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Alarm Circuits, Page 1
Alarm Circuits designed by Dave Johnson:
Page 2
Circuits designed by David
Johnson, P.E.
Last Updated on:
Friday, March 12, 2010 12:07 PM
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DOORKNOB ALARM
Many companies offer simple alarm devices for personal use in bedrooms or hotel
rooms. A metal chain attached to a box holding the electronics is placed around
the inside doorknob of a wood door. Anyone grabbing the knob from the outside is
detected by the electrical capacitance change that occurs from the human hand
contact between the knob and the box. Almost all of the commercial devices sold
use a more expensive and power consuming radio frequency circuit approach to
detect the capacitance change. But, a very inexpensive and micro power technique
can also work. This circuit schematic should dramatically reduce the cost of the
device and allows it to operate for many years from one set of batteries.
Also see
Completed Project
by Matthew C. Ruschmann
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- Earthquake
Alarm Circuit
Geophones are dandy motion sensors. They are metal cans containing a
powerful magnet, suspended with springs inside a coil of wire. The magnet
functions as a floating mass. The slightest vertical motion of the device
causes the coil to move relative to the magnet. This induces a small
voltage in the wire coil...
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Fake Car Alarm Light
Whenever the car’s ignition is turned off, this circuit activates a flashing
LED, which can be positioned to appear as an active alarm system. (added 7/06)
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Freezer Over-Temperature Alarm
This electronic hobby circuit below is designed to activate a beeper alarm,
whenever the outside air temperature is below 0 degrees C (32F). A 9v battery
powers the hobby circuit. The average 9ua current is so low that the battery
should last for many years....
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Freezing Temperature Alarm
This circuit turns on a beeper whenever the outside air temperature is less than zero
degrees Centigrade. The circuit draws only a few microamps from a 9 volt battery.
It uses a glass bead thermistor accurate to 1 degree C.
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Human Traffic Footstep Alarm
This circuit processes the amplified signals from the footstep detector circuit and
activates a microprocessor when the circuit detects footsteps. At has an
adjustable sensitivity.
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Simple Security Wire Loop Alarm
A wire loop is used to protect valuable objects in this simple alarm circuit. The
electronic hobby circuit is powered by a 9v battery. The alarm beeper is activated
if the wire loop is severed. The standby current is so low that the 9v battery
should last for many years.
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- Wire Security
Loop Alarm
A wire loop is used to protect valuable objects in this alarm circuit. The circuit
is powered by a 9v battery. (added 7/06)
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More Alarm Circuits
Master Category List - Dave's Circuits
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