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 Assembly

I  recommended that you use a metal box to house the monitor electronics. A telescoping whip antenna makes an excellent probe for collecting the electric field changes near the monitor. The antenna can easily be raised or lowered to increase or decrease sensitivity. A banana plug soldered onto the end of the whip antenna can be plugged into a matching insulated banana jack, mounted on top of the box. The metal chassis forms an electric shield around the electronic circuits inside and forms a reference capacitor plate with a large cross-sectional area. As the discussion above suggests, the metal chassis and the whip antenna form the two vertical points in space needed to detect the electric field change.

Figures 2 and 3 (seen above) suggest the placement for the components that need to be mounted to the front and rear of the metal box. The metal box selected has a top section that slides over the bottom section in two U-shaped pieces. All of the chassis mounted parts can then be mounted onto the top section. The circuit board can be mounted upside down to the inside top of the box. Four one inch long metal standoff legs should be used to suspend the circuit board.

The front-end amplifier circuit should be enclosed within a metal can that is soldered to the circuit board and connected to circuit ground. The shielded can is easily be constructed using sections cut from tin plated steel sheets which can be purchased from any hobby store. The can will help prevent other signals generated inside the enclosure from being picked up by the very sensitive front-end circuit. The open top of the can should be made about one inch high so it nearly touches the inside of the metal enclosure. It should be positioned so it surrounds the banana jack, that the whip antenna plugs into. You can connect the banana jack terminal to the front-end circuit by feeding a wire soldered to the jack through a hole in the circuit board.

The signal output jack and the earth ground terminal should be installed on the rear of the box. A 9 volt battery clip is used to secure the 9 volt battery, inside of the box. Although having the battery inside the box is less convenient, a fresh battery should provide many days worth of experiments. As shown in the drawing, the three LED indicator lights, the piezoelectric beeper, the alarm level control knob and the two toggle switches should be mounted on the front of the panel. Connections from the circuit board to the alarm LEDs, beeper, rear output signal jack and rear alarm jack should all be made using shielded cables.   Click here for a detailed Material List for this project.

When completed, the disturbance monitor will resemble a portable radio with a single large whip antenna protruding out its top. By gluing a 1/4 - 20 nut on the bottom of the box, the assembly can be attached to a standard metal camera tripod.

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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