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.