Electric Field Disturbance Monitor -- Hobby Corner

Before we get into the details of the circuit, let's cover some of the basics first -- the phenomenon of the earth's electric field gradient. In his Lectures on Physics, Richard Feynman stated that as you go up from the surface of the earth, the electrical potential increases by about 100 volts per meter. Thus a vertical electric field gradient of 100 volts/meter exists in the air. As a means of explanation, imagine the existence of a very sensitive voltmeter that could measure the voltages present in the open air. If you pushed the negative terminal of the instrument's probe into the earth's surface and you positioned the positive lead one meter above the surface, about 100 volts would be detected. If you then moved the probe vertically by another meter above the surface, the voltmeter would measure 200 volts. This voltage difference would continue to increase as you moved the positive probe upward until it reached the top of the atmosphere, some 150,000 feet (46,000 meters) up. At that point, the instrument would finally measure an average potential difference of about 4 million volts. This naturally occurring 100 volts per meter electric field gradient exists everywhere in the earth's atmosphere and can even penetrate inside most buildings. You might ask: "If such voltages exist in open air, then why isn't the average 2 meter tall human shocked by the 200 volts that should be present between his feet and the top of his head?" The reason you don't feel anything is that the air is too poor a conductor of electricity to allow enough current to be delivered by the voltage. Also, since the human body is filled with salt water, which is a good conductor, the body actually distorts the voltages as it moves through the field, reducing the actual potential difference across the body. To illustrate this effect, suppose an air potential voltmeter was positioned to measure the atmospheric voltage between ground level and a position two meters above the ground surface. Without any conducting objects nearby, the meter would measure a potential of 200 volts. But, when a conducting human walked next to the probe the voltage is shunted to a near ground level potential and the instrument's reading would drop toward zero volts. As the human walked away from the meter, the field would again gradually be restored to the 200-volt reading. In addition to the average voltage changes as a person walks near the detector, the shifting contact of the human's foot on the ground, as well as arm motion, causes small higher frequency fluctuations in the measured voltage. Such signal changes are used as the basis for detecting human motion near the detection circuit. TOP
Field Disturbance Monitor Design The field disturbance monitor I designed is illustrated in Figures 2 and 3 below. It uses a telescoping whip antenna that is mounted on top of a metal box, to probe the air for field changes. By raising and lowering the antenna you can increase or decrease the field change sensitivity. The design I chose uses an off the shelf metal box to house the electronics. The circuit is powered by a standard 9 volt battery. Three LED light emitting diode indicator lights provide system status. One of the LEDs indicates a positive field change while another indicates a negative field change. A third power indicator light doubles as a battery voltage indicator. If the light fails to turn on, it is an indication that the 9 volt battery needs to be replaced. An alarm sensitivity dial can set the minimum disturbance level to trigger an alarm. A loud piezoelectric type beeper sounds whenever the alarm level is exceeded. A toggle switch allows the alarm feature to be turned off. An output jack at the rear of the monitor can be used to connect the monitor to a remote alarm device if desired. I have also included a output jack that can be used to send the monitor's processed disturbance signal to some remote recording device. Finally, to insure consistent operation, an earth ground jack is also included at the rear of the monitor's enclosure. Connecting the ground jack to a true earth ground improves disturbance sensitivity.


