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DiscoverCircuits.com -- Hobby Corner
Last Updated on:
Tuesday, June 01, 2021 03:06 PM
Hobby Circuits List
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More
Alarm Circuits
Beepers & Buzzers
Door Bell Circuits
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Door Bell Sounds like a Knock on
the Door -- 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 Circuit Below to view PDF of Schematic |
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