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Circuits
designed by David Johnson, P.E.
Last Updated on:
Thursday, December 01, 2011 07:04 AM
Master Category List - Dave's Circuits
Text & Graphics Copyright ©
David A. Johnson, PE - ALL Rights Reserved.
LINKING to Dave's circuits is permitted but DO NOT COPY to your WEB SITE server! |
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Alarm Circuits,
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3V Sweeping Siren
Alarm
The circuit uses a LTC1799 precision frequency generator from Linear Technology.
A 74HC14 hex Schmitt trigger from Texas Instruments is also used to perform several
other functions. One section is wired as a simple 7Hz square wave oscillator.…
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AC Line Under/Over Voltage Alarm - Power lines, which deviate much beyond
normal voltages can damage expensive electronic equipment. The circuit below
sounds an alarm whenever the line voltage is higher or lower than normal. I
set the alarm limits at about +-15% from standard levels. The circuit
rectifies and filters the power line signal. I set the resistor values, so the
DC voltage produced is close to 1% of the RMS value of the line. Thus, a
120vac line would yield about 1.2v DC. That voltage is fed to a pair of
voltage comparators. The outputs of the comparators decide if the sampled
voltage is above, below or acceptable....
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Chain Link Fence Alarm - Long stretches of steel fence can be monitored using
the simple circuit below. An insulated wire is woven through the links in the fence.
Where the fence begins, the circuit is connected to an earth ground and the security
wire. At the end of the fence, the wire is connected to one of the steel posts, held in
the ground. The typical earth ground resistance will be in the 1000s of ohms
range, which is low enough to keep the n-channel FET turned off in the circuit below.
If the wire is cut or broken,....
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Cut Phone Line Alarm - Burglars, rapists and other criminals will often cut a
phone line before they break into a home. The cut phone line will prevent many
alarm systems from alerting the police. The battery powered circuit below will
sound an alarm when the unit no longer....
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Door Knock Beeper - In some apartment buildings and homes, not equipped with a
door bell, it may be hard to hear someone knocking on the front door. The circuit
below provides a means to activate a loud beeping sound inside, whenever someone knocks
on the door from the outside. The circuit is powered by a 9v battery, which should
provide years of service. An on/off switch allows the device to be turned off if
desired....
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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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