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Alarm Circuits, Page 2
Alarm Circuits designed by Dave Johnson:
Page 1
Circuits designed by David
Johnson, P.E.
Last Updated on:
Friday, March 12, 2010 12:08 PM
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MACHINE POWER LOSS BEEPER
For some medical equipment it is important for an operator when power is lost to
the machine. The beeper is powered from a 9v battery and requires the machine to
have a power switch with a third set of contacts.
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MICRO POWER OVER-TEMPERATURE ALARM
The circuit is powered for years by a single 3 volt lithium battery. It sounds
an alarm when the temperature exceeds a certain point. With some minor changes
the circuit could also be configured for an under temperature (freeze) alarm.
The circuit uses a cheap but accurate thermistor as the temperature sensor.
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Motion Alarm
Using a piezoelectric device, this circuit will activate a beeper whenever the
circuit is moved. It could be used as an
earthquake alarm.
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Motion Shadow Alarm
This simple circuit uses two inexpensive PIN photo diode light detectors to
detect nearby motion. Any shadow which is cast over the two diodes from a
moving object produces a slight voltage change between the two diodes.
When this change is detected, an alarm switch is activated.
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Motion Alarm Using Piezoelectric Device
An inexpensive piezoelectric device is used as a motion sensing device for this
motion alarm. This circuit will activate a beeper whenever the circuit is moved.
It could be used as an
earthquake alarm.
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Pump Motor Monitor
This circuit can turn on a light or
sound a beeper, whenever an AC line powered pump motor turns on. It is
designed to detect the motor current using a small inductor placed on the
outside of the motor’s power cable. No direct wire contact is needed.
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Simple Security Wire Loop Alarm:
A wire is looped through various objects which are to be monitored. The loop wire
is typically an aircraft cable but could also be any insulated stranded wire. The
loop is attached to a table or desk. Should the loop be cut, the alarm is sounded.
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TIME TO
DUST INDICATOR
I thought about this circuit when I heard that a lot cleaning personal in hotels were
either dusting rooms more often than necessary or not enough. I have not yet built and
tested this circuit completely but in concept it should work. The circuit draws very low
current from a +3v battery and could be housed in a package similar to a small ashtray.
The assembly might be placed in a suitable out of the way area to collect dust. It would
alert a maid when it was time to dust the room. The circuit detects dust with an
infrared LED that is pulsed so its light shines onto a smooth flat plate. Any dust
settling onto the surface is detected by a phototransistor, mounted at a 90 degree angle
from the LED. When the dust reaches a particular level, sufficient light is reflected
into the phototransistor to change the logic state of the circuit to an alarm condition.
The alarm output could be connected to a beeper, a flashing LED or to one of the LED
flashing circuits in this hobby circuit section.
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UNPLUGGED POWER CORD ALARM
Many medical devices, such a portable X-ray and ultrasound equipment, carry their own
batteries. However, the frantic pace of some hospital emergency rooms might cause the
device to be shoved off into a corner without being plugged back into an AC power source
to recharge their batteries. So, I designed this circuit to emit an audible alarm
whenever the device’s power cord was unplugged from the wall for a period of time. The
device was designed to snap onto the outside of a power cord, where it senses the AC
electric field emitted. No direct connection to the internal wires of the power cord is
needed. I also included a low battery voltage monitor.
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WATER
SEEPAGE ALARM
This simple circuit sounds a beeper when its electrodes detect water. It is powered by a
single 1.5v N cell. A small 1.5v button battery will also work.
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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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