VCO to get an appropriate frequency. Now slowly rotate the tube by about 180° and
listen for a frequency change. If there is one, it indicates that the sensors are misaligned. I found an effective method is to rotate the
tube to find the highest output frequency; with the tube held at this position adjust the sensor to reduce the frequency. Continue this
until you can rotate the tube with little or no frequency change.
FIGURE 12. Adjustable Sensor Assembly
FIGURE 13. Sensor Connection
56nH
33mF
VCC1
GND
Ouput
+
ã1999-2002 Carl W. Moreland Revision 1.2 Page 8
Options
This mag has only two knobs: volume and “threshold.” The threshold knob can be used to reduce the minimum frequency at
idle for better sensitivity. Doing this creates non-linearities in the DAC voltage to VCO current translation and reduces the
maximum frequency somewhat, but neither effect is important for an audio mag. You may choose to omit this potentiometer and
put a short in its place. As mentioned before the PC board includes extra solder pads at the digital outputs of the SCL007 for
interfacing to a computer. This permits use of the mag design for more accurate mapping with appropriate software. The analog
output of the DAC also has convenient solder pads for driving another analog indicator but it is important to use a high input
impedance buffer for this signal as any current drawn from this node will get applied directly to the VCO capacitor and cause large
frequency errors.
The Motorola MC34119 was used for the audio amplifier and is the
amp supplied by Fat Quarters. This device is a little difficult to find so the
PC board has been laid out to accommodate the more popular (and slightly
higher power) LM386. Figure 14 shows the schematic for this as well as
the parts placement. Note that you need to clip pin 7 off the LM386 and
add a ground short for pins 2 and 4 (pads are provided).
The fluxgate sensors are extremely sensitive to magnetic fields and the
use of a moving coil speaker caused initial concern. However, I’ve found
that the dynamic electromagnetic field of the speaker does not seem to
affect the sensors. You may choose to use headphones instead of or in
addition to the speaker, or to use a piezo-based audio device or a meter
driven from the DAC analog output.
Usage
You can carry the mag one of two ways: horizontal or vertical. Vertical orientation seems to be more sensitive while horizontal
can allow you to cover more ground by making a wider swath. In the vertical position the sensor with the wires coming out towards
the end cap (the adjustable module) is more sensitive and should be the one nearest the ground. When using the mag in the
horizontal position, remember that a target signal could be coming from either of the two sensors. You will either have to
determine this by moving the mag around, or you can use the sign bit from the SCL007 to drive an LED to distinguish the sensors.
With a little practice you should be proficient in no time. The fluxgate mag is extremely easy to use and will make a nice (and
inexpensive!) addition to your treasure hunting arsenal.
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