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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 circuit turns on a beeper whenever the inside temperature of a freezer is greater then 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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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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- 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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