The Design Corner / Hobby Corner - Electronic Field Disturbance Monitor
designed by David A. Johnson, P.E.
Updated on:  Thursday, December 17, 2015 01:44 PM

  Electronic Field Disturbance Monitor:  Introduction - Monitor Design - Monitor Operation

Circuit Description:    Front-end Circuit Section - Alarm Threshold Detector 
Power Supply Voltage Regulators
Battery Voltage Monitor - Monitor Assembly
Material List                       

Hobby Corner:   Electronic Tips - Hobby Circuits - Electronic Field Disturbance Monitor - 100+ Construction Ideas

Monitor Operation

The system works best when the metal box is attached to a metal tripod. If a plastic or wood tripod is used, an earth ground reference should be established by connecting a wire from the unit's earth ground terminal to a metal rod, that can be driven into the ground. An old screwdriver with the wire attached makes a convenient grounding tool. If the unit is used indoors, a good earth ground can be obtained by connecting the monitor's ground terminal to a water pipe or the ground terminal of a power outlet. The monitor will still work, even without an earth ground, but it will lack overall sensitivity, especially to the low field change frequencies. I should also mention that the monitor works much better in dry environments. Moist air tends to be slightly more conductive than dry air and therefore less static electricity is produced by walking humans.

Because the monitor's frequency response extends down to once cycle in five seconds, you should expect the monitor to go into an alarm condition for several seconds after the power is first turned on. Likewise, when the monitor is subjected to a large field change, that may saturate the front-end circuit, the alarm may sound for several seconds, even after the field has stabilized.

A standard phone jack mounted on the rear panel can be used to route the disturbance signals to a data collection device. When using the signal output jack, a shielded audio type cable should be used to route the signal to the recording device. The output signal is designed to have a 2.5 volt center bias point, so the signals of interest will swing above and below 2.5 volts. The voltage range is ideal for many digital recording systems that are connected to a computer.

When you are satisfied that the circuit board and all the components are wired correctly,  attach a fresh 9v battery to the battery clip. Make sure the unit is turned when you connect the new battery. Extend the whip antenna to a mid 24 inch length. Lay the monitor on a high wood table or on top of a wood shelf. Position the sensitivity dial to the mid point. Switch the alarm switch to the on position. Switch the power switch to the on position and back away about six feet from the box. You need to stay perfectly still and not move. The alarm should sound for several seconds. After the alarm stops, you should be able to move one of your feet and see the two LED indicator lights turn on and off while the alarm beeper sounds. Try walking by the unit and notice how the two indicator lights turn on and off matching you foot steps.

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