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Circuits designed by David Johnson,
P.E.
Last Updated on:
Monday, December 25, 2017 02:08 PM
Master Category List - Dave's Circuits
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More
Alarm Circuits
Beepers & Buzzers
Door Bell Circuits
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Door Knock Beeper -- June 14,
2009
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| 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. It is
suggested that the circuit be housed in a plastic box with a 9v battery holder. A
Serpac box from Mouser, part number 635-211-I-G is just right for this
application. It is about 4 inches long and 2.6 inches wide. It has a
nice compartment for a 9v battery. |
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The door knock is detected with a piezoelectric wafer, glued to the inside bottom of
the box, as shown below. The resistor R5 determines the knock sound sensitivity.
The value shown should work in most cases. The box could be hard mounted to the door
or suspended at about the middle of the door, by a string from the top of the door in
such a way that the box rests against the door. Whenever a knock occurs, the dual
comparator circuit will activate the beeper for a few seconds, and then turn off.
You can use just about any type of beeping device you wish. Some people prefer the
piezoelectric beepers with their higher frequency, while others may like the lower
frequency electromechanical buzzers. |
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Click on Drawing Below to view PDF version of Schematic |
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