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Sunday, February 27, 2011

Deer Repellent/ Seismic Sensor

Here is a simple sensor which can detect the seismic vibrations caused by a person or
large animal walking nearby. A representative application for the sensor is a deer
repellent for the vegetable garden. When a deer steps near the sensor a loud buzzer or
beeper sounds for a few seconds startling the would-be vegetable thief away. The
sensor also makes an effective intruder detector to catch trespassers as soon as they
step on the property!


The unit is designed to consume minute amounts of power when not rattling to provide
many months of unattended protection (practically set by the battery shelf life). The
seismic sensor is built from an ordinary 2 inch speaker by gluing a mass to the speaker
cone to lower its resonance frequency. A lid from a baby food jar with a little extra
weight glued to the inside will work fine.

The CA3094 I.C. is an unusual op-amp consisting of a programmable transconductance
amplifier connected to a darlington transistor. In this circuit the darlington is combined
with a pnp transistor to form a monostable timer which determines how long the buzzer
sounds. When the ground shakes, the vibration sensing speaker generates a small voltage
which is amplified causing the voltage on pin 1 to go high. The darlington in the IC and
the 2N4403 turn on with regenerative feedback provided by the diode. The 2N4401 turns
on, powering the buzzer until the monostable resets.

The circuit may be used to activate a variety of devices including a relay to control line
voltage devices, a transmitter to telemeter an alarm from a remote location, a battery powered walkway light, or even one of those battery powered squirt guns! The output
transistor may be replaced with a power darlington transistor for directly controlling
higher current loads.

The circuit may be built into ordinary plumbing PVC pipe or practically any
weatherproof enclosure. A larger detection area may be accomplished by burying a long
pole or PVC pipe just below the surface of the ground with the seismic sensor located
above the pole. Vibrations will readily travel down the pole whenever a footstep occurs
anywhere along its length.

The seismic sensor may be replaced with other sensors for different applications. A
photocell/ resistor divider will sense changes in light level, a microphone will sense fairly
low sound levels, and a diode detector will sense a low-level RF field.

Deer Repellent/ Seismic Sensor

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