	Figure 2	Figure 3
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CIRCUIT DESCRIPTION Front-end Circuit Section -- Some of the experiments I performed about 25 years ago indicated that most moving objects, including humans, produce electric field disturbances with frequencies that range between 0.1Hz and 15Hz. However, when the disturbance monitor is used indoors, those signals must compete with rather large fields produced by nearby 50Hz and 60Hz power lines and appliances. Since the motion signals of interest can be as much as 1000 times smaller than the signals produced by power lines, much of the monitor's electronic circuit is dedicated to removing most of the unwanted power line frequencies. If the monitor is to be used only outside, a less aggressive filter design can be used. ( In later discussions I will include an exclusive outdoor version of the disturbance monitor. I might also include some super low power monitor designs that will operate for years off a 9 volt battery. ) A complete indoor/outdoor monitor circuit schematic is shown in Figures 4 and 5 (Adobe Acrobat pdf files). The circuit can be broken into several sections. The most important part of the design is the front-end section. Many different front-end circuits were tried over the years. The circuit included is both simple and effective. The circuit uses an operational amplifier A1 that is wired as an impedance amplifier. The circuit has a very high input impedance and a low output impedance. The small voltage signals collected by a telescoping whip antenna, caused by an object moving near the monitor, are routed to the amplifier circuit. The one gigaohm feedback resistor R2 provides the amplifier with a DC feedback path while the capacitor C1, in parallel with resistor R2, reduces the gain of the amplifier at high frequencies. Without the capacitor C1, the amplifier would easily be swamped by AC fields, associated with any 50Hz/60Hz power lines or line powered devices nearby. The 100K resistor R1 is placed between the antenna and the amplifier, to protect the amplifier from being damaged by any high voltage static charge that could be picked up by the antenna. The 1 gigaohm resistor value is important for studying human motion signals. Higher resistance values, going up to 100 gigaohms, have been tried and cause the monitor's frequency response to be excessively low. Electric field changes from nearby rain storms can be measured with high resistance values. A resistance value lower than 1 gigaohm makes the monitor more sensitive to higher frequencies.
Passive Filter Stages -- The signal that emerges from the front-end circuit will contain a large amount of power line noise. Even when the monitor is used outside, away from visible power lines, there will still be some unwanted power line signals collected. The passive filter section after the front-end stage contains three networks. One low pass filter network begins the process of attenuating the unwanted high frequencies. One high pass filter network is designed to block the slow DC shift that will occur at the front-end circuit. The values selected start rejecting frequencies below 0.1Hz. To remove much of the fundamental 50/60Hz noise signals, a third notch filter network is used. As shown on the schematic, I have selected the circuit components for a 55Hz notch filter frequency. The selection is a compromise between the 50Hz and 60Hz international power line frequency standards in use around the world. The notch filter should reduce the power line frequency noise by a factor of 1/50 (-34db). First Buffer Stage -- The output of the notch filter is connected to a non-inverting operational amplifier (A2a). The values chosen give the signals of interest a gain of about x6 while rejecting some of the unwanted higher frequencies. Second Buffer State with Active Filter Network -- The output of the buffer stage is connected to a pseudo three pole active low pass filter. The combination of the passive components and the operational amplifier (A2b) boost the signals of interest with a gain of x6 while filtering the high frequency signals that may still remain. The overall gain for the two amplifiers is about x36 (+31db). Note that all three of the operational amplifier stages are biased at 2.5 volts. The signals of interest will therefore swing above and below 2.5 volts. External Signal Output -- The output of the second buffer stage is routed to a phono jack at the rear of the monitor enclosure. Using a shielded cable connected to the phono jack, the signal can be fed to a strip chart recorder or to a digital recorder that is connected to a computer. Using some digital signal processing schemes a lot of information can be squeezed from the raw signals generated by the monitor. As stated above, individual human identification is possible by carefully monitoring the frequency signature produced by a person's arm and leg motion during walking. Also, certain animals and insects can be identified.
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Alarm Threshold Detector Circuits For some experiments, you may want to know if the signals exceeded a certain level. This feature is especially useful if the monitor is to be used for a motion alarm application. The signals that emerge from the signal processing circuits are routed to two comparators (A3a and A3b). The two comparators determine if the signal has sufficient amplitude to be considered an alarm condition. Comparator A3a is referenced above the 2.5v bias point, while the low A3b stage is referenced below the 2.5v bias voltage. The upper comparator is triggered when the signal swings above the upper threshold (positive voltage change) and the lower comparator is triggered when the signal swings below the lower threshold (negative voltage change). The variable resistor R17 symmetrically controls both thresholds and allows a single knob to set the trigger sensitivity levels. With the values chosen, the comparators can be triggered from voltage changes as small as +-0.05 volts or as high as +-1.5 volts. An LED connected to the output of each comparator provides an indication of either a positive or a negative disturbance. Both LEDs are mounted on the front side of the monitor's metal panel. Diodes D3 and D4 sum the two comparator outputs and with the aid of another comparator A4a drive the transistor Q1 and Q2. The two transistors Q1 and Q2 are used to drive the piezoelectric alarm connected to the monitor or an external alarm that is connect by a shielded cable to the remote alarm output jack at the rear of the monitor. The external alarm output can sound a remote beeper or close a relay. If desired, the local alarm feature can also be turned off when the alarm selector switch is placed in the "off" position. In the off position, the switch disconnects the 9 volt source to the local beeper alarm and the two indicator LEDs. When the system is operated in the alarm off mode, the overall current will be much less and will extend the operating time from the battery.

Power Supply Voltage Regulators To provide the system with two well regulated voltages from the 9 volt battery, two voltage regulators are used. The regulator A5 generates 5 volts while A6 produces 2.5 volts. The 2.5 volt supply is used to generate the mid-supply bias voltage used in the signal processing circuits while the 5 volt supply is used in the alarm and the voltage monitor circuits. Since the current demands of both supplies are very low, low power regulators are used.

Battery Voltage Monitor Another voltage comparator A4b is wired as a battery voltage monitor. Should the battery voltage drop below about 6.8 volts, the power indicating LED will not turn on, giving the user an indication that the battery needs to be replaced

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 cameratripod. TOP
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 phono 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. TOP

INTRODUCTION Field Disturbance Monitor Design Circuit Description Front-end Circuit Section Alarm Threshold Detector Power Supply Voltage Regulators Battery Voltage Monitor Monitor Assembly Material List Monitor Operation	ELECTRIC FIELD DISTURBANCE MONITOR
	Experiments with changes in the Earth's natural electric field and detects the movement of animals, vehicles and humans.
	INTRODUCTION
	About twenty five years ago, an inventor friend of mine showed me some illustrations in a college physics book. The book was one of a three volume collection of physics lectures given by the noble prize winner Richard Feynman. The book illustration was very similar to the figure below. The picture depicted how the earth's natural electric field would be distorted by an electrically conducting object, such as a human body, positioned above the earth's surface.