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

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 Figures 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 phone jack at the rear of the monitor enclosure. Using a shielded cable connected to the phone 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